© 2006 International Society of Travel Medicine, 1195-1982Journal of Travel Medicine, Volume 13, Issue 6, 2006, 329–333

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Rabies is an acute progressive encephalitis trans-mitted 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 com-mon in the United States in wild animal reservoirs, domestic animal and human cases are rare. Interna-tional travelers to areas where canine rabies is en-demic, including Asia, Africa, and Latin America, are

ORIGINAL ARTICLES

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

Megan Ranney , MD , * Robert Partridge , MD , MPH , DTM , † Gregory D . Jay , MD , PhD , ‡ Deborah Eastman Rozzoli , MD , * and Prativa Pandey , MD § * Department of Emergency Medicine , † Division of International Emergency Medicine, Department of Emergency Medicine , and ‡ Department of Emergency Medicine/Research, Rhode Island Hospital/Brown Medical School Providence, Rhode Island ; § CIWEC Clinical Travel Medicine Center, Kathmandu, Nepal

DOI: 10.1111/j.1708-8305.2006.00067.x

Background . Rabies preexposure immunization is recommended for international travelers who are at risk for expo-sure 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 fl uorescent 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 purifi ed 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 signifi cant 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 vacci-nation 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 infl uence rabies antibody titer. Rabies vaccination with HDCV and PVRV vaccine was effective in stimulating adequate seroprotection in this sample of travelers.

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

Corresponding Author: 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

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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 at tributable to exposure to domesticated animals abroad. 1 – 5

Preexposure rabies immunization is not de-signed to provide long-term immunity but to better induce immune memory that can be boosted after exposure to a potentially rabid animal. Antibody re-sponses are usually used to demonstrate adequately boostable immune memory after vaccination. Indi-viduals with a minimum antibody titer would be expected to demonstrate full immunity after a post-exposure booster dose was given. The US Centers for Disease Control has recommended the mini-mum protective antibody titer as either ≥ 1:5 (using the rapid fl uorescent 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 pre-exposure vaccination if they are likely to come into contact with animals in areas where dog rabies is en-zootic and immediate availability of post exposure medical care, including immunoglobulins and vac-cines, may be limited. 7,8 Although the overall risk to travelers of exposure to rabies is low, rabies trans-mission 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 purifi ed chick embryo cell culture vaccine. The pu-rifi ed 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 seropro-tection 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. CI-WEC 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 con-sisted 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 in-formed consent and a questionnaire to each eligible patient. He or she also copied the patient ’ s vaccina-tion 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 Veteri-nary School (KSU). Measurement of rabies virus neutralizing antibody levels [using the rapid fl uores-cent focus inhibition test (RFFIT)] was performed at KSU . Subjects were considered to have successfully maintained seroprotection for rabies virus neutraliz-ing 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 com-pared to serum antibody titers using STATA soft-ware. Statistical signifi cance 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 confi rm accuracy of the test) and four had incomplete data. Only one eligible patient refused consent.

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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 differ-ent 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 vacci-nation 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 sub-jects 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%) demon-strated 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 ob-served. 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 signifi cant ( � 2 = 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 re-ceived. 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.

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 signifi cant ( t -test 1.43, p = 0.20) (see Table 1, for additional details).

Discussion

It is of the utmost importance that people who have received the rabies prevaccination series are ade-quately boostable. After a rabid animal ’ s bite, pre-vaccinated patients receive only booster vaccine to induce an amnestic B cell response against the ra-bies 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 hu-man or equine rabies immunoglobulin (RIG) treat-ments. For travelers in remote locations, effective PEP against rabies is both diffi cult to access and ex-pensive. 10 Many health centers in low-income coun-tries 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 alone 11 , and some nonvaccinated, exposed travelers reportedly forgo PEP because of the cost. 12

As defi ned by the Advisory Committee on Im-munization 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 consid-ered “ poor responders ” to the preexposure vacci-nation. 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 (defi ned 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 in-adequate 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

Figure 1 Distribution of serum titers across the study population.

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revaccination. (Strady et al. do not address this is-sue.) 13 – 15 However, there are no reports of travelers preimmunized with rabies vaccine who died of ra-bies 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 associ-ated with lower titers of serum neutralizing antibod-ies, both with 16 and without 17,18 boosters. However, another study 19 observed equivalent immunity in patients vaccinated via the ID and IM routes. It is thought that this difference may be due to practitio-ner experience in ID vaccine administration. Be-cause 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 nee-dle or via leaking from the dermal needle track could result in inadequate immunization. ID administra-tion is preferred in many countries, though, pre-cisely because it requires less vaccine and is therefore less expensive. Considering the large variation in country of vaccination among our study partici-pants, the lack of association between ID vaccina-tion and low titers in our study is reassuring.

Previous studies have also observed lower anti-body titers in the elderly 20 and in those more than 1.5 to 2 years post-IM vaccination. 17 In contrast to these reports, our study demonstrates no statisti-cally signifi cant variation in titer according to age, at least within our study group range of 19 to 60 years old. There was also no statistically signifi cant 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 signifi cant differences in titer levels according to the type of vaccination.

There are some signifi cant limitations to this study. Most important is the small study size. Al-though we were able to observe trends, it is nearly impossible to make defi nitive conclusions about strength of associations given the very small num-ber of travelers with low RFFIT titers. We also did not try to boost the travelers and measure the re-sponse, something that would have defi ned 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 in-cluded in the study. Moreover, of those travelers at-tending 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 fi nal limitation is that, due to the small study size, we were unable to correlate place of immuni-zation (eg, registered travel clinic in home country vs primary doctor ’ s offi ce) 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

Table 1 Vaccinee characteristics by RFFIT Level

RFFIT (IU/mL) Type of vaccine received Route of vaccine administration

Mean time since last vaccine (mo)

Mean age of traveler (y)

<0.5 HDCV = 1, PVRV = 0 ID = 1, IM = 0 19 28 0.5 – 1.4 HDCV = 1, PVRV = 1 ID = 1, IM = 1 6.5 34.5 1.5 – 4.9 HDCV = 0, PVRV = 0 ID = 0, IM = 0 — — 5.0 – 9.9 HDCV = 1, PVRV = 2 ID = 0, IM = 3 6.3 25.6 10.0 – 19.9 HDCV = 1, PVRV = 2 ID = 1, IM = 2 26 34.6 20.0 – 39.9 HDCV = 4, PVRV = 7 ID = 3, IM = 8 18.7 32.6 40.0 – 99.9 HDCV = 0, PVRV = 0 ID = 0, IM = 0 — — ≥ 100.0 HDCV = 5, PVRV = 13 ID = 4, IM = 14 18.1 37.2

HDCV = human diploid cell vaccine; ID = intradermal; IM = intramuscular; PVRV = purifi ed Vero cell rabies vaccine; RFFIT = rapid fl uorescent focus inhibition test.

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(97.4%) had adequate rabies antibody titers. This level of response to preexposure rabies immuniza-tion 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 infl uence rabies anti-body titers. Rabies vaccination with HDCV and PVRV vaccine was effective in stimulating ade-quate seroprotection in this sample of interna-tional 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 confl icts of interest.

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