Safety and Immunogenicity of ID vs IM Rabies Vaccine

Epidemiology of Rabies:

Rabies is a vaccine-preventable zoonotic viral disease that primarily targets the central nervous system, leading to severe neurological symptoms and, if untreated, is 100% fatal. It is transmitted through the bite or saliva of an infected animal, with dogs being the most common source of human infections worldwide. The virus travels from the site of infection through the peripheral nerves to the brain, where it causes encephalitis, characterized by confusion, agitation, paralysis, and ultimately, coma and death.

Globally, rabies is responsible for around 59,000 deaths each year, with the majority occurring in Asia and Africa. An alarming 40% of those affected are children under the age of 15. This infection comes with an economic burden of $8.6 billion annually. A similar scenario prevails in Bangladesh. Bangladesh is ranked third globally in terms of rabies infections, right after China and India. In 2009, rabies-related human deaths in Bangladesh were estimated to exceed 2,000. However, the death toll has steadily declined to 26 in 2020, owing to the implementation of the 'National Rabies Elimination Program' beginning in 2010, which included the introduction of the cell culture vaccine.

Treatment of Rabies from a Bangladeshi Perspective:

Despite being ultimately fatal when symptoms show, rabies is 100% preventable by vaccination. Louis Pasteur was the first to administer the rabies vaccine to a patient in 1885. Currently, two types of vaccines are available for protection against rabies: nerve tissue vaccines and cell culture vaccines. The World Health Organization (WHO) recommends replacing nerve tissue vaccines with more effective and safer cell culture-based vaccines as soon as possible. Recent advancements in cell culture vaccines have made them more affordable and require smaller doses. Cell-culture-based rabies vaccines can be administered through two different routes: intramuscular (IM) and intradermal (ID). As per the National Guideline for Animal Bite Management in Bangladesh, (i) a 1-site IM regimen or 2-site ID regimen for Pre-exposure Prophylaxis (PrEP), and (ii) a 1-site IM regimen or 2-site ID regimen for Post-exposure Prophylaxis (PEP) is available for Rabies prevention. The IM regimen is more commonly used in clinical settings, as observed in various studies. However, the IM regimen has been found to elicit a serious adverse event (SAE) in a previous study. Seven days after receiving an intramuscular rabies vaccine regimen, Bell's Palsy was reported as a suspected unexpected SAE. Moreover, multiple studies conducted worldwide have demonstrated the superiority of the ID regimen over IM in terms of seroprotection.

The introduction of an intradermal rabies vaccine regimen in Bangladesh could have significant public health implications. As a low-resource country with a high burden of rabies, Bangladesh could benefit greatly from a more cost-effective vaccination method. Given that the intradermal route uses significantly less vaccine (up to 80% less per dose) and reduces costs by about 75% compared to intramuscular injection, its adoption could lead to much wider vaccine coverage within budgetary limits and a substantial decrease in the economic burden of rabies. This could potentially lead to increased accessibility of rabies prevention, especially in rural and underserved areas where cost is a major barrier to vaccination. Furthermore, the dose-sparing nature of the intradermal route could help mitigate vaccine shortages, which are occasionally reported worldwide. By conducting this trial, we aim to provide local evidence to support the adoption of this WHO-recommended approach, potentially revolutionizing rabies prevention strategies in Bangladesh.

This study seeks to determine the safety and immunogenicity of intradermally administered Inj. Rabivax ID/SC by Popular Pharmaceuticals PLC. (Test) and Inj. Rabivax IM (2.5 IU/ml) by the same manufacturer (Reference) in healthy participants in pre-exposure prophylactic doses.

What is seroconversion? The production of antibodies in an individual following vaccination or exposure to any antigen is called seroconversion. According to WHO, an adequate antibody response is usually defined as 0.5 IU/ml or more at 14 and 28-30 days following the final vaccination dose. The Advisory Committee on Immunization Practices (ACIP) has endorsed this Rabies Virus Neutralizing Antibody (RVNA) level, thereby replacing the previously established minimum acceptable rabies antibody titer of 0.1-0.3 IU/ml (16). This higher limit offers a more conservative approach to determining inadequate response and booster requirements. RVNA level is usually assessed using one of the two serological assays: rapid fluorescent focus inhibition test (RFFIT) or fluorescent antibody virus neutralization (FAVN). In this study, we will focus on the RFFIT for the assessment of RVNA level. RFFIT is a serological assay that quantifies the neutralizing capacity of rabies-specific antibodies, reflecting their ability to inhibit viral infection of susceptible cells. This investigational procedure utilizes serial fivefold dilutions of patient serum, which are subsequently combined with a standardized challenge dose of live rabies virus. Following incubation, the serum/virus mixtures are introduced to susceptible cell cultures. The presence of RVNA is determined by assessing the inhibition of viral cytopathic effect through fluorescent antibody staining. The endpoint titer is calculated based on the highest serum dilution that demonstrates significant neutralization. Since the World Health Organization (WHO) and the Advisory Committee on Immunization Practices (ACIP) consider an RVNA level of complete neutralization at a 1:5 serum dilution by the RFFIT test as proof of a sufficient immune response to rabies vaccination, this test will provide us with the accurate immunogenicity of the test vaccine.

The assessment of the seroconversion rate among the enrolled participants will be one of the primary outcome measures of this study. The seroconversion rate is defined as the proportion of subjects with RVNA level ≥0.5 IU/mL, measured 4 weeks after vaccination completion. We will perform a comparative analysis to determine the non-inferiority of seroconversion rates between participants receiving intradermal and intramuscular rabies vaccinations.

Safety Profile Rabies vaccine production involves three principal types: the now largely obsolete nerve tissue vaccines, contemporary cell culture vaccines, and vaccines derived from embryonated eggs. WHO guidelines endorse cell culture and embryonated egg rabies vaccines over nerve tissue vaccines since the latter pose risks of severe adverse events and reduced immunogenicity, including the potential for rabies transmission due to incomplete virus inactivation. In comparison, cell culture and embryonated egg vaccines exhibit a favorable safety profile and are generally well-tolerated. In our study, we will use purified Vero cell vaccine (PVRV) for both the test and reference groups. By utilizing a serum-free medium and an animal-origin-free trypsin-like enzyme, this cell culture vaccine is designed to minimize the risk of hypersensitivity and the transmission of contaminants, including bacteria, fungi, and mycoplasma. Furthermore, Frazatti-Gallina et al. demonstrated the safety of PVRV in mouse models. A Chinese trial by the manufacturers of our study vaccines established the safety of this vaccine in murine models. In addition, the vaccine has proved to be safe in healthy participants of different age groups. The safety of these rabies vaccines in pregnant women has also been demonstrated in prior studies. These purified Vero cell vaccines produce mild to moderate adverse events such as erythema, pain, swelling at the administration site, and fever, which usually resolve spontaneously without treatment. Serious adverse events related to the PVRV vaccine are rare, with an estimated incidence of 1 per 100,000 people.

Because of its proven safety record, this vaccine can be reasonably administered to healthy individuals. The established safety profile justifies our undertaking a non-inferiority trial in healthy individuals. However, our skilled staff will administer the vaccines properly while maintaining safety and asepsis to minimize any complications. We will conduct a follow-up assessment six months after the final vaccination to determine if any long-term adverse events have occurred and provide proper treatment if needed.

Vaccine Potency The quantifiable biological activity elicited by a vaccine antigen is known as its potency. According to the WHO, the potency of rabies vaccines should be 2.5 IU/intramuscular dose to elicit an adequate immune response once administered. 1 ml inj. Rabivax IM contains 2.5 IU of rabies antigen as per the potency recommended by WHO. On the other hand, the intradermal rabies vaccine regimen only requires a dose of 0.2 ml (0.1 ml in each arm), which corresponds to a potency of ≥0.5 IU/dose. Several studies showed a potency range of 0.22-2.32 IU/intradermal dose. Since the intradermal regimen requires multiple site administration and fractional dosing due to the abundance of antigen-presenting cells in the dermis, this antigen level in the vaccines is sufficient to produce immunogenicity similar to the levels of the IM regimen.

Appropriate Dosing According to the WHO guidelines, the intramuscular rabies vaccine regimen is given at a dose of 1 ml, while the intradermal regimen is given at a dose of 0.2 ml (0.1 ml in each arm). We will adhere to the guidelines since administration of 0.1 ml per intradermal site is considered appropriate for practical reasons and produces similar seroprotection levels. We will also administer 1 ml inj. Rabivax IM to the reference group per the recommended dosing schedule. As the WHO has already established and standardized the doses for both intradermal and intramuscular administration, we will not include an assessment of dose responses.

Appropriate Vaccine Schedule The dosing schedule for both intradermal and intramuscular rabies vaccine regimens has been included in the national guidelines. We will follow the 2-dose regimen for both vaccines. In the test group, 0.1 ml Inj. Rabivax will be administered intradermally on both the deltoid regions on days 0 and 7. We will administer 1 ml inj. Rabivax (2.5 IU/ml) vaccine intramuscularly on the deltoid region of one arm on days 0 and 7 in the reference group according to the instructions in the national guidelines.

Rationale Rationale of the non-inferiority trial According to the DGDA's regulatory requirements, a previously licensed and approved drug product requires additional approval to be marketed for administration via an alternate route. Therefore, a clinical trial comparing the non-inferiority of the intradermally administered rabies vaccine by Popular Pharmaceuticals PLC. to the intramuscularly administered rabies vaccine in terms of immunogenicity and safety can provide the foundation and evidence for its use in the Bangladeshi population.

Rationale of the age group The purified Vero cell vaccine for rabies has been proven to be remarkably safe and well-tolerated in children of all ages. This vaccine is already in use for people of all ages in Bangladesh, according to the National Guideline for Animal Bite Management. Therefore, we will assess the non-inferiority of the intradermal vaccine in terms of safety and immunogenicity across all age groups to strengthen the evidence generated by the trial, which is required for the vaccine's marketing authorization by the DGDA.

Rationale of the follow-up schedule We will conduct follow-up visits with the participants on days 21, 35, and 187. Since the RVNA level following vaccination peaks at 2-4 weeks, the first two follow-up visits will allow us to analyze the accurate seroconversion rate in the participants. We will p

• Number of participants with treatment-related serious adverse events as assessed by CTCAE v5.0 [Срок оценки: Participants will be followed up 14 days, 28 days, and 180 days following the completion of vaccination doses. They will also contact the research team for self-reporting of adverse events.]
• Number of participants with post-vaccination laboratory abnormalities [Срок оценки: The number of participants with post-vaccination laboratory values will be assessed on day 21 and day 187 of the study.]
• Number of participants with post-vaccination abnormalities in vital signs [Срок оценки: Vital signs will be evaluated on the day of check-in (day 0), day 7, and during post-vaccination follow-up (day 21, 35, and 187); 30 minutes before dosing on vaccination days]
• Number of participants with post-vaccination abnormalities found in physical examination [Срок оценки: General and systematic examination will be conducted on check-in (Day 0), day 7, and during post-vaccination follow-up (Days 21, 35, and 187) and]