Neutralization potency of sera from Vietnamese patients with Japanese encephalitis (JE) against genotypes I and V JE viruses.

Japanese encephalitis virus (JEV) is classified into five genotypes (GI, GII, GIII, GIV, and GV), and the GI and GIII strains have been the most widely distributed in JE endemic areas. In recent years, GV JEV has been detected in China and Korea, suggesting that GV JEV may invade other JE endemic areas, including Vietnam, and that more attention should be paid to JEV strains circulating in these areas. In this study, we investigated the neutralization ability of the sera collected from 22 Vietnamese JE patients who lived in northern Vietnam against GI and GV JEV strains. In most cases, the ratios of the titer against GV to that against GI (GV:GI) were equal to or less than 1:4. However, the titer against GV JEV was equivalent (1:1) to that against GI JEV in only a few cases, and no serum had a ratio higher than 1:1. Thus, our results did not show convincing evidence that GV JEV have been emerging in northern Vietnam in 2014.