Abstract
Current guidelines for rabies pre-exposure prophylaxis (PrEP) recommend multiple vaccine doses. Travellers sometimes present for pre-travel consultation with insufficient time to complete standard PrEP schedules. We investigated the efficacy of one-dose intramuscular (IM) vaccine in priming the immune system (as PrEP) by measuring antibody response to simulated post-exposure prophylaxis (PEP).
A quasi-experimental pre–post intervention clinical trial was conducted at a specialist travel clinic in Australia. Adults (≥18 years) without a history of rabies vaccination were included. At Visit 1, seronegative status was confirmed and one dose of 0.5 ml IM rabies vaccine (Verorab®) administered. At Visit 2 (≥60 days after Visit 1), serology was repeated and a simulated PEP dose (0.5 ml IM) given on this day and again 3 days later (Visit 3). Serology was repeated at Visit 4 (7 days after Visit 2).
A total of 94 antibody-negative participants were included (<50 years [n = 50]; ≥50 years [n = 44]). At Visit 2, 38.0 and 31.8% of participants aged <50 and ≥50 years were antibody-positive (≥0.5 EU/ml). At Visit 4, all participants were antibody-positive; 82.0 and 47.7% of participants aged <50 and ≥50 years had antibody levels >4 EU/ml, respectively.
One-dose IM vaccine was effective as PrEP for priming the immune system in both age groups, resulting in rapid development of antibodies 7 days after commencing simulated PEP. If there is insufficient time to complete a standard PrEP schedule, one-dose IM could be considered as an alternative schedule for short trips, rather than not offering travellers any doses at all.
Clinical trials registration: ACTRN12619000946112.
Introduction
Rabies is an invariably fatal viral zoonosis in humans, posing a threat to over 3 billion people worldwide, resulting in an estimated 59 000 human deaths and 3.7 million disability-adjusted life years (DALYs) per annum.1 Travellers to endemic areas are at risk of rabies infection if bitten or scratched by infected mammals. Rabies is not entirely preventable by education or awareness, but vaccination provides protection in the event of a rabies-prone exposure. Although travellers are often counselled to avoid contact with animals while in rabies-endemic areas, many travellers who present for treatment after rabies-prone exposure report that they did not initiate contact with the animal.2
The number of reported deaths among travellers is low—between 2013 and 2019, Gautret et al.3 identified 23 published reports of cases of rabies in travellers. Although rabies deaths in travellers are rare, the number of animal exposures with potential risk of rabies infection is not, and ranges from 0.3 to 157 per 1000 travellers per month.4 The rate of animal exposures in travellers has remained relatively constant over time. In 1994, Phanuphak et al.5 found that the incidence of animal bites among travellers in Thailand was 1.3%. In 2020, in a German airport study Heitkamp et al.6 reported similar risks across the rabies-endemic continents: 2.3% of travellers in Asia, 1.9% in Latin America, 1.7% in the Caribbean and 1.6% in Africa had a rabies-relevant mammal exposure. Therefore, Steffen and Hamer7 advocate that rabies pre-exposure prophylaxis (PrEP) needs to be prioritized during pre-travel consultations. They estimated that the number of travellers seeking rabies post-exposure prophylaxis (PEP) overseas was 22- and 12-times higher than those contracting hepatitis A or typhoid fever, respectively.7
For immunologically naïve individuals, recommended PEP includes wound management, multiple doses of rabies vaccines (wide range of schedules used across the world) and rabies immune globulin (RIG). Unfortunately, accessing RIG is challenging in many low-income countries.2,8 Using GeoSentinel data, Gautret et al.9 found that two-thirds of travellers with an indication for RIG, but who started PEP overseas, did not receive RIG. The current World Health Organization (WHO) guidelines for PrEP recommends multiple doses (either intradermal [ID] or intramuscular [IM]) to provide sufficient priming of the immune system.1 PrEP can be given before travel, is safe and effective, and simplifies PEP. In the event of an at-risk exposure, travellers who have completed a course of PrEP require less rabies vaccine doses for PEP and do not need RIG, making treatment simpler and safer. The urgency for seeking treatment of rabies-risk exposures can lead to immense disruption of travel plans. The lack of easy access to recommended treatment may also lead to significant anxiety, not to mention the possibility that inadequate treatment may be fatal.
There are many logistical challenges associated with providing rabies PrEP to travellers. Travellers often present for travel medicine advice very close to departure with insufficient time for the recommended PrEP doses.10,11 Despite shorter 1-week PrEP schedules now being recommended, 12% of travellers attend a pre-travel consultation with less than a week prior to departure.11 Travellers are time-poor, may be disinclined to return for multiple visits, and are sensitive to the cost of immunization.12 The WHO has set ‘… 1-visit PrEP in rabies endemic settings, including special populations and their response to simulated PEP …’1 as a research priority. Since rabies is fatal, the standard number of vaccine doses recommended is high (more than what would be required for most people) to ensure seroconversion in the small number of ‘low responders’ (3% of the healthy population who produce low antibody levels, probably due to genetic factors).13 It may be possible to reduce the number of doses required for PrEP, and still adequately prime the immune system, so that it can be rapidly boosted with PEP in the event of a rabies-prone exposure. Furthermore, it is known that age impairs the speed and magnitude of the immune response to rabies vaccination.14–16 Thus, it is important to document the efficacy of one-visit PrEP schedules in all ages, especially in persons ≥50 years as they are more likely to elicit a lower immunological response.14,15
Therefore, we conducted this study to investigate the efficacy of one-dose IM rabies vaccine as PrEP in priming the immune system by measuring antibody responses after simulated PEP using two IM doses of rabies vaccine (administer three days apart) in subjects aged <50 and ≥50 years.
Methods
Informed consent and registration
Information sheets were provided to all participants and written informed consent was obtained before enrolment. The study was approved by the Australian National University Human Research Ethics Committee (2019/451) and prospectively registered in the Australian and New Zealand Clinical Trials Registry (ACTRN12619000946112).
Study design
A quasi-experimental, pre–post intervention clinical trial without a control group was conducted to assess the efficacy of one-dose IM rabies vaccine in priming the immune system after simulated PEP.
Sample size
The sample size was calculated based on the primary objective, to estimate the proportion of antibody-positive individuals after a one-dose IM rabies vaccine followed by two simulated IM PEP doses. Based on the findings from a previous meta-analysis, we assumed a 95–100% antibody response after the boosters,17 thus 94 participants were needed to generate a confidence interval of seroconversion rate of between 88 and 98%. From previous studies conducted at the clinic,18 we anticipated a loss to follow-up of ~ 5%; thus the targeted sample size for this study was 100 participants.
Setting and study population
The study was conducted at a specialist travel medicine clinic, Dr Deb The Travel Doctor in Brisbane, Australia. As travellers seldom present >3 months prior to departure, the clinic advertised for participants though its contacts and website.
Participants were enrolled between 17 January 2020 and 2 October 2020, and the last follow-up was completed on 15 December 2020. Adults (≥18 years) without a history of rabies vaccination were eligible (confirmed by antibody levels <0.1 Equivalent Units [EU]/ml at enrolment). Exclusion criteria included participants with uncontrolled medical conditions (e.g. diabetes, asthma, hypertension); taking medications that impaired the normal functioning of the immune system; pregnancy or planning pregnancy; and travelling or planning to travel to areas of high risk for rabies exposure within 90 days. Given the paucity of information in older individuals and their lower immunological response, we aimed to include 50% of participants aged ≥50 years.
Study intervention and data collection
At enrolment (Visit 1), participants completed a baseline questionnaire (i.e. demographic data, medical history, allergies), blood samples were collected for antibody testing to confirm that they were seronegative, and one-dose 0.5 ml IM rabies vaccine was administered. To simulate an animal exposure that required PEP, participants returned to the clinic on Visit 2 (≥60 days after Visit 1) and Visit 3 (3 days after Visit 2) for one-dose 0.5 ml IM each time. Antibody testing was repeated on Visit 4 (7 days after Visit 2, when simulated PEP commenced) (Figure 1).
Flowchart of the timing of enrolment, vaccine doses and serology. Visit 1 (day 0), Visit 2 (Day 60), Visit 3 (Day 60) and Visit 4 (Day 67). IM intramuscular; IQR interquartile range; PEP post-exposure prophylaxis.
The vaccine used in the study was a purified Vero cell rabies vaccine (Verorab®, Sanofi Pasteur Ltd), which contains ≥2.5 International Unit (IU) lyophilized inactivated WISTAR Rabies PM/WI38 1503-3 M strain per 0.5 ml. All vaccinations were performed with the same vaccine batch (lot no. R1E732M). Antibody testing was conducted at Sullivan Nicolaides Pathology (Brisbane, Australia) using Platelia Rabies II enzyme-linked immunosorbent assay (ELISA), the only rabies serology test readily available in Australia. Results were reported in EU/ml, and antibody-positive and adequate booster response after the two simulated PEP doses were defined as antibody levels of ≥0.5 EU/ml. Results below 4 EU/ml were quantified by the laboratory, and those above 4 EU/ml were only reported as >4 EU/ml.
Participants were advised to telephone the clinic if they experienced any adverse events following immunization (AEFI). They were also directly asked about AEFI when they returned to the clinic at Visits 2 and 3 prior to administering the simulated PEP doses, and at Visit 4 prior to post-PEP antibody testing. In addition, an active SMS-based surveillance system of AEFI, SmartVax,19 was routinely used at the clinic for all patients who received vaccines. The SmartVax system automatically sends a text message 2 days after each dose with a short questionnaire regarding AEFIs, and the clinic receives regular summaries of patients’ responses.
Statistical analysis
Descriptive statistics were used to report the characteristics of the participants. The outcomes of interest (antibody-positive on Visit 1 and Visit 4, and AEFIs) were estimated as the proportion of the participants with the outcomes over the total number of participants. The analyses were stratified by age groups (<50 and ≥50 years). Antibody levels were categorized into <0.5, 0.5–2.49, 2.5–4.0, >4.0 EU/ml; differences between participants aged <50 and ≥50 years were compared using Chi2 test, and changes in antibody levels between Visit 2 and Visit 4 were compared using the Stuart–Maxwell test. All tests were 2-tailed, and a P-value < 0.05 was deemed statistically significant. Analyses were conducted using Stata MP version 14 (StataCorp, College Station, Texas).
Results
Characteristics of the participants
A total of 104 participants were enrolled in the study; one participant had detectable rabies antibodies (≥0.1 EU/ml) on Visit 1 and was excluded. Eight participants withdrew from the study after receiving one-dose 0.5 ml IM rabies vaccine on Visit 1 (six withdrew due to coronavirus disease 2019 (COVID-19) lockdown and movement restrictions, one due to a family emergency, and one did not specify a reason); thus 94 participants completed the study and were included in the analyses (Figure 1).
The majority of the participants (67%) were females, the median age was 47.1 years (IQR 32.3–61.7 years, range 21.8–75.2 years) and 44 participants (46.8%) were aged ≥50 years. A total of 27 participants (28.7%) reported having at least one well-controlled comorbidity; the most common were hypertension (12.8%), asthma (7.4%), anxiety/depression (6.4%) and diabetes mellitus (6.4%) (Table 1).
Characteristics of the 94 participants included in the study
| Number (%) | |
|---|---|
| Demographics | |
| Female sex | 63 (67.0) |
| Median age (IQR) | 47.1 (32.3–61.7) |
| Age groups | |
| <30 years | 19 (20.2) |
| 30–39 years | 14 (14.9) |
| 40–49 years | 17 (18.1) |
| 50–59 years | 15 (16.0) |
| 60–69 years | 21 (22.3) |
| ≥70 years | 8 (8.5) |
| Comorbidities | 27 (28.7) |
| Hypertension | 12 (12.8) |
| Asthma | 7 (7.4) |
| Anxiety/depression | 6 (6.4) |
| Diabetes mellitus | 6 (6.4) |
| ADHD | 2 (2.1) |
| History of DVT/pulmonary embolism | 2 (2.1) |
| Hashimoto’s disease | 1 (1.1) |
| Hyperthyroidism | 1 (1.1) |
| Migraine | 1 (1.1) |
| Psoriasis | 1 (1.1) |
| Number (%) | |
|---|---|
| Demographics | |
| Female sex | 63 (67.0) |
| Median age (IQR) | 47.1 (32.3–61.7) |
| Age groups | |
| <30 years | 19 (20.2) |
| 30–39 years | 14 (14.9) |
| 40–49 years | 17 (18.1) |
| 50–59 years | 15 (16.0) |
| 60–69 years | 21 (22.3) |
| ≥70 years | 8 (8.5) |
| Comorbidities | 27 (28.7) |
| Hypertension | 12 (12.8) |
| Asthma | 7 (7.4) |
| Anxiety/depression | 6 (6.4) |
| Diabetes mellitus | 6 (6.4) |
| ADHD | 2 (2.1) |
| History of DVT/pulmonary embolism | 2 (2.1) |
| Hashimoto’s disease | 1 (1.1) |
| Hyperthyroidism | 1 (1.1) |
| Migraine | 1 (1.1) |
| Psoriasis | 1 (1.1) |
IQR interquartile range; ADHD attention-deficit/hyperactivity disorder; DVT deep vein thrombosis
Characteristics of the 94 participants included in the study
| Number (%) | |
|---|---|
| Demographics | |
| Female sex | 63 (67.0) |
| Median age (IQR) | 47.1 (32.3–61.7) |
| Age groups | |
| <30 years | 19 (20.2) |
| 30–39 years | 14 (14.9) |
| 40–49 years | 17 (18.1) |
| 50–59 years | 15 (16.0) |
| 60–69 years | 21 (22.3) |
| ≥70 years | 8 (8.5) |
| Comorbidities | 27 (28.7) |
| Hypertension | 12 (12.8) |
| Asthma | 7 (7.4) |
| Anxiety/depression | 6 (6.4) |
| Diabetes mellitus | 6 (6.4) |
| ADHD | 2 (2.1) |
| History of DVT/pulmonary embolism | 2 (2.1) |
| Hashimoto’s disease | 1 (1.1) |
| Hyperthyroidism | 1 (1.1) |
| Migraine | 1 (1.1) |
| Psoriasis | 1 (1.1) |
| Number (%) | |
|---|---|
| Demographics | |
| Female sex | 63 (67.0) |
| Median age (IQR) | 47.1 (32.3–61.7) |
| Age groups | |
| <30 years | 19 (20.2) |
| 30–39 years | 14 (14.9) |
| 40–49 years | 17 (18.1) |
| 50–59 years | 15 (16.0) |
| 60–69 years | 21 (22.3) |
| ≥70 years | 8 (8.5) |
| Comorbidities | 27 (28.7) |
| Hypertension | 12 (12.8) |
| Asthma | 7 (7.4) |
| Anxiety/depression | 6 (6.4) |
| Diabetes mellitus | 6 (6.4) |
| ADHD | 2 (2.1) |
| History of DVT/pulmonary embolism | 2 (2.1) |
| Hashimoto’s disease | 1 (1.1) |
| Hyperthyroidism | 1 (1.1) |
| Migraine | 1 (1.1) |
| Psoriasis | 1 (1.1) |
IQR interquartile range; ADHD attention-deficit/hyperactivity disorder; DVT deep vein thrombosis
Impact of COVID-19 pandemic on the study
Due to COVID-19 lockdowns and movement restrictions in Australia, 33 participants (35.1%) could not return to the clinic for Visit 2 at the recommended time (60 days after Visit 1). The median delay for these participants was 122 days (interquartile range [IQR]: 27–130 days). Once the simulated PEP dose commenced on Visit 2, there were no delays in returning for Visits 3 and 4 in relation to the timing of Visit 2. There were no differences in baseline characteristics or immune responses between participants who were or were not affected by the delays in receiving the simulated PEP.
Immunogenicity and boostability
None of the included participants had detectable antibodies on Visit 1 (<0.1 EU/ml). After receiving one dose of IM rabies vaccine on Visit 1, 35.1% of participants were antibody-positive on Visit 2. There was no difference in the proportion of antibody-positive participants if Visit 2 was (33.3%) or was not (37.7%) delayed by COVID-19 movement restrictions. Among participants aged <50 years, 38.0% (34.0% with titres of 0.5–2.49 EU/ml; 4.0% with titres > 4 EU/ml) were antibody-positive, while 31.8% (29.5% with titres of 0.5–2.49 EU/ml; 2.3% with titres of 2.5–4.0 EU/ml) of participants aged ≥50 years were antibody-positive. There was no statistical difference in titre levels between the two age groups (P-value = 0.384) (Figure 2).
Percentage of participants with rabies antibody titre levels of <0.5, 0.5–2.49, 2.5–4.0 and >4.0 EU/ml by age groups at Visit 1 (Day 0), Visit 2 (Day 60) and Visit 4 (Day 67).
Two IM doses of simulated PEP ≥60 days after one-dose of IM rabies vaccine as PrEP showed a rapid anamnestic response in 100% of participants. Titre levels were higher among participants aged <50 years than ≥50 years (P-value = 0.002). Among participants aged <50 years, 8 and 82% had titre levels of 2.5–4.0 and >4 EU/ml, respectively, whereas in participants with ≥50 years, 22.7 and 47.7% reached titre levels of 2.5–4.0 and >4.0 EU/ml, respectively (Figure 2).
Adverse events following immunization
Overall, 22 participants (23.4%) reported one or more AEFI. The most common AEFIs were pain, swelling or bruising on the injection site (11.7%), fatigue (4.3%) and nausea/diarrhoea (2.1%). None of the AEFIs reported by the participants were severe enough to interfere with their normal daily activities.
Discussion
Our study provides valuable evidence on the efficacy of a one-dose IM as PrEP that could be used in last-minute travellers or other settings when complete PrEP schedules cannot be completed. We demonstrated that one-dose IM rabies was highly effective in priming the immune system, resulting in the rapid development of antibodies after two simulated PEP doses in all participants, including those participants aged ≥50 years. The advantages of a one-dose PrEP schedule include the ability to provide PrEP to last-minute travellers, convenience, cost-saving and dose-saving (during times of vaccine shortage).20
A major strength of our study is the inclusion of older participants. Previous studies only included young participants (median age <25 years),21,22 who are known to develop a more robust immune response to vaccines. The median age of the participants in our study was 47 years, 66% were over the age of 40, and we had a wide range of ages (21 to 75 years). Our study is the first to demonstrate that one-dose IM is highly effective (100%) in priming the immune system in individuals ≥50 years (n = 44). In this study, we focused on the IM delivery route, as vaccination using the IM route is simpler to administer in a non-travel clinic setting, and thus its use could be more widely implemented.
Our findings corroborate previous studies on one-visit PrEP. In young participants (aged 18–29 years), Khawplod et al.22 found that two-site 0.1 ml ID and one-dose IM with Rabipur® can be adequately boosted by a simulated four-dose ID PEP or two-dose IM PEP at one year. Jonker et al.21 reported similar findings using either one-visit 0.1 ml ID (1-site, 2-sites and 3-sites) or one-dose IM with Verorab® to prime the immune system, followed by two-dose IM PEP in young participants (aged 19–31 years). In a large study with 303 individuals aged 18–54 years, Soentjens et al.23 found that two-site 0.1 ml ID with Rabipur® can be boosted with two-site and four-site 0.1 ml ID PEP. In a dose–response meta-analysis, our previous study found that antibody levels with one-dose IM declined below 0.5 IU/ml after six months24; however, in another meta-analysis Langedijk et al.17 found that the antibody responses after booster immunization, including some after one-dose boosting schedules, were close to 100%. Based on our findings, we agree with Soentjens and Croughs,20 and we believe that current available evidence is sufficiently compelling to break the existing dogma of ‘no time for full PrEP is no PrEP’.
Although one-visit PrEPs are currently considered incomplete schedules by WHO,1 the WHO expert consultation report acknowledges that there is evidence to support these ‘alternative’ schedules in special circumstances such as when the two-visit PrEP cannot be completed.25 The purpose of PrEP is not necessarily to generate persistently detectable antibodies. Instead, the main objective of PrEP is to prime the immune system, so that in the event of a rabies-prone exposure, there is rapid development of adequate concentrations of antibodies after PEP.26 Even though 65% of participants in our study were seronegative on Visit 2 after one-dose IM PrEP, they all developed adequate antibody responses to PEP doses 7 days later (Visit 4), indicating that a single IM dose is able to initiate a robust immune memory that rapidly responds to restimulation. The current and previous studies17,21,22 demonstrate that one-dose IM is effective in priming the immune system, resulting in adequate antibody titres through anamnestic response after PEP (e.g. one-dose IM on days 0 and 3, without RIG). Therefore, clinical management after a rabies-prone exposure would be expected to have a similar outcome in individuals who received one-dose IM compared to those who received standard PrEP.
There were some limitations in the study. Antibodies were measured using ELISA instead of rapid fluorescent focus inhibition test, which is considered the gold standard but not readily available in Australia. However, ELISA has a good discriminatory capacity (>95% sensitivity and specificity) to detect seroconversion.27 Quantifying the increase in titre levels before and after the two simulated PEP doses was not possible because ELISA results are capped at 4 EU/ml; our analyses were therefore limited to seroconversion after the simulated PEP doses, defined as antibody levels of ≥0.5 EU/ml. Due to the COVID-19 pandemic and lockdowns, there were unavoidable delays in 35% of participants returning for the first simulated PEP dose (Visit 2). However, this should not have impacted the results given the similar characteristics between participants who were and were not affected by the delays related to the lockdown.
It is acknowledged that travellers who present for PEP in rabies-endemic countries after a one-dose priming course may find that the local medical system is unfamiliar with alternative schedules of PrEP, and insist on offering the traveller full PEP. Careful counselling of travellers would be necessary to ensure they are forewarned of this possibility. Given RIG is often unavailable and rabies is a fatal disease, an extra vaccine dose is less dangerous than insufficient doses, and having a primed immune system at the time of a significant bite would likely be advantageous.
Our study provides favourable evidence to support the use of one-dose IM for PrEP if there is insufficient time to complete a standard course of PrEP. However, further studies are needed to understand how long the immune priming is effective for, especially in older adults, and if this schedule can be generalized to longer trips. Khawplod et al.,22 Jonker et al.21 and Soentjens et al.23 found that one-visit PrEP can be successfully boosted with two IM or ID doses after one year in young individuals. Mansfield et al.28 demonstrated that standard PrEP primes the immune system in both young and older people, and can be boosted decades after the primary vaccine course. Furthermore, previously vaccinated individuals have been shown to respond to one or more booster doses even if the initial series of PrEP or PEP was administered several years before.29 However, previous anamnestic studies were conducted in persons who likely were antibody-positive after their course of three doses of vaccines. In persons who have only had one dose, with no detectable antibodies (e.g. about 65% in our study), we do not know how long their immune memory will persist. Future studies are needed to investigate the long-term boostability 5 or 10 years after one-dose IM PrEP. If exposed to a rabies-prone injury during their trip, individuals who received one-dose IM are likely to have received adequate immune priming, as our study and previous ones21,22 have demonstrated that antibodies develop rapidly after simulated PEP. However, one-dose IM rabies vaccine as PrEP should not be considered in people with who are at high risk of rabies (e.g. bat handler or laboratory technicians). Given the encouraging results from the current and previous studies,21–23 the acknowledgment of WHO Expert Consultation on Rabies: Third report of one-visit PrEP schedules as alternative regimes25; national and international clinical guidelines should evaluate the entire body of evidence on one-visit PrEP and consider updating their recommendations.
In conclusion, if there is insufficient time to complete a standard rabies PrEP schedule, the use of one-dose IM rabies vaccine should be considered for immunocompetent adult travellers departing on trips of <60 days. One-dose IM may be useful as an alternative schedule (instead of no PrEP) as it may facilitate post-exposure management by removing the need for RIG, which is often unavailable.
Acknowledgement
We would like to sincerely thank clinic staff, nurses, and doctors who enrolled and followed up patients, collected samples and administered the vaccines as part of the study. We also thank all the participants who so kindly participated in the research, and without whom these findings would not have been possible.
Authors’ contribution
Conception and design of the study: D.J.M., C.L.L., L.F.K.
Collection and assembly of the dataset: D.J.M., C.M., L.F.K.
Analysis of the dataset and interpretation of results: D.J.M., C.L.L., L.F.K.
Manuscript writing: D.J.M. and L.F.K.
Reviewing and editing the manuscript: All authors.
Final approval of manuscript: All authors.
Funding acquisition: D.J.M.
Funding
D.J.M. received funding from Sanofi Pasteur (RAB00054) to conduct the study. C.L.L. and L.F.K. were supported by Australian National Health and Medical Research Council Fellowships (APP1193826 and APP1158469). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.
Conflicts of interest
The authors do not have any conflicts of interest to declare.
