Two Japanese Encephalitis Vaccines Can Be Safely Mixed, Landmark Indian Trial Finds

A large randomized trial conducted across five Indian medical college hospitals has found that two domestically produced inactivated Japanese encephalitis vaccines can be used interchangeably in the country’s childhood immunization program without compromising immune protection or safety. The phase 4 study, published in eClinicalMedicine, enrolled more than 900 infants and demonstrated that children who received one dose of one vaccine followed by a dose of the other mounted neutralizing antibody responses that were statistically non-inferior to those seen in children who received the same product for both doses. The finding addresses a practical gap that has long concerned immunization managers in endemic regions, where supply interruptions, procurement changes, and family migration can leave children facing a second dose of a different vaccine product than the one that primed their immune system.

Japanese encephalitis is a mosquito-borne viral encephalitis caused by a flavivirus transmitted predominantly by Culex mosquitoes, and it remains the leading vaccine-preventable cause of viral encephalitis in the Asia-Pacific region. An estimated 3 billion people living in 24 countries are at risk of infection. In 2019, an estimated 57,000 cases and 20,642 deaths occurred globally, with India contributing the largest share of the burden at 39.1 percent, followed by Bangladesh and China. Although most infections are asymptomatic, roughly one in 250 infected people develop severe neuroinvasive disease. Among those who develop encephalitis, case fatality rates range between 20 and 30 percent, and 30 to 50 percent of survivors are left with long-term neurological, cognitive, or psychiatric sequelae. Childhood vaccination therefore remains the cornerstone of prevention in endemic countries.

India’s Universal Immunization Programme relies on two inactivated Vero cell-derived vaccines. JENVAC, manufactured by Bharat Biotech, is derived from an Indian JE virus strain known as Kolar-821564XY, while JEEV, produced by Biological E, is based on the SA 14-14-2 strain that underpins several internationally used JE vaccines. Both are administered as a two-dose intramuscular schedule, with the first dose given at 9 to 12 months of age and the second at 16 to 24 months. Although individual states generally receive a single vaccine type for routine immunization, programmatic realities mean that some children inevitably receive a second dose of a different product. Until now, no trial had evaluated such mixed-product schedules under India’s routine two-dose program, and previous studies of JE vaccine interchangeability had mostly examined other vaccine platforms or shorter dosing intervals.

The trial was conducted between November 2023 and September 2025 at hospitals in Kolkata, Wardha, Bhubaneswar, Bibinagar, and Patna. Healthy infants aged 9 to 12 months attending immunization clinics were eligible, while those with severe malnutrition, systemic disorders, neurological conditions, or prior JE vaccination were excluded. A total of 937 infants were screened and 911 were randomly assigned in equal proportions to one of four schedules: JENVAC followed by JEEV, JENVAC followed by JENVAC, JEEV followed by JENVAC, or JEEV followed by JEEV. Randomization was performed centrally using a computer-generated permuted block sequence, and although the study was open-label, laboratory staff performing the immunogenicity assays were masked to treatment allocation. The trial was funded by the Indian Council of Medical Research, which had no role in the design, analysis, or reporting.

The vaccines themselves differ subtly in composition. Each 0.5 milliliter dose of JENVAC contained 5 micrograms of purified inactivated JE virus protein from the Kolar strain, adsorbed onto 0.25 milligrams of aluminum hydroxide with thiomersal as a preservative, whereas each dose of JEEV contained 3 micrograms of inactivated virus from the SA 14-14-2 strain with aluminum hydroxide. Participants received their assigned vaccine in the thigh alongside the routine vaccines scheduled at the same visits, including measles-rubella vaccine, fractional inactivated polio vaccine, and pneumococcal conjugate vaccine at the first visit, and measles-rubella and DPT booster at the second. Blood samples were collected before vaccination, after the first dose, before the second dose, and 28 days after the second dose.

The primary outcome was seroconversion, measured by the 50 percent plaque reduction neutralization test, or PRNT50, against a wild-type JE virus strain isolated during a large 2005 outbreak in Uttar Pradesh. Seroconversion was defined as a rise in antibody titer from below 1:10 to at least 1:10, or a four-fold or greater rise among children with detectable baseline antibodies. In the modified intention-to-treat analysis of 841 children with evaluable samples, seroconversion 28 days after the second dose reached 89.8 percent in the JENVAC-JEEV group, 90.2 percent in the JENVAC-JENVAC group, 90.9 percent in the JEEV-JENVAC group, and 89.2 percent in the JEEV-JEEV group. The differences between mixed and same-product schedules were small and their confidence intervals fell entirely above the prespecified non-inferiority margin of minus 10 percentage points, in both the intention-to-treat and per-protocol analyses.

Geometric mean neutralizing antibody titers followed the same pattern, climbing from around 2 before vaccination to between roughly 68 and 74 after the second dose across all four groups, with no meaningful separation between mixed and same-product schedules. Among children who were seronegative at baseline, seroconversion rates after two doses ranged from about 92 to 95 percent. Among the smaller subset who were seropositive at baseline, fewer met the strict four-fold rise criterion, but the overwhelming majority nonetheless remained seroprotected. In a randomly selected subset, responses were also tested against the two vaccine-homologous strains, and titers were generally higher against these than against the wild-type strain, particularly after the first dose, a pattern consistent with the strain-specific bias that earlier trials using homologous assays may have introduced.

Safety results were reassuring. Roughly 36 percent of children in each group reported at least one solicited adverse event within seven days of vaccination, with fever the most common, followed by runny nose and cough. The vast majority of events were mild and resolved without sequelae. Twenty participants experienced 23 serious adverse events during follow-up, but none was assessed as related to vaccination according to the World Health Organization causality assessment algorithm. A Data and Safety Monitoring Board oversaw participant safety throughout the trial, and written informed consent was obtained from a parent or legal guardian of every enrolled child.

One finding warrants attention: only about a third of children, between 34 and 41 percent, remained seroconverted just before receiving the second dose, roughly seven months after the first. This suggests a possible window of reduced antibody-mediated protection in the second year of life. Surveillance data from India’s Virus Research and Diagnostic Laboratory Network indicate that children aged 12 to 24 months accounted for 3.4 to 6.5 percent of JE cases diagnosed annually between 2014 and 2025, without an apparent increasing trend, but the trial’s authors note that age-specific incidence data are needed to determine whether the longer dosing interval increases risk. They emphasize that timely completion of both doses remains essential.

The public health implications extend well beyond immunology. In large-scale programs, children may not always receive the same product for both doses because of migration, stock availability, procurement cycles, or supply changes, and in the absence of interchangeability evidence, program managers might defer or withhold second doses unnecessarily. By showing that JENVAC and JEEV can be swapped without sacrificing short-term immunogenicity or safety, the trial removes a barrier to maintaining high two-dose coverage in endemic districts. The authors caution that immune responses were assessed only one month after the second dose, leaving the durability of protection after the extended-interval schedule an open question, and that cell-mediated immunity, which may contribute to protection, was not measured. Still, for the hundreds of millions of children living in JE-endemic Asia, the message from this trial is straightforward: whichever of the two licensed inactivated vaccines is available when the second dose falls due, it can be given with confidence.

Subject of Research: Interchangeability of two inactivated Japanese encephalitis vaccines in India's childhood immunization schedule

Article Title: Immunogenicity and safety of an interchangeable vaccination schedule of two inactivated Japanese encephalitis vaccines: a multicentric, phase IV, open-label, randomized controlled trial

Article References: Vivian Thangaraj, J. W., Reddy, H., Sapkal, G. N., Velusamy, S. K., Jain, M. A., Dwibedi, B., Kanungo, S., Kundapur, R., Pandey, S., Deshpande, G. R., Saritha, A., Kadavath, V., Kanagasabai, K., Maliye, C., Parida, S. P., Majumdar, A., Reddy, M., Singh, C., Lade, A. R., … Murhekar, M. V. (2026). Immunogenicity and safety of an interchangeable vaccination schedule of two inactivated Japanese encephalitis vaccines: a multicentric, phase IV, open-label, randomized controlled trial. eClinicalMedicine, 100, Article 104235. https://doi.org/10.1016/j.eclinm.2026.104235

Image Credits: AI Generated

DOI: 10.1016/j.eclinm.2026.104235

Keywords: Japanese encephalitis, JENVAC, JEEV, vaccine interchangeability, immunogenicity, PRNT50, non-inferiority trial, India, Universal Immunization Programme, flavivirus, childhood vaccination, vaccine safety