Background Travelers’ diarrhea (TD) occurs at high frequency in individuals from industrialized countries visiting destinations in nonindustrialized countries and may result in chronic complications such as Guillain–Barré syndrome.
Methods We distributed a questionnaire requesting information on physicians’ perceptions of currently available TD products, hypothesized TD vaccines, and aspects of travelers’ behavior. Some physicians also were interviewed by telephone.
Results We obtained completed questionnaires from named individuals at 68 UK/US clinics. These individuals reported seeing a total of approximately 76,500 travelers per year, and estimated that 61% (UK) and 77% (US) of travelers present 2 weeks or more predeparture. More US (92%) than UK (43%) travelers are advised to purchase TD products. In both countries, 85% of travelers would be prescribed an ideal TD vaccine, but only ∼47% (UK) and ∼65% (US) would purchase this vaccine. About 80% of physicians would recommend an hypothesized 100% effective Campylobacter vaccine for travelers visiting regions where 30% of TD cases are caused by Campylobacter.
Conclusions Physicians support the concept of TD vaccines, including campylobacteriosis vaccines. An ideal TD vaccine might be purchased by up to 16% (UK) and 28% (US) of relevant travelers. A 100% effective Campylobacter vaccine might be purchased by over 2 million UK/US travelers per year.
Travelers’ diarrhea (TD) is usually defined as the passage of at least three unformed stools in 24 hours, together with symptoms such as fever, vomiting, or abdominal pain.1,2 Common causes of TD in adults include enterotoxigenic Escherichia coli, Shigella, and Campylobacter. Typically, TD occurs in individuals from industrialized countries when visiting destinations in nonindustrialized countries. The incidence of TD among these travelers is often said to be 20% to 50% within 2 weeks of arrival.2–5
Furthermore, TD is one of the most common problems encountered by deployed military personnel6–8 and, in some cases, pathogens such as Campylobacter may cause chronic complications. Thus, campylobacteriosis has been associated with the development of myocarditis9 and acute pancreatitis,10,11 and occasionally progresses to one of the most serious TD-associated sequelae, namely Guillain–Barré syndrome (GBS).12 GBS is a peripheral nerve disorder characterized by paralysis12 and has been noted in international travelers.13 Given the high incidence of Campylobacter-associated TD in some regions (up to 40% of TD cases in parts of Africa and Asia14,15), and its potential to cause serious chronic conditions,12 it may be concluded that a Campylobacter vaccine would be of use for at-risk groups such as international travelers visiting destinations in Asia.
Therefore, the literature indicates that TD occurs at high frequency among travelers, may significantly disrupt work/travel plans, and has the potential to cause serious chronic conditions. We have carried out a survey to determine current practice and perceptions regarding available and proposed TD treatments and prophylactics. The data we have collected, together with preexisting information, allow an estimation of the probable take-up of proposed TD vaccines.
We designed a postal questionnaire for distribution to UK and US physicians active in the field of travel medicine. Respondents were asked to provide information on the number of travelers to TD high-risk regions (“relevant travelers”) seen per annum, on perceptions of currently available TD products, and on perceptions of hypothesized TD vaccines.
Questions relating to physicians’ perceptions of currently available TD products included requests to identify the nature of TD products, if any, recommended to travelers to TD high-risk regions; to estimate the proportion of relevant travelers that are advised to purchase a TD product; and to estimate the proportion of relevant travelers so advised who actually purchase one or more of the recommended TD products.
For the purposes of the survey, we postulated two hypothetical vaccines, namely an ideal TD vaccine (ie, a TD vaccine that is safe and effective against all TD pathogens in all travelers, with an administration schedule that is suitable for all patients, and that may be purchased at a price similar to that of other travel vaccines) and a 100% effective Campylobacter vaccine. We asked physicians to estimate the proportion of travelers to TD high-risk regions to whom the clinic would strongly recommend an ideal TD vaccine, estimate the proportion of these travelers (based on the physician’s experience of his or her patient population) that would then purchase the hypothesized ideal TD vaccine, and estimate the extent to which the clinic would recommend a 100% effective Campylobacter vaccine at destinations with different defined levels of campylobacteriosis (as a percentage of all TD cases at the destination). The campylobacteriosis incidence levels that we postulated were as follows: 1%–5%, 6%–10%, 11%–20%, 21%–30%, 31%–40%, and >40%.
In both the UK and the United States, named individuals with an active interest in travel medicine were identified by internet searches (eg, to identify travel clinics, yellow fever vaccination centers, attendees at travel medicine conferences, and members of relevant professional organizations) and by literature searches (to identify individuals who are publishing in the field). Where clinics but not named individuals were identified by this method, the clinic was contacted by telephone to obtain the contact details for an appropriate individual. This methodology identified 45 individuals throughout the UK (Scotland, England, Wales, and Northern Ireland).
Many potential contacts were identified in the United States; however, for time and budget reasons, we chose to limit the total UK + US surveyed population to 300. Therefore, 255 named individuals were selected from the pool of potential US contacts, according to the following two principles. First, we attempted to contact at least one individual from each state of the United States, to provide broad geographic coverage (for the more densely populated states, several individuals per state were contacted). Within each state, we attempted to include individuals associated with clinics in large population centers, for example, state capitals. This strategy was expected both to provide reasonable geographic coverage and also to provide information from some of the largest and busiest clinics in the United States.
The questionnaire was emailed or faxed to each of the US and UK contacts that we identified above. After 2 weeks, all those that had not responded were sent the questionnaire a second time, by fax. Thereafter, nonresponders were, where possible, contacted by telephone and encouraged to return the questionnaire.
Data were entered into a Microsoft Excel spreadsheet for analysis. Where the responses provided a range of numerical values, the midpoint of that range was used.
After the initial data analysis, those respondents who had indicated that they would be willing to participate in a verbal interview in return for an honorarium were (where possible) contacted by telephone to discuss their responses in more detail. The precise number and identity of the physicians that participated in verbal interviews were largely determined by their availability within the timescale of this project.
Follow-up questions posed during verbal interviews included requests to estimate the percentage of patients traveling to TD high-risk regions that present 2 weeks or more prior to departure; to discuss reasons why patients refuse to purchase TD products in general and TD vaccines in particular; to discuss the significance of campylobacteriosis; and to discuss the factors that would influence the physicians’ willingness to recommend proposed TD vaccines.
Response rates and respondent characteristics
The questionnaire was sent to 300 named individuals at clinics throughout the UK and the United States. Of these, 71 returned the questionnaire, providing 68 usable questionnaires (a response rate of about 23%). Of the 68 respondents, 15 were from the UK and 53 from the United States. Within the UK, six respondents were from the London region, six from regions of England outside London (Ashford, Kent; Nottingham; Gloucester; Liverpool; Alcester, Warwickshire), and three from Scotland (Edinburgh, Aberdeen, and Glasgow). Within the United States, responses were received from 20 states across a broad area of the United States, including the Eastern United States [28 respondents from, respectively, PA (5), NY (6), MA (7), RI (1), NJ (4), MD (1), WV/VA (2), FL (1), and TN (1)], Western states [12 respondents from, respectively, OR (2) and CA (10)], and Central states [12 respondents from, respectively, IN (1), OH (3), WY (1), NE (1), CO (1), OK (1), TX (1), WI (1), and MI (2)]. There was one anonymous response from an unidentified US location.
The 15 UK respondents indicated that they see approximately 28,435 travelers to TD high-risk regions per year, and the 53 US respondents see approximately 48,058 travelers to TD high-risk regions per year. The total number of relevant travelers seen per annum by the respondents to our survey therefore is 76,493. The smallest number of relevant travelers seen per annum was 10 (a US respondent); the largest number of relevant travelers seen was 10,000 per annum (a UK respondent).
In conclusion, the 23% response rate achieved with the postal questionnaire was satisfactory. The geographical distribution of respondents in both the UK and the United States seems to be reasonable, suggesting that the data should not have been significantly skewed by spatial factors.
Respondents’ attitudes to currently available TD products
The postal questionnaire asked respondents to identify the TD products (prophylactics or treatments to be carried in case of need) recommended to travelers visiting regions at high risk for TD. The responses indicate that virtually all physicians in both the United States and the UK recommend that at least some travelers carry antibiotics (Table 1). However, there are some differences in the responses provided by UK and US physicians. Thus, the survey suggests that the cholera vaccine is prescribed by only UK physicians and that rehydration salts and loperamide are more commonly prescribed by UK than by US respondents. Bismuth is prescribed by only US respondents.
Table 1. TD products recommended to travelers visiting regions at high risk of TD*
% UK physicians (n= 15)
% US physicians (n= 53)
% Total physicians (n= 68)
TD = travelers’ diarrhea.
The postal questionnaire provided physicians with an opportunity to list TD products that they recommend to travelers visiting high-risk TD regions; responses are summarized above. The main differences between the United States and the UK appear to be related to the availability in the UK, but not in the United States, of an approved cholera vaccine, which is sometimes suggested to protect against some forms of TD;26,27 and the approval in the United States, but not in the UK, of bismuth products. In addition, UK physicians seem to be significantly more willing than their US colleagues to recommend symptomatic treatments, such as loperamide and rehydration salts. The great majority of physicians in both countries recommend antibiotics to be carried in case of need.
Loperamide or similar
Proportion of relevant travelers advised to purchase TD products, and respondents’ estimations of Take-Up of recommended TD products
The postal questionnaire asked physicians to identify the proportion of relevant travelers that are advised to purchase a TD product. The responses are summarized in Figure 1. It can be seen that nearly half of UK physicians recommend purchase of a TD-related product to only 1% to 20% of their patients traveling to high-risk TD regions; however, 33.3% of UK physicians recommend purchase of TD products to 81% or more of such travelers, and over 13% of UK physicians recommend purchase of a TD-related product to 100% of relevant travelers. Among US physicians, over 90% recommend purchase of a TD-related product to 81% or more of relevant travelers, and about 70% of US physicians recommend purchase to 100% of relevant travelers. No US physicians recommend purchase of a TD product to less than 41% to 60% of relevant travelers. These results suggest a mirror-image difference in prescribing behavior (Figure 1) between the UK and the United States in terms of the numbers of physicians advising given proportions of their traveling patients to purchase a TD-related product.
However, not all patients who are advised by their physician to purchase a product will actually make that purchase. Our questionnaire also requested physicians to estimate the proportion of relevant travelers that will purchase a TD product when advised to do so. Responses suggest that, among the patient populations seen by the physicians that responded to this survey, in the UK about 43% of patients are advised to purchase a TD product, and about 37% actually purchase a TD product. In the United States, by contrast, over 92% of the patient population represented by this survey are advised to purchase a TD product, and about 77% are thought to actually make the purchase. This suggests that similar proportions of relevant travelers act on the advice given by their physicians in both the UK (86%) and the United States (84%). However, far more relevant travelers are advised to purchase TD products by US than by UK physicians (92% vs 43%).
Proportion of relevant travelers that would be strongly advised to purchase an ideal TD vaccine, and respondents’ estimations of subsequent Take-Up of an ideal TD vaccine
Our postal questionnaire also requested physicians to provide estimations of the extent of likely recommendation of a postulated ideal TD vaccine and, based on their experience of their patient population, to provide estimations of the proportion of travelers that would actually buy the ideal TD vaccine when recommended to do so. Responses suggest that the proportion of patients likely to be advised to purchase an ideal TD vaccine is very similar in the United States (∼84%) and the UK (∼86%). However, only ∼47% of the UK patient population seen by physicians that answered this question (n= 28,435) is thought likely to purchase the postulated vaccine, as opposed to 65% of the US patient population seen by physicians that answered this question (n= 48,058).
None of the UK physicians that responded to the survey indicated that they would never recommend purchase of an ideal TD vaccine, and only one US physician indicated that he/she would never recommend an ideal TD vaccine. Thus, the postal survey suggests that there is no significant inherent resistance among UK/US physicians to the concept of TD vaccines. However, the survey also suggests that both UK and US physicians believe that significant proportions of their patients would refuse to purchase even an ideal TD vaccine.
Proportion of physicians that would recommend purchase of a 100% effective campylobacter vaccine
In our postal survey, we postulated a 100% effective Campylobacter vaccine and asked physicians to indicate the extent to which they would recommend vaccination for travelers visiting regions where Campylobacter was responsible for various specified percentages of all TD cases. The responses obtained are summarized in Figure 2. The data are presented as cumulative totals, that is, the percentage of physicians that would recommend purchase of a Campylobacter vaccine at a given rate of campylobacteriosis incidence is the sum of the percentage of physicians that would recommend purchase of the vaccine at the given rate of incidence and of the percentage of physicians that would recommend purchase of the vaccine at lower rates of incidence.
The data suggest (Figure 2) that the US and UK populations of travelers to high-risk TD regions would be subject to very similar prescriber behavior with regard to a putative 100% effective Campylobacter vaccine. Thus, the proportion of physicians that would recommend purchasing the Campylobacter vaccine is comparable in the United States and the UK, at all levels of campylobacteriosis incidence.
The survey results also indicate that a 100% effective Campylobacter vaccine may be recommended by up to 20% of physicians even when travelers are visiting destinations with campylobacteriosis incidence levels of only 1% to 5% (Figure 2). Clearly, many travel physicians feel that campylobacteriosis is sufficiently significant to merit prevention by vaccination, even for travelers visiting regions with a relatively low incidence of Campylobacter-mediated TD.
Verbal interviews with physicians
We carried out verbal interviews with 7 physicians in the UK (seeing a total of 16,250 relevant travelers per year) and 13 physicians in the United States (seeing a total of 20,900 relevant travelers per year). We asked physicians to estimate the proportion of travelers to regions of high TD risk that present themselves for pretravel consultation 2 weeks or more prior to departure. Responses suggest that, in terms of percentages of the total numbers of patients seen per annum by the physicians that were interviewed, 61% of relevant travelers from the UK and 77% of relevant travelers from the United States present 2 weeks or more prior to departure. US physicians report a range of 25% to 90% of patients meeting the 2-week window, and UK physicians report a range of 50% to 90%.
We also asked physicians to comment on the factors that influenced their decisions on what products to prescribe for travelers to regions at high risk of TD. Information volunteered by physicians during the interviews suggests that in the UK, the main factors affecting physicians’ willingness to recommend existing TD products for travelers to regions at high risk of TD are individualized risk assessments (specified by four of seven UK physicians), such as the nature of intended travel and activities. The high frequency of TD was mentioned as a factor by one of seven UK physicians.
By contrast, in the United States, the main factors affecting physicians’ willingness to recommend existing TD products are (1) a desire to avoid local health services and/or to provide the ability to self-medicate (specified by 7 of 13 US physicians) and (2) the high frequency of TD (mentioned by 6 of 13 US physicians). Although 1 of 13 US physicians mentioned individualized risk assessments as an influencing factor, 3 of 13 indicated that they undertook little or no risk assessment, for example, because of data suggesting that behavior has little effect on TD frequency.
In summary, in the UK, physicians tend to recommend TD products on the basis of an individual risk assessment. By contrast, in the United States, the general pattern is for TD products to be automatically recommended to all patients, in view of the high frequency of TD and the perceived poor relationship between behavior and TD incidence. US physicians also frequently expressed a desire to help their patients avoid using local health care systems by equipping them with the ability to self-medicate. The verbal interviews with physicians therefore provide a basis for the apparent differences in UK and United States prescribing behavior illustrated in Figure 1.
Physicians were also asked to provide their views on the reasons for travelers refusing to purchase TD products when advised to do so. Responses suggest that, in the UK, physicians believe that the main factor affecting travelers’ purchasing decisions with regard to TD products is cost, including the prevailing “NHS culture”, that is, the expectation that all health care should be free. Thus, cost was specified as a factor by five of seven UK physicians; however, it should also be noted that two of seven UK physicians explicitly stated that cost was not an issue. Other factors affecting a UK traveler’s decision to purchase a TD product were said to include (1) a perception that TD can be avoided (mentioned by three of seven UK physicians); (2) a perception that TD is mild (mentioned by two of seven UK physicians); and (3) a perception that TD is inevitable (mentioned by one of seven UK physicians).
Similarly, in the United States, physician interviews suggest that the main factor affecting travelers’ purchasing decisions for TD products is cost. Thus, cost was mentioned as a factor by eight of thirteen US physicians, and six of thirteen said that it was the main factor behind refusing to purchase; however, one of thirteen said that cost was not an issue. Another factor commonly mentioned by US physicians (four of thirteen) was the perception among some travelers (eg, those visiting friends/family) that TD can be avoided.
Physicians were also asked to comment on factors that would influence their willingness to prescribe a hypothetical ideal TD vaccine. Responses given suggest that, in the UK, the most common factor affecting a physicians’ willingness to recommend an hypothesized ideal TD vaccine would be the individual risk assessment (mentioned by four of seven UK physicians). Other factors included (1) the duration of protection afforded by the vaccine (mentioned by two of seven UK physicians); (2) cost or the need to prioritize other vaccines within a limited budget (three of seven); (3) the potential for side effects (two of seven); and (4) the vaccine administration regime (one of seven).
In the United States, by contrast, interview responses indicated that many clinics would routinely recommend an ideal TD vaccine for travelers to regions at high risk of TD (seven of thirteen US physicians gave this response). However, four of thirteen US physicians suggested that they would prescribe an ideal vaccine on the basis of a risk assessment per traveler. Other factors affecting the US physicians’ willingness to recommend a proposed ideal TD vaccine included (1) concerns about side effects (one of thirteen) and (2) prioritization of other vaccines, for example, if the traveler’s budget is limited (one of thirteen).
The postal survey clearly indicated that most physicians would expect a proportion of their patients to refuse even an ideal TD vaccine recommended by their physician. Physicians therefore were asked to provide their views on the reasons why travelers might refuse to purchase an ideal TD vaccine. Responses from UK physicians suggested that the main factor likely to influence a traveler to refuse even an ideal vaccine strongly recommended by his/her physician is cost (specified by six of seven UK physicians). Other factors affecting UK patients’ willingness to purchase an ideal TD vaccine include (1) an antivaccine attitude and/or fear of too many vaccinations (mentioned by four of seven UK physicians); (2) a fear of side effects (mentioned by one of seven UK physicians); (3) injection pain (one of seven, but note that one of seven also explicitly said that the route of administration was not an issue); and (4) an expectation of good health care at the destination (one of seven).
Responses from US physicians were quite similar to those from the UK. Again, the most common reason suggested for a traveler refusing to purchase an ideal TD vaccine strongly recommended by a physician was cost (specified by nine of thirteen US physicians). Other factors that were mentioned include (1) an antivaccine attitude (specified by four of thirteen US physicians); (2) needle phobia or a dislike of injections (four of thirteen); (3) a limited tolerance of multiple vaccines (two of thirteen); and (4) fear of side effects (two of thirteen).
Physicians were also asked to comment on factors that would influence their willingness to prescribe a hypothetical Campylobacter vaccine. Results suggest that a significant proportion of UK physicians regard the incidence of Campylobacter-mediated TD at the destination as only one of several influencing factors. Thus, two of seven UK physicians explicitly volunteered that extent of coverage is not the sole factor behind deciding whether to vaccinate and one of seven mentioned that reported incidence rates are unreliable, although they would have some influence. Similarly, two of seven UK physicians could not say what level of coverage would trigger a vaccination recommendation.
In the United States, by contrast, at least nine of thirteen physicians appeared to be strongly influenced by the level of campylobacteriosis incidence at the destination, but one of thirteen was skeptical about the reliability of reported incidence values, and one of thirteen volunteered that vaccination recommendations are not made on the basis of extent of coverage alone. The increasing frequencies of antibiotic resistance in Campylobacter were noted as an influencing factor by five of thirteen US physicians, and four of thirteen said the vaccination decision would depend on a risk assessment per patient.
Finally, physicians were asked to comment on the significance of Campylobacter as a pathogen. Among UK physicians, four of seven volunteered that they regarded Campylobacter as having the potential to cause significant and serious disease, and three of seven indicated that they would be keen to have a vaccine. By contrast, three of seven UK physicians said that a Campylobacter vaccine was not essential and one of seven UK physicians said that Campylobacter was only responsible for mild, self-limiting disease. Similarly, in the United States, seven of thirteen physicians indicated that they felt that campylobacteriosis can be serious, while two of thirteen said that Campylobacter was not common and/or not serious.
All surveyed UK and US physicians identify antibiotics as one of the TD products recommended to relevant travelers. However, although antibiotics appear to be the most commonly prescribed TD products, the literature suggests that they are not always useful or beneficial. In particular, the growing frequency of resistance is a concern. For example, fluoroquinolone resistance appears to be a particular problem with Campylobacter,5 especially in Southern Asia.16
Our survey suggests some US–UK differences regarding prescription of TD products for relevant travelers. The main differences seem to be due to the different availability of drugs in the respective countries; thus, bismuth but not cholera vaccine is available in the United States, while the converse holds for the UK. However, there does seem to be a genuine US–UK divergence in the extent to which physicians recommend purchase of TD products to relevant travelers. For example, nearly half of UK physicians recommend purchase of a TD-related product to only 1% to 20% of relevant travelers, whereas no US physicians reported recommending the TD products to less than 41% to 60% of relevant travelers, and about 70% of US physicians recommend purchase to 100% of relevant travelers (Figure 1). Data from telephone interviews suggest that the difference is due to UK physicians giving more weight to individual assessments (eg, only recommending TD products for travelers likely to indulge in higher risk behavior), while US physicians seem more likely to take the view that the high frequency of TD, and the ineffectiveness of behavioral precautions,17 indicate that TD products should be routinely recommended for travelers to high-risk regions. Hence, in the UK, about 43% of patients are advised to purchase a TD product, while in the United States, by contrast, over 92% of the patient population represented by this survey are advised to purchase a TD product.
The proportion of patients that would be advised to purchase an ideal TD vaccine is practically identical in the UK (∼86%) and the United States (∼84%). Given that we hypothesized a vaccine with ideal characteristics, the high projected prescription rate is not surprising. Of greater interest is the finding that even a TD vaccine with ideal characteristics, sold at a similar price to other travel vaccines, would not have a 100% prescription rate. Data from verbal interviews suggest that this could be due to a number of factors, including the need to prioritize other vaccines within a limited budget, and an assessment of the extent to which an individual traveler is likely to benefit from a vaccine given the intended destination, duration of stay, nature of activities, and mode of travel. In any case, the clear implication is that even an ideal TD vaccine will never be prescribed to more than about 85% of relevant travelers.
Our survey also suggests that the percentage of physicians that would recommend purchase of the Campylobacter vaccine is practically identical in the United States and the UK, at all levels of campylobacteriosis incidence, and approaches 100% at incidence levels of 31% or more (Figure 2). An interesting finding to emerge from this survey is the potential for some patients to be advised to purchase a Campylobacter vaccine even at incidence rates of only 1% to 5%. The apparent willingness of some physicians to recommend a putative Campylobacter vaccine to patients traveling to destinations with a low incidence of campylobacteriosis may be at least partly explained by the influence of the individual’s risk assessment; for example, some travelers, by virtue of type of travel/duration of stay, will be perceived as being at greater risk than others and prescribed the vaccine accordingly.
The above suggests that a high proportion of relevant travelers are advised by physicians that perceive Campylobacter as a significant pathogen. This interpretation is largely supported by data from verbal interviews, which indicate that a majority of UK and US physicians believe that Campylobacter can cause serious disease. However, in both countries, there appears to be a significant minority that believes that the pathogen causes only mild, self-limiting disease and/or that a vaccine is not essential. Our findings therefore suggest that there is not universal consensus in the travel medicine community (both within the UK and the United States) with regard to the significance of Campylobacter.
We also asked physicians to discuss the factors that might lead to a decision to prescribe a Campylobacter vaccine. The responses suggest that most US physicians would recommend the vaccine based primarily on incidence at the traveler’s destination, with the level of antibiotic resistance at the destination and individual risk assessments also being significant factors. In the UK, although campylobacteriosis incidence rates have some influence on the decision to recommend a vaccine, the survey suggests that many physicians would use an individualized risk assessment as a primary basis for their prescription decision. Our conclusion is that the decision to recommend vaccination is influenced by many interacting factors, including a risk assessment of the patient (eg, duration of exposure, type of travel); features of the vaccine (eg, duration of protection, side effects, administration regime); and cost (including the necessity to prioritize vaccines within a limited budget).
Physicians’ perceptions of factors that influence travelers’ behavior
Data from the verbal interviews that we carried out suggest that most physicians, both in the UK and the US, believe that cost is a common reason for travelers to refuse purchase of a vaccine. This perception is partly supported by recent airport surveys18–21 suggesting that between 12 and 20% of travelers may perceive vaccines as expensive. However, the existence of this perception does not necessarily indicate that travelers are unwilling to pay the prices charged. Responses given by the travelers themselves indicate that the percentages refusing to purchase a travel vaccine on the basis of cost range from 2% for typhoid and 3% for cholera22 to 4% for hepatitis A and influenza.22,23 Similarly, our survey indicates that some respondents believe that cost is not a significant factor in travelers’ decisions to purchase or decline vaccines and that other reasons may include an antivaccine attitude, which, in both the UK and the United States, was the second most commonly cited reason for refusing vaccination.
Likely extent of TD vaccine uptake
The commercial development of TD vaccines requires the existence of a market sufficient to provide an adequate return on the investment necessary to develop and manufacture the product. We suggest that the percentage of relevant travelers that would purchase a TD vaccine may be estimated from (1) the percentage of relevant travelers that seek pretravel medical advice from a general practitioner (GP) or travel clinic (since only these will be prescribed a TD vaccine); (2) the percentage of these travelers that present themselves to the GP/clinic in sufficient time for the vaccine to be administered; and (3) the percentage of travelers that are advised to purchase the vaccine and that subsequently act on that advice.
Existing studies suggest that 30% to 70% (average 56%) of Western travelers seek pretravel medical advice from a GP or travel clinic.18,21,23–25 Of these travelers, the vaccine ideally will be prescribed to those that present themselves in sufficient time for the vaccine course to be completed prior to their travel. For the purposes of illustration, we assume that a TD vaccine requires administration of two doses 2 weeks apart. Physicians’ estimates suggest that 61% (UK) and 77% (United States) of relevant travelers present themselves 2 weeks or more prior to departure.
However, even for an hypothesized ideal TD vaccine, with an administration regime suitable for all patients, our survey suggests that only 47% of UK and 65% of US relevant travelers are expected to purchase the vaccine. Therefore, assuming that these percentages also hold for the subpopulation of patients that present themselves 2 weeks or more prior to departure, we can estimate the uptake of an ideal TD vaccine to be 16% of relevant UK travelers (56% × 61% × 47%) and 28% of relevant US travelers (56% × 77% × 65%).
World Travel Organization data (not shown) suggest that during the period 1999 to 2003, an average of 6 million UK travelers and 45 million US travelers arrived each year in regions associated with a high risk of TD. If we reduce these figures by 50% to allow for double counting and other sources of error, and multiply by 16% or 28%, respectively (cf above), we find that up to 0.5 million UK travelers and 6.3 million US travelers might purchase an ideal TD vaccine each year.
We undertook a similar exercise with regard to the potential uptake of a 100% effective Campylobacter vaccine. We relied upon the incidence levels of campylobacteriosis reported for different geographical areas,14,15 and figures from the World Travel Organization rearranged to reflect international arrivals in these areas (data not shown), together with physicians’ estimates of the percentage of relevant travelers that would buy an ideal vaccine when advised to do so (this survey), and the projected Campylobacter vaccine prescription rates suggested by our survey (Figure 2). This exercise estimates the uptake for a 100% effective Campylobacter vaccine to be in the region of 0.3 million UK travelers and 1.8 million US travelers per year, that is, over 2 million vaccinations per annum in the US and UK markets alone.
Our survey suggests that relevant travelers seen by UK physicians are less likely to be advised to purchase a TD product than are relevant travelers seen by US physicians. This may be due to a perception in the United States that TD occurs at high frequency, regardless of behavioral precautions, in regions with unreliable health care systems, such that travelers should be given the ability to self-medicate. In the UK, by contrast, there appears to be more emphasis on prescription according to an individual’s risk assessment. Both UK and US physicians estimate that ∼85% of relevant travelers purchase a TD product when advised to do so by their physicians, but far more US than UK travelers are advised to purchase a TD product.
We did not detect any significant skepticism among physicians with regard to the concept of TD vaccines. None of the UK respondents indicated that they would never recommend purchase of an ideal TD vaccine. Only one US physician indicated that he/she would never recommend an ideal TD vaccine, and this physician reported seeing only ∼60 relevant travelers per year (representing 0.12% of the relevant travelers seen by US respondents). Indeed, the survey suggests that an ideal TD vaccine would be recommended to about 85% of relevant travelers in the United States/UK.
Only ∼47% of the UK patient population and ∼65% of the US patient population represented by this survey is thought likely to purchase an ideal TD vaccine recommended by their physician. Physicians who responded to our survey commonly believe that cost is a frequent reason to decline vaccination, but reports from travelers suggest that although many perceive vaccines as expensive, cost is not a common reason to decline vaccination (∼4% at most). Several other reasons were proposed by physicians, and have been reported in the literature, to explain why travelers may refuse a vaccine. We conclude that there are likely to be many interacting factors influencing a traveler’s vaccination decision.
Our survey suggests that both in the UK and in the United States, approximately 80% of physicians would recommend a postulated Campylobacter vaccine to travelers visiting regions where up to 30% of TD cases are caused by Campylobacter. This may be due to a perception among many physicians that Campylobacter is a significant pathogen that is increasingly resistant to antibiotics. We conclude that many physicians perceive a need for a Campylobacter vaccine for travelers visiting regions with reported Campylobacter-mediated TD.
Using the results from our survey, and data published in the literature, we estimate that a broad-spectrum TD vaccine would be purchased by up to 16% of relevant travelers from the UK and up to 28% of relevant travelers from the United States. Similarly, by reference to published campylobacteriosis rates at different locations, and using data on international arrivals at those locations, we can estimate that a 100% effective Campylobacter vaccine would be purchased by about 0.3 million UK travelers and about 1.8 million US citizens every year. We conclude that over 2 million units per year of a Campylobacter vaccine might be sold in the US and UK markets alone.
We thank the physicians who gave up their time to participate in this study.
Declaration of interests
This study has been sponsored by ACE Biosciences. Otherwise the authors state they have no conflicts of interest.