Rabies Antibody Seroprotection Rates Among Travelers in Nepal: “Rabies Seroprotection in Travelers”

Authors


  • This paper has been presented at the May 2001 Society for Academic Emergency National Meeting University Emergency Medicine Support.

Robert Partridge, MD, MPH, DTM, Department of Emergency Medicine/Samuels 2, Rhode Island Hospital, 593 Eddy Street, Providence, RI 02903, USA. E-mail: robert_partridge_md@brown.edu

Abstract

Background Rabies preexposure immunization is recommended for international travelers who are at risk for exposure to rabid animals, especially in areas where postexposure treatment may be limited. Rabies antibody seroprotection rates among international travelers has not been previously investigated.

Objective To assess preexisting rabies seroprotection among travelers presenting to a health clinic in Nepal.

Methods A prospective convenience sample of international travelers evaluated at a health center in Kathmandu, Nepal during a 2-month period. Subjects were eligible for inclusion if they had received rabies preexposure vaccination within the previous 5 years. Demographic information and vaccination records of rabies preexposure prophylaxis were obtained. Consenting subjects provided serum for rabies antibody measurement measured using the rapid fluorescent focus inhibition test. A dilution greater than or equal to 1:5 (0.5 IU/mL) was considered positive. Data were analyzed using chi-squares and two-sample t-tests with unequal variances.

Results A total of 43 patients consented to enroll. Complete data were available for 38 patients. Subjects had received human diploid cell vaccine (HDCV) or purified Vero cell rabies vaccine (PVRV) vaccine, either via the intradermal or intramuscular route. All patients had adequate antibody titers except one, who had a titer below 0.5 IU/mL. There was no statistically significant relationship between antibody titer and type of vaccine, route of administration, time since vaccination, number of vaccinations, or patient age.

Conclusions Among 38 travelers to Nepal who had received documented preexposure rabies HDCV or PVRV vaccination series, 37 demonstrated adequate titers of ≥0.5 IU/mL and would be considered boostable if exposed to rabies virus. One traveler had a titer of <0.5 IU/mL. Type of vaccine, method of administration, number of vaccinations, and time since vaccination did not influence rabies antibody titer. Rabies vaccination with HDCV and PVRV vaccine was effective in stimulating adequate seroprotection in this sample of travelers.

Rabies is an acute progressive encephalitis transmitted in the saliva of infected mammals that is almost universally fatal in humans. Worldwide, dogs are the major reservoirs of rabies, and bites from rabid dogs result in tens of thousands of deaths annually and the emergent prophylactic treatment of millions of patients.1,2 Although rabies is common in the United States in wild animal reservoirs, domestic animal and human cases are rare. International travelers to areas where canine rabies is endemic, including Asia, Africa, and Latin America, are at increased risk of exposure to rabies. Since 1990, only 47 cases of rabies have been reported in the United States. Of these, nine cases were attributable to exposure to domesticated animals abroad.1–5

Preexposure rabies immunization is not designed to provide long-term immunity but to better induce immune memory that can be boosted after exposure to a potentially rabid animal. Antibody responses are usually used to demonstrate adequately boostable immune memory after vaccination. Individuals with a minimum antibody titer would be expected to demonstrate full immunity after a postexposure booster dose was given. The US Centers for Disease Control has recommended the minimum protective antibody titer as either ≥1:5 (using the rapid fluorescent inhibition technique) or ≥0.5 IU/mL (using the mouse neutralization test).6 Antibody levels are rarely checked after standard pre- or postexposure vaccination series as these vaccinations are assumed to be effective if properly administered.

International travelers are candidates for preexposure vaccination if they are likely to come into contact with animals in areas where dog rabies is enzootic and immediate availability of postexposure medical care, including immunoglobulins and vaccines, may be limited.7,8 Although the overall risk to travelers of exposure to rabies is low, rabies transmission to international travelers has been reported, including a fatal case in an American woman who was bitten by a street dog in Nepal and did not follow through on a recommendation to receive postexposure prophylaxis (PEP).1,4,6

In the United States, two rabies vaccines are in use: the human diploid cell vaccine (HDCV) and a purified chick embryo cell culture vaccine. The purified Vero cell rabies vaccine (PVRV) is also safe and effective and is widely used outside of the United States. Vaccines that are derived directly from neural tissue are poorly immunogenic and have a much greater risk of severe side effects. Some of these vaccines are still in use in developing countries and should be avoided by travelers.

The rate of preimmunization against rabies among international travelers to Nepal has been shown in a previous study to be 21%, and the risk of possible exposure to rabies by international travelers in Nepal was shown to be about 1 in 6,000 travelers.9 This investigation was initiated to determine the seroprotection rates and neutralizing antibody response in travelers presenting to a health clinic in Nepal. Immunogenicity of international travelers given preexposure prophylaxis has not previously been published.

Methods

Study Setting

The study took place at the Canadian International Water and Energy Consultants (CIWEC) Clinic Travel Medicine Center in Kathmandu, Nepal. CIWEC Clinic is a Western-style outpatient clinic, which provides the majority of medical care for expatriates and tourists in Nepal.

Study Design

This study was a prospective convenience sample of international travelers. The study population consisted of foreign (non-Nepalese) patients, aged 18 to 65 years old, who presented to CIWEC travelers clinic during the months of December 1999 and January 2000 and had received rabies preexposure vaccination within the previous 5 years. The human research institutional review board of Rhode Island Hospital, Providence, RI, approved the study protocol and informed consent form before the study began.

A nurse or medical practitioner administered informed consent and a questionnaire to each eligible patient. He or she also copied the patient’s vaccination record as evidence of the date, route (intradermal [ID] or intramuscular [IM]), and type of preexposure vaccination. Each consenting subject’s serum was obtained for antibody testing, frozen, and shipped on dry ice, via DHL international courier, with customs notice to expedite clearance to Kansas State Veterinary School (KSU). Measurement of rabies virus neutralizing antibody levels [using the rapid fluorescent focus inhibition test (RFFIT)] was performed at KSU. Subjects were considered to have successfully maintained seroprotection for rabies virus neutralizing antibody if they achieved an RFFIT titer ≥0.5 IU/mL (a dilution greater than or equal to 1:5).

The variables (age, time since immunization, type of vaccine, route of vaccination) were then compared to serum antibody titers using STATA software. Statistical significance was calculated using the chi-squared method, the two-sample t-test with unequal variances, and the two-sample rank-sum (Mann–Whitney) test.

Results

Demographics

Of the 43 patients who consented, one was a control (known to have no preexposure rabies vaccination, included only to confirm accuracy of the test) and four had incomplete data. Only one eligible patient refused consent.

A total of 38 subjects were enrolled, of whom 13 had been vaccinated with HDCV and 25 with PVRV within the previous 5 years. The patients had received their vaccinations or boosters in 14 different countries. The majority of study participants (20/38) had received their vaccinations in Europe; eight had received their vaccinations exclusively in Nepal, and one had been vaccinated in both Nepal and India. Ten of the 38 received their rabies vaccination ID; the other 28 had been vaccinated via the IM route. Of these 38 subjects, 36 had received at least three doses of vaccine, although not necessarily in the standard (day 0, 7, and 21) sequence; two subjects received only two vaccinations. A total of 19 had received more than three vaccinations; 7 had received these extra doses as part of the standard postexposure booster series.

In this sample, 37 of 38 subjects (97.4%) demonstrated adequate antibody titers (eg, ≥0.5 IU/mL). Distribution of serum titers across the population is shown in Figure 1. The subject with a titer of <0.5 IU/mL had received the 3-part ID HDCV vaccine series. A small clustering of titers near 1:15 was observed. Although titers greater than or equal to 0.5 IU/mL are adequate for preexposure prophylaxis, we were interested in whether the lower titers shared any characteristics with the inadequate (<0.5 IU/mL) titer. Analysis of whether vaccine type (HDCV vs PVRV) was associated with this cluster was not significant (χ2= 1.52; p= 0.22). The method of vaccination (ID vs IM) was not associated with a low titer, nor was the number of vaccinations received. Furthermore, the distribution of adequate titers did not show any relationship to the type of vaccine received, Pearson’s chi-square F(2,35) = 0.85, p= 0.43.

Figure 1.

Distribution of serum titers across the study population.

The mean ± SD elapsed time since immunization among the 38 subjects was 17.8 ± 19.3 months. The three subjects in the cluster of titers near 0.5 IU/mL received their rabies immunoprophylaxis 10.6 ± 4.2 months prior to enrollment, which was shorter than 18.4 ± 3.4 months for the rest of the subjects but not significant (t-test 1.43, p= 0.20) (see Table 1, for additional details).

Table 1.  Vaccinee characteristics by RFFIT Level
RFFIT (IU/mL)Type of vaccine receivedRoute of vaccine administrationMean time since last vaccine (mo)Mean age of traveler (y)
  1. HDCV = human diploid cell vaccine; ID = intradermal; IM = intramuscular; PVRV = purified Vero cell rabies vaccine; RFFIT = rapid fluorescent focus inhibition test.

<0.5HDCV = 1, PVRV = 0ID = 1, IM = 01928
0.5–1.4HDCV = 1, PVRV = 1ID = 1, IM = 16.534.5
1.5–4.9HDCV = 0, PVRV = 0ID = 0, IM = 0
5.0–9.9HDCV = 1, PVRV = 2ID = 0, IM = 36.325.6
10.0–19.9HDCV = 1, PVRV = 2ID = 1, IM = 22634.6
20.0–39.9HDCV = 4, PVRV = 7ID = 3, IM = 818.732.6
40.0–99.9HDCV = 0, PVRV = 0ID = 0, IM = 0
≥100.0HDCV = 5, PVRV = 13ID = 4, IM = 1418.137.2

Discussion

It is of the utmost importance that people who have received the rabies prevaccination series are adequately boostable. After a rabid animal’s bite, prevaccinated patients receive only booster vaccine to induce an amnestic B cell response against the rabies virus. Nonvaccinated patients must be given not only the full series of rabies vaccines but also a series of passive immunizations in the form of human or equine rabies immunoglobulin (RIG) treatments. For travelers in remote locations, effective PEP against rabies is both difficult to access and expensive.10 Many health centers in low-income countries do not have RIG available; in Nepal, for instance, CIWEC is the only clinic that consistently stocks RIG. A course of PEP is estimated to cost at least $1,500 for the medicine alone11, and some nonvaccinated, exposed travelers reportedly forgo PEP because of the cost.12

As defined by the Advisory Committee on Immunization Practices, the minimum protective antibody titer is ≥0.5 IU/mL.6 In this study, we observed one patient with a less-than-adequate titer and two patients whose titers were near the cutoff (eg, greater than 0.5 but less than 1.5 IU/mL). Subjects with low titers have been considered “poor responders” to the preexposure vaccination.13,14 However, a low antibody titer does not necessarily mean that a subject is not boostable. Strady et al have suggested that standard boosters (eg, at day 365 after initial vaccination series) may be necessary to assure adequate immune memory in poor responders (defined as those with antibody levels <30 IU/mL) with chronic rabies exposures.13,14

Since rabies postexposure treatment is always needed in the event of exposure in preimmunized individuals, the main issue is whether those with inadequate rabies titers are boostable, preferably within 5 days. In our study, we did not assess whether the subject with inadequate titers was boostable. Some researchers have reported that some vaccinees may be slow responders to postexposure boosters, unable to develop adequate titers within 5 days of revaccination. (Strady et al. do not address this issue.)13–15 However, there are no reports of travelers preimmunized with rabies vaccine who died of rabies after receiving a booster. Further research is needed in this area.

We found no association between low titers and site of vaccination (ID vs IM). (Of note, in our study population, all ID-vaccinated subjects had received HDCV vaccine; this use of HDCV is currently off-label in the United States.) There has been debate over the long-term effectiveness of the ID route. Some studies suggest that ID vaccination is associated with lower titers of serum neutralizing antibodies, both with16 and without17,18 boosters. However, another study19 observed equivalent immunity in patients vaccinated via the ID and IM routes. It is thought that this difference may be due to practitioner experience in ID vaccine administration. Because the ID technique requires only one tenth of the amount of vaccine (0.1 mL vs 1.0 mL), loss of a small amount of vaccine within the bore of the needle or via leaking from the dermal needle track could result in inadequate immunization. ID administration is preferred in many countries, though, precisely because it requires less vaccine and is therefore less expensive. Considering the large variation in country of vaccination among our study participants, the lack of association between ID vaccination and low titers in our study is reassuring.

Previous studies have also observed lower antibody titers in the elderly20 and in those more than 1.5 to 2 years post-IM vaccination.17 In contrast to these reports, our study demonstrates no statistically significant variation in titer according to age, at least within our study group range of 19 to 60 years old. There was also no statistically significant variation in titer in relation to the length of time since vaccination (range = 1 month to 6 years). Larger prospective studies may challenge current understanding of the relationship of age and time since vaccination on expected antibody titers in travelers. Finally, our study accorded with other studies in observing no significant differences in titer levels according to the type of vaccination.

There are some significant limitations to this study. Most important is the small study size. Although we were able to observe trends, it is nearly impossible to make definitive conclusions about strength of associations given the very small number of travelers with low RFFIT titers. We also did not try to boost the travelers and measure the response, something that would have defined whether the travelers would have been adequately protected against rabies in the case of an exposure, despite a low antibody level.

There may also have been an inclusion bias as only travelers visiting the CIWEC Clinic were included in the study. Moreover, of those travelers attending the CIWEC Clinic, only those visiting during the hours of the travel clinic—those who are presumably more proactive about their health—were enrolled. Of those potential subjects who did not enroll, it is impossible to know how many of them declined, and why. It is conceivable that those travelers in the study were, for some reason, more likely to have received adequate preexposure rabies vaccinations.

A final limitation is that, due to the small study size, we were unable to correlate place of immunization (eg, registered travel clinic in home country vs primary doctor’s office) with titer levels. This correlation could be interesting, given the frequent discussion of why ID vaccine administration may or may not be less effective.

Conclusions

Among a population of 38 travelers to Nepal who had received documented preexposure HDCV or PVRV rabies vaccination series, 37 travelers (97.4%) had adequate rabies antibody titers. This level of response to preexposure rabies immunization is consistent with other studies that described poor responders to rabies immunization. Only one traveler had an inadequate titer of <0.5 IU/mL, and although not tested, this individual might still be boostable. Type of vaccine, method of administration, number of vaccinations, and time since vaccination did not influence rabies antibody titers. Rabies vaccination with HDCV and PVRV vaccine was effective in stimulating adequate seroprotection in this sample of international travelers.

Acknowledgments

The authors wish to thank Deborah Briggs of Kansas State University for performing laboratory analysis on study participant’s serum samples, Anne Barylick for her invaluable assistance in enrolling patients in the study, and David Shlim, MD, for his detailed review and suggestions on this article.

Declaration of Interests

The authors state that they have no conflicts of interest.

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