Are Japanese Encephalitis Vaccines Interchangeable?

There are currently 15 Japanese Encephalitis vaccines in use worldwide. The most widely administered of these is the live attenuated lyophilized vaccine, CD.JEVAX. New generation, Vero-cell based inactivated vaccines are increasingly used to provide long-lasting immunity. IXIARO is an inactivated cell-derived vaccine based on the same SA 14-14-2 strain of Japanese Encephalitis as CD.JEVAX.

JENVAC, an inactivated vero-cell adapted vaccine, was produced using the Kolar strain 821564XY. A new study in The Journal of Infectious Diseases comparing immunogenicity, safety, and interchangeability of JENVAC to IXIARO reports that JENVAC offers statistically higher immunological measures. The research found that a single dose of JENVAC was found to induce protective titers persisting up to 1 year.

The investigators previously reported that a 2-dose regimen of JENVAC, administered 4 weeks apart, was well tolerated and immunogenic. The new study examined a single dose of JENVAC in comparison with IXIARO, the SA 14-14-2 vaccine, as well as interchangeability of a 2-dose regimen of either vaccine.

The new study was a phase 4, multi-center, randomized, single blind, clinical trial conducted from June 2014 to July 2016 at 4 sites in India.

Children enrolled were equally randomized to receive either JENVAC or IXIARO and were followed for 1 year after receiving a single dose of either vaccine.

Samples taken at various time points throughout the study were subjected to the determination of 50% plaque reduction neutralization titer (PRNT₅₀). Seroprotection was defined as a PRNT₅₀ of ≥10 U/mL.

Among the 360 children initially randomized, 329 completed the single dose trial. At day 360 post-vaccination, 81.7% (95% confidence interval [CI]: 74.9, 87.3) in the JENVAC group and 47.9% (CI: 40.1, 55.8) in the IXIARO group achieved seroprotection.

JENVAC administration led to statistically higher immune responses at day 28 and subsequent time points.

After the follow-up period, parents and guardians of the children were contacted for their willingness to participate in an interchangeability study.

At that point, 178 children were assigned to receive a booster of the same vaccine previously administered or interchanged to receive the other vaccine.

Among the participants, 96 were placed in the JENVAC group and 82 were placed in the IXIARO group.

At day 720, 88.5% (95% CI: 80.4, 94.1) in JENVAC group achieved seroprotection, compared to 68.3% (57.1, 78.1) in the IXIARO group. At day 748, the group that received 2 doses of JENVAC had the highest geometric mean concentration of antibody.

Results that show a single dose of JENVAC produces robust immune response are encouraging for clinicians and public health officials seeking to protect at-risk populations from this dangerous condition.

Adverse events were similar between the JENVAC and IXIARO groups, with 57 and 62 adverse events reported, respectively. The most common adverse event noted was fever. No participant in either group had an illness associated with Japanese Encephalitis during the 3 years of study.

The investigators recommended further research comparing immune responses between JENVAC and other inactivated Japanese Encephalitis vaccines in endemic settings. They also suggested evaluating the duration of protection for travelers in non-endemic settings as a future area of research.