Japanese encephalitis vaccines: current vaccines and future prospects.

Vaccination against JE ideally should be practiced in all areas of Asia where the virus is responsible for human disease. The WHO has placed a high priority on the development of a new vaccine for prevention of JE. Some countries in Asia (Japan, South Korea, North Korea, Taiwan, Vietnam, Thailand, and the PRC) manufacture JE vaccines and practice childhood immunization, while other countries suffering endemic or epidemic disease (India, Nepal, Laos, Cambodia, Bangladesh, Myanmar, Malaysia, Indonesia and the Philippines) have no JE vaccine manufacturing or policy for use. With the exception of the PRC, all countries practicing JE vaccination use formalin inactivated mouse brain vaccines, which are relatively expensive and are associated with rare but clinically significant allergic and neurological adverse events. New inactivated JE vaccines manufactured in Vero cells are in advanced preclinical or early clinical development in Japan, South Korea, Taiwan, and the PRC. An empirically derived, live attenuated vaccine (SA14-14-2) is widely used in the PRC. Trials in the PRC have shown SA14-14-2 to be safe and effective when administered in a two-dose regimen, but regulatory concerns over manufacturing and control have restricted international distribution. The genetic basis of attenuation of SA14-14-2 has been partially defined. A new live attenuated vaccine (ChimeriVax-JE) that uses a reliable flavivirus vaccine--yellow fever 17D--as a live vector for the envelope genes of SA14-14-2 virus is in early clinical trials and appears to be well tolerated and immunogenic after a single dose. Vaccinia and avipox vectored vaccines have also been tested clinically, but are no longer being pursued due to restricted effectiveness mediated by anti-vector immunity. Other approaches to JE vaccines--including naked DNA, oral vaccination, and recombinant subunit vaccines--have been reviewed.