Clinical

Japanese encephalitis vaccines effective against all major virus sub-types

Last Updated: 2012-10-31 11:00:16 -0400 (Reuters Health)

By C. Vidyashankar, MD

NEW YORK (Reuters Health) - Though the currently used Japanese Encephalitis (JE) vaccines are type-specific, they offer cross-protection against major pathogenic genotypes, according to a new study from Finland.

Researchers found that the JE genotype III vaccine elicited protective levels of antibodies against genotypes I to IV.

"Our study showed that the different viruses (gI-gIV) resemble each other so much that the body develops a protective immune response against all genotypes even if we only vaccinate with a vaccine containing one genotype," Dr. Anu Kantele of Helsinki University Central Hospital, who led the study, told Reuters Health by email.

Japanese encephalitis is a RNA flavivirus infection acquired through mosquito bites. It affects around 700,000 people every year, predominantly in Asia, and has no specific treatment.

While vaccines are available against genotype III, genotypes I, II and IV are increasingly responsible for epidemics, the researchers explained in their paper, published online October 16 in Clinical Infectious Diseases.

"We have two Japanese encephalitis vaccines in use for travelers. The traditional one, JE-GCC has been associated with safety concerns and the newer vaccine, Ixiaro, has replaced it. Ixiaro is not associated with any significant adverse effects," Dr. Kantele said.

The team studied the cross-protective effect of two JE genotype III vaccines in previously healthy and uninfected adult volunteers from Finland and Sweden. Twenty-nine adults received the Ixiaro vero-cell vaccine (Intercell AG, Vienna, Austria) and 12 received the Nakayama JE GCC mouse-brain vaccine (Green Cross, South Korea) prior to travel to JE endemic areas.

The GCC vaccine was given as three doses intramuscularly on day 0, 7, and 30 and the Ixiaro as two intramuscular doses 4 weeks apart. JE antibody titers were measured from serum samples prior to and 4 to 8 weeks after vaccination using the plaque-reduction neutralization test (PRNT); a PRNT-50 antibody titer of greater than 10 was considered protective.

The vast majority of volunteers in both groups developed protective levels of antibodies against genotypes I through IV, the researchers report. The seroconversion rates were similar and ranged from 83%-100% with the GCC vaccine and from 93%- 97% with Ixiaro vaccine.

Both groups recorded highest titers against JE genotype II and the lowest levels, though protective, against virus genotype I. The mean protective antibody titers with Ixiaro vaccine was 55 and 811 against virus genotypes I and II, respectively. The corresponding titers with GCC vaccine were 50 and 580.

"As to protective efficacy, our study showed that both vaccine types provide protection against all strains tested," Dr. Kantele said.

As genotype I is becoming increasingly prevalent in Asia, the relatively low levels of protective antibodies "calls for special attention in the future," the researchers say. The duration of protection and the need for booster doses also needs consideration in future studies, they add.

SOURCE: http://bit.ly/St5GA7

Clin Infect Dis 2012.