Early warning: current Japanese encephalitis vaccine might not protect against recently emerged virus strain
Japanese encephalitis virus (JEV) is the leading cause of viral encephalitis (infection of the brain) in Asia. There is no specific treatment for Japanese encephalitis (JE) which can cause death or serious long-term disability, and WHO recommends JEV vaccination in all areas where the disease is recognized as a public health priority. A study published in PLOS Neglected Tropical Diseases suggests that current vaccines may fail to protect individuals against an emerging strain of the virus.
An estimated 3 billion people live in 24 South-East Asian and Western Pacific countries where the virus is present. JE viruses come in different ‘flavors’: there are five different genotypes (G1-G5), defined by differences in the ‘envelope’ gene that codes for proteins covering the virus surface. Strain G5 was originally isolated from a patient and described in 1951, but then not seen again until found recently (in 2009) in China and subsequently in Korea.
No specific treatment exists against the JE virus, but a number of vaccines are used to protect local populations and travellers. All of the vaccines are based on G3 virus strains and have been shown to work well against G1 through G4 strains. However, their efficiency against the previously rare but possibly re-emerging G5 strain is not clear.
Guodong Liang, from the Chinese Center for Disease Control and Prevention, in Beijing, China, and colleagues were the first to report the re-emergence of the G5 strain. In this study, they compared G3 and G5 viruses and tested whether the vaccine commonly used in China can protect against G5 viruses.
Having found the two strains similar in their ability to cause disease in mice, the researchers vaccinated mice and tested whether they were protected against a dose of virus that would be lethal to unvaccinated animals. They found that the (G3-based) vaccine protected all the mice against a lethal challenge with G3 virus, but only 50% of the mice infected with G5 virus survived.
Next, the researchers looked for inactivating (or neutralizing) antibodies in vaccinated two-year-old children. They examined blood samples from 26 children that had been collected both before and 28 days after JE vaccination. Following vaccination, they were able to detect neutralizing antibodies against G3 strains in all the children, but only 35% of them also had antibodies that could neutralize G5 strains.
The geographic distribution of Japanese encephalitis (in yellow)