Varicella vaccination in Japan: necessity of implementing a routine vaccination program.

Varicella-zoster virus (VZV) is the causative agent of varicella (chickenpox). It shows extremely high infectivity and is spread by airborne, droplet, and contact transmission. After a person is infected with VZV, the virus remains dormant in the dorsal root ganglia, but can be reactivated under circumstances where specific immunity declines, leading to the development of herpes zoster (shingles). Although varicella is a disease that usually resolves after about 1 week, it can cause various complications such as secondary bacterial skin infection, pneumonia, and encephalitis. In addition, varicella can become severe in immunocompromised persons, whereas VZV infection transmitted from an infected mother can cause the congenital varicella syndrome or serious neonatal varicella. In 1974, a live varicella vaccine (Oka strain) was developed in Japan for the prevention of varicella, and clinical trials performed during the development were mainly focused on high-risk children. In 1985, the Oka strain was recognized as the best varicella vaccine strain by the World Health Organization (WHO). Today, all the varicella vaccines used worldwide to immunize approximately 32 million people annually contain the Oka strain. In Japan, it has been commercially available since 1987 for the voluntary vaccination program, in which children over the age of 1 year with no history of previous varicella infection receive a single dose. In addition to healthy children, this vaccine can be used for immunocompromised children, and vaccination of elderly persons can also be done to enhance their immunity against VZV. Varicella vaccine is a highly safe vaccine with sufficient immunogenicity. The preventive effect of single-dose vaccination is believed to be approximately 80 % for all types of varicella, including mild cases; it is 95 % or greater for moderate to severe disease. Implementation of a two-dose vaccination schedule has proved to be effective against breakthrough varicella, which is observed in approximately 20-30 % of children vaccinated with a single dose. Because it is administered as part of the voluntary vaccination program, the varicella vaccination coverage rate in Japan has remained low until recently at around 20-30 %, with no sign of a decrease in the number of varicella patients. It is necessary to maintain a vaccination rate of 90 % or higher to prevent varicella epidemics. To achieve this goal, implementation of a routine vaccination program for varicella and introduction of a two-dose vaccination schedule, which is more effective than a single-dose schedule, would be highly desirable.