Japanese encephalitis vaccine. Useful for selected adult travellers

Japanese encephalitis is a very rare but potentially severe disease for travellers who spend prolonged periods in rural areas of Asian countries, in which the disease is endemic. There is no satisfactory treatment. An inactivated vaccine targeting the virus responsible for Japanese encephalitis (strain SA14-14-2) has been authorised for use in the European Union. It replaces the Biken vaccine (based on a different strain) which is no longer manufactured. A comparative study in 867 adults showed that the new vaccine is at least as immunogenic as its predecessor. Preventive efficacy in terms of cases of Japanese encephalitis has not been assessed. Among 181 adults monitored for one year, 83% of vaccinees still had detectable antibodies at the end of the study period. There are no data on longer-term immune response or on the effect of a booster vaccination. About 1% of adults enrolled in clinical trials discontinued vaccination because of adverse effects. In one study the new vaccine caused fewer local reactions than its predecessor (54% versus 61%), and fewer cases were severe (3% versus 14%). In another trial the frequency of systemic adverse effects was similar in the vaccine and placebo groups. However, follow-up is too short to rule out the possibility of rare but serious adverse effects, which were reported with the old vaccine. In a study including 192 adults, concomitant vaccination against Japanese encephalitis and hepatitis A did not undermine the immunogenicity of either vaccine. The new Japanese encephalitis vaccine comes in ready-to-use syringes and is administered in 2 injections, 4 weeks apart. In practice, vaccination against Japanese encephalitis should be restricted to adults visiting Asia who are likely to be at high risk of contracting the virus.