The Immunogenicity of a Modified Intradermal Pre-exposure Rabies Vaccination Schedule—A Case Series of 420 Travelers

Abstract

Background. Current Australian recommendations for rabies pre-exposure vaccination involve the use of cell-culture-based rabies vaccines, which are administered via intramuscular (IM) or intradermal (ID) routes. ID vaccination is more affordable for travelers, but is only recommended if there is sufficient time to perform serology 2 to 3 weeks post-vaccination and confirm immunity prior to travel. We report the immunogenicity of a modified ID schedule that can be completed in less time than the standard ID schedule, and allow more travelers to be vaccinated prior to departure.

Methods. Travelers were offered a modified schedule if they were unable to afford standard IM vaccinations, and did not have time to complete a standard ID course. The modified schedule consisted of two ID injections of 0.1 mL of human diploid cell rabies vaccine administered on days 0 and 7, and serology was performed to determine immune status at a time between day 21 and 28.

Results. A total of 420 travelers aged between 10 and 65 years were vaccinated using the modified ID course. The overall seroconversion rate was 94.5%, with 397 travelers developing antibody levels of >0.5 IU/mL when tested at approximately 21 days post-vaccination.

Conclusion. The modified ID schedule used in this case series was highly effective, had similar immunogenicity to the standard ID schedule, and should be considered in travelers who are unable to complete standard IM or standard ID courses of rabies vaccines.

Rabies is an invariably fatal viral zoonosis in humans, posing a threat to over 3 billion people around the world, and causes an estimated 55,000 human deaths each year.¹ Travelers to rabies-endemic areas are at risk of infection if bitten or scratched by animals, and the estimated incidence of animal bites in travelers to developing countries is 2 to 4 per 1000 per month.² Phanuphak and colleagues reported an animal bite incidence of 13 per 1000 in travelers who spent an average of 17 days in Thailand.³

Travelers can be protected from rabies either by pre-exposure vaccination prior to traveling to an endemic area or post-exposure prophylaxis (PEP) after animal bites or scratches. Pre-exposure vaccination simplifies the management of a potentially rabies-infected bite by precluding the need for rabies immunoglobulin and reducing the number of doses of rabies vaccines required.

Although travelers should be advised to avoid contact with animals while in rabies-endemic areas, many bites occur without any initiation of contact by the victims. At our Australian travel medicine clinic, approximately one third of travelers who present for PEP after an animal bite or scratch overseas reported that they did not initiate contact with the animal (DJ Mills, personal communication, February 2011).

Recommendations for pre-exposure rabies vaccination vary between countries. The World Health Organization (WHO) recommends either intramuscular (IM) or intradermal (ID) administration of rabies vaccines.¹ The current Australian National Health and Medical Research Council (NHMRC) Immunization Guidelines recommend one of two options for pre-exposure rabies vaccination:⁴ (1) IM injections (1.0 mL) at 0, 7, and 28 days; or (2) ID injections (0.1 mL) at 0, 7, and 28 days, followed by serology 2 to 3 weeks after the last dose to confirm immunity. The ID route is only recommended for use in clinics where staff members are trained in administering ID injections. The Centers for Disease Control and Prevention, USA, currently recommends the IM route for rabies pre-exposure prophylaxis.⁵

Pre-exposure rabies vaccination is safe and effective, but the cost of IM vaccines can be prohibitive for some travelers (approximately AUD$400 for a primary course of three vaccine doses at the time of writing). ID vaccination is approximately one third of the cost and has been shown to be a safe and effective option.^1,6–8 Antibody levels after ID vaccination have also been shown to respond well to subsequent boosters,^9,10 and provide long lasting immunity.¹¹

Although ID rabies vaccination is safe, effective, and affordable for many, it poses a number of challenges. Current recommendations for ID vaccination require at least 7 weeks to complete the course of vaccines, perform serology 2 to 3 weeks later, and for results to be available. Many travelers present for pretravel advice less than 7 weeks prior to departure. Also, some travelers are not compliant with the recommendation to have post-vaccination serology performed, and vaccine non-responders are therefore not identified.

Ideally, pre-exposure rabies vaccination should be safe, effective, affordable, and rapidly immunogenic. In this paper, we present a case series of travelers who were unable to be vaccinated using the standard IM or ID rabies schedules, and were consequently offered rabies vaccination using a modified ID schedule. We describe the immunogenicity of the modified ID schedule, and the factors that influenced vaccine efficacy.

Methods

Subjects

The data were collected at a travel medicine clinic in Brisbane, Australia. All nurses at the clinic are experienced with administering vaccines through ID route. Travelers who attend the clinic are routinely counseled regarding the risk of rabies if traveling to endemic areas. They are advised about the advantages of pre-exposure vaccination, and offered the standard IM or ID course of vaccines recommended by the NHMRC.⁴

Travelers who could not afford a course of IM vaccines and were not able to complete the requirements for standard ID vaccination were offered a modified course of ID rabies vaccines. All travelers were informed that this was an “off label” use of the vaccine, and given an explanation and written information about why the nonstandard ID schedule was being offered. The modified ID schedule was not offered to children under the age of 10 years.

From June 2007 to November 2010, 420 travelers were vaccinated using the modified ID course of rabies vaccines. During this same time period, more than 2000 travelers were vaccinated using the standard IM or ID schedules at the clinic.

Vaccine

The Merieux Inactivated Rabies Vaccine (human diploid cell vaccine for rabies, Sanofi Pasteur SA, Lyon, France) containing at least 2.5 IU/mL was used for all patients.

Modified pre-exposure ID rabies vaccination schedule

The modified ID rabies vaccination schedule offered to travelers in this case series was named Travelers Rabies Intradermal 2 site (TRID2), and involved three visits to the clinic. The schedule involved two 0.1 mL ID injections on each of day 0 (clinic visit 1) and day 7 (clinic visit 2), and one 0.1 mL ID injection and rabies serology at a time between day 21 and 28 (clinic visit 3). Table 1 compares the doses of rabies vaccines given at each clinic visit for standard IM schedule, standard ID schedule, and the TRID2 schedule used in this case series.

Table 1.  Rabies vaccination schedules

Rabies vaccine schedules	Clinic visit 1 Day 0	Clinic visit 2 Day 7	Clinic visit 3 Day 21 to 28	Clinic visit 4 Day 35 to 42
	Day 0	Day 7	Day 21 to 28	Day 35 to 42
IM = intramuscular; ID = intradermal; TRID2 = Travelers Rabies Intradermal 2 site.
Standard IM	1 mL	1 mL	1 mL	Not required
Standard ID	0.1 mL	0.1 mL	0.1 mL	Serology
TRID2 (This study)	2 × 0.1 mL (Doses 1 and 2)	2 × 0.1 mL (Doses 3 and 4)	0.1 mL (Dose 5) and Serology	Only if not immune on serology done at visit 3

With the TRID2 schedule, it was proposed that the two 0.1 mL ID doses given at clinic visits 1 and 2 would provide adequate and more rapid immunity than the standard ID schedule, and allow time for seroconversion to be confirmed prior to departure. Blood samples were collected at a time between day 21 and 28 (clinic visit 3) to measure rabies antibody levels and determine immune status. Travelers were considered immune if rabies antibody levels were at least 0.5 IU/mL.¹ Another 0.1 mL ID dose (Dose 5) was given at clinic visit 3 because there is currently insufficient evidence to show that the ID doses given on clinic visits 1 and 2 of the TRID2 schedule are sufficient to induce an adequate immune response.

Non-responders

Travelers who did not develop an adequate antibody response on serology performed at clinic visit 3 were informed of their result, and advised that they should consider themselves nonimmune to rabies. They were asked to return to the clinic for an extra vaccine dose (Dose 6) if they had not already departed on their travel, and repeat serology was performed on the same day to assess antibody response to “Dose 5.” If the second serology test showed adequate rabies antibodies, the need for further serology after “Dose 6” was avoided.

Laboratory analysis and antibody levels

Rabies serology was performed at Sullivan and Nicolaides Pathology laboratories (Brisbane, Australia) using the PLATELIA™ RABIES II ELISA method. The maximum rabies antibody level measured was 4 IU/mL, and levels higher than this were reported as >4 IU/mL. Results were generally available within 1 week, and all travelers were contacted to advise them of their immune status.

Although the WHO recommends the use of rapid fluorescent focus inhibition test (RFFIT) or the fluorescent antibody virus neutralization (FAVN) test,¹ these are not readily available in Australia. Serology results using the ELISA method are comparable to the RFFIT method, and the ELISA is considered to be a reliable alternative when the RFFIT is unavailable.^12,13

Statistical analysis

All data analyses were performed using STATA 11.1 (Statacorp, College Station, Texas, USA). The outcome measures used were seroconversion rates and antibody levels. Differences in outcomes were analyzed for each of the independent variables: age, gender, type of vaccine schedule, timing of vaccine doses, and the timing of rabies serology. Chi-square tests were used to assess the effect of each independent variable on the outcome measures. p Values of <0.05 were considered statistically significant, and 95% confidence intervals (CI) were calculated for seroconversion rates.

As the laboratory did not quantify antibody levels above 4 IU/mL, and it was not possible to calculate the mean or standard deviation for antibody levels. For the purposes of statistical analysis, rabies antibody levels were interpreted as categorical variables as follows: <0.5 IU/mL; 0.5 to 1.49 IU/mL; 1.5 to 2.49 IU/mL; 2.5 to 4 IU/mL; and >4 IU/mL.

Definitions

• •
  “TRID2 standard” schedule: Travelers who received the TRID2 doses on days 0, 7, and 21 to 28.
• •
  “TRID2 nonstandard” schedule: Travelers who received the vaccines at different times to “TRID2 standard” schedule.
• •
  “Seroconversion” or “Immune”: Rabies antibody levels of at least 0.5 IU/mL.
• •
  “Non-responder”: Rabies antibody level of <0.5 IU/mL on clinic visit 3.

Results

Demographics of cases

A total of 420 travelers were given the TRID2 course over a period of approximately 3.5 years from June 2007 to November 2010, with 227 (54%) females, and 193 (46%) were males. The mean age was 32.4 years, with a range of 10 to 65 years.

Vaccine schedules and timing of serology

Most travelers (63.8%) received the “TRID2 standard” schedule, and there were no significant differences in age and sex distribution between the “TRID2 standard” group, and the “TRID2 nonstandard” group. Figure 1 shows the age distribution of travelers in this case series, by “TRID2 standard” and “TRID2 nonstandard” schedules status.

Figure 1. Age distribution of travelers in the case series.

For travelers who received the “TRID2 nonstandard” schedule, the time interval between clinic visits 1 and 2 ranged from 6 to 30 days, with a median of 8 days; and the time interval between clinic visit 2 and serology ranged from 2 to 37 days, with a median of 21 days.

Seroconversion

Compliance with serology was 100%, and the overall seroconversion rate was 94.5% (95% CI: 91.9 to 96.5) at clinic visit 3, with no significant difference between males and females. Seroconversion rate was significantly lower with increasing age (correlation coefficient = −0.05, p < 0.001), and rates for each age group are shown in Figure 2.

Figure 2. Seroconversion rates in different age groups.

The seroconversion rate was 94.4% (95% CI: 90.9–96.8) in the “TRID2 standard” group, and 94.7% (95% CI: 89.9–97.7) in the “TRID2 nonstandard” group. There was no significant difference in seroconversion rates between the two groups (χ² = 0.02, p = 0.89). In addition, there was no difference in seroconversion rates between “TRID2 standard” and “TRID2 nonstandard” cases in any of the age groups.

The time interval between clinic visits 1 and 2 in this study did not have any significant effect on seroconversion rates (correlation coefficient = 0.03, p = 0.78). The seroconversion rate was higher in those who had their serology performed later, but this was not statistically significant (correlation coefficient = 0.06, p = 0.15). The variation in seroconversion rates with the timing of serology is shown in Figure 3.

Figure 3. Seroconversion by timing of blood test.

Antibody levels

Of the 420 cases, 23 (5.5%) had antibody levels below 0.5 IU/mL, and were considered nonimmune. The distribution of antibody levels measured at clinic visit 3 is shown in Figure 4.

Figure 4. Distribution of antibody levels in males and females.

Females had significantly higher antibody levels than males (χ² = 11.96, p = 0.02), but the clinical significance of this finding is uncertain. The percentage of cases in each antibody category for males and females is shown in Figure 4.

Antibody levels were significantly lower in the older age groups (χ² = 41.30, p = 0.003), and the variation in antibody levels between age groups is shown in Figure 5.

Figure 5. Distribution of antibody levels in different age groups.

There were no significant differences in antibody levels with variations in vaccine schedule (χ² = 4.83, p = 0.30), the timing of clinic visit 2 (correlation coefficient = 0.07, p = 0.09), or the timing of serology (χ² = 11.84, p = 0.76).

Non-responders

Twenty-three cases did not show an adequate antibody response with serology performed on the blood samples from clinic visit 3. One of these cases had no detectable rabies antibody, but the other 22 cases had detectable levels less than 0.5 IU/mL. The traveler with no detectable rabies antibodies was also known to be a non-responder to hepatitis B immunization after nine doses of the vaccine. Of the 23 non-responders, 12 (52%) had their first blood tests done before day 28, and 10 (44%) were over 50 years of age.

Five of the non-responders did not return for a booster vaccine dose or a repeat serology test, and were advised to consider themselves nonimmune. Of the remaining 18 cases, 16 had antibody levels of >0.5 IU/mL when tested at a later date (range 3–51 d after clinic visit 3), indicating that they developed adequate antibody levels after “Dose 5” given at clinic visit 3, and/or had developed higher antibody levels with time. The other two cases developed adequate antibody levels after “Dose 6,” and one of these cases had chronic lymphocytic leukemia and Type 2 diabetes mellitus.

Overall seroconversion

Taking into account the 397 travelers who seroconverted on the first serology test performed at clinic visit 3, and the 16 travelers who seroconverted after “Dose 5,” the overall seroconversion rate using the TRID2 schedule was 98.3% (95% CI: 96.6–99.3) after three clinic visits and five ID vaccine doses.

Adverse events

There were no reports of significant side effects with the TRID2 schedule, and the two vaccine doses required at clinic visits 1 and 2 were acceptable to travelers.

Conclusion

This case series demonstrated that the TRID2 schedule is highly effective, inducing immunity in 94.5% of travelers after the first two clinic visits, and immunity in 98.3% of travelers after three clinic visits. The major advantage of the TRID2 schedule over the standard ID schedule is that travelers were able to complete the course of vaccines and have their immunity confirmed in a shorter time (4 wk compared with 7 wk). Also, only three clinic visits were required for the TRID2 schedule, compared to four visits with the standard ID schedule. We found the TRID2 schedule to be a safe, convenient, acceptable, and affordable way of protecting travelers from rabies, and the majority of travelers had their immunity confirmed prior to travel.

Accelerated schedules of ID rabies vaccines have been shown to be safe and effective for pre-exposure vaccination,^8,10,11,14 and are routinely used for rabies PEP in some countries. In the post-exposure setting, the Thai Red Cross regimen involves two 0.1 mL ID doses given on day 0, and repeated on days 3, 7, and 28 is one of the PEP schedules recommended by the WHO.¹ The schedule used in the TRID2 course should therefore also be safe and effective.

Previous studies have demonstrated that 0.1 mL ID doses given at days 0, 7, and 21 to 28 were effective,^6,7 and “Dose 5” of the TRID2 schedule would therefore ensure that travelers are afforded at least as much protection as those who are immunized with the standard ID course. In addition, serology results can take up to a week to be reported, by which time many of the TRID2 travelers have departed on their trips, and will not be able to return for extra vaccine doses if they were not immune. “Dose 5” will increase the chances of seroconversion even if travelers were not immune at clinic visit 3.

In our travel medicine clinic, a significant number of travelers would not have been protected against rabies if the TRID2 vaccine schedule had not been offered to them. Taking into account the cost of the vaccines and the number of clinic visits, the total cost of administering the TRID2 vaccine schedule is currently approximately the same as for the standard ID course.

Variations in timing in the “TRID2 nonstandard” group were largely caused by travelers being busy with work or personal commitments at the time of the recommended clinic visit days, and these variations occurred more frequently during busy times such as Christmas and public holidays. In the real world, pretravel preparation of travelers often involves planning vaccine doses around other commitments, and it is reassuring to know that irregular timing of vaccine doses in the “TRID2 nonstandard” schedule in this case series did not affect immunogenicity.

The overall seroconversion rate of 98.3% after three clinic visits and five ID doses is similar to the immunogenicity of the standard ID schedule found in studies in similar travel clinics in Australia and New Zealand, which have reported seroconversion rates of between 95.1 and 99.5%.^6–8

At our Brisbane travel medicine clinic, 317 travelers received the standard ID schedule between 1999 and 2005. This series of travelers had a seroconversion rate of 99.4% (D Mills, personal communication, April 2011), which is not statistically different to the 98.3% seroconversion achieved using TRID2 (p = 0.21). The seroconversion rate of 94.5% after two clinic visits of the TRID2 schedule is significantly lower than seroconversion rate with the standard ID schedule (p = 0.00), but TRID2 has the advantage of providing earlier confirmation of immunity to travelers, and should be considered as an option in those departing in less than 7 weeks. A comparison of antibody levels measured after a standard ID course versus a TRID2 course showed that travelers who received a standard ID course had significantly higher antibody levels, with 74.5% having levels of >4.0 IU/mL (p = 0.00) at an average of 22 days after the third ID vaccine dose. However, the clinical significance of higher antibody levels is unclear, and it is difficult to make direct comparisons of levels because serology was performed at different times in the two groups.

TRID2 was more effective in the younger age groups, inducing higher seroconversion rates as well as antibody levels. Over half (62.9%) of the travelers in this study were aged between 20 and 40 years of age, and larger numbers of cases are required to accurately assess the immunogenicity of the TRID2 schedule in other age groups. Females were found to have higher antibody levels in this case series, but this difference is unlikely to be of any clinical significance.

Although previous studies have demonstrated that various ID rabies vaccine schedules provide long lasting immunity,^10,11 the persistence of antibodies after a TRID2 schedule warrants further investigation. The antibody response to subsequent vaccine boosters after the TRID2 schedule also needs to be assessed, but it is reassuring that other studies have shown good response to boosters a year or more after standard and abbreviated rabies ID vaccination schedules.^9,10,14,15 The immunogenicity of TRID2 should also be compared to other abbreviated schedules using ID rabies vaccines.^10,14,16 The use of the ELISA technique rather than the WHO recommended gold standard RFFIT method should also be taken into account when interpreting the results of this study.^1,12

The TRID2 schedule should be considered an option for pre-exposure rabies vaccination in clinics with staff who are experienced at administering ID vaccines. Further research is required to confirm the findings in this case series, assess the variation in response between different age groups and gender, and determine the optimal timing of vaccine doses and serology. If such additional work supports our findings, it may become appropriate to consider revisions to the current vaccination guidelines to include a modified ID pre-exposure rabies vaccination schedule.

Acknowledgments

We would like to thank the staff at Dr Deb—The Travel Doctor, Brisbane, Australia for collecting the data, and Justine Jackson (RN) for managing and collating the data.

Conflicts of Interest

This study was not subsidized, funded or associated with the vaccine manufacturers in any way.

D. J. M. and C. L. L. are doctors at privately owned, independent travel medicine clinics, and provide rabies vaccines to travelers.

The other authors state they have no conflicts of interest to declare.