Long-term clinical studies of varicella vaccine at a regional hospital in Japan and proposal for a varicella vaccination program

In 1974, a live varicella vaccine (Oka strain) was developed in Japan for the prevention of varicella. 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. From before approval up to the present, we have been carrying out long-term studies in healthy children at a regional hospital to assess the immunogenicity, safety, and efficacy of the varicella vaccine. This vaccine is very safe, and serious adverse reactions have not been observed since the year 2000 when it changed gelatin-free. In the past three studies, seroconversion was detected in around 95% of subjects by the immune adherence hemagglutination (IAHA) test, and this high rate was considered to indicate good immunogenicity. Breakthrough varicella is observed in approximately 20–30% of children who receive a single dose of the vaccine, but most cases are mild. Although recent vaccination has generally been effective, the IAHA test has shown that immunogenicity is somewhat lower than was previously demonstrated. The sensitivity of the IAHA test has been shown to be adequate when compared with the neutralization test, so the current testing system is sufficient for the maintenance of immunity levels. An additional vaccination increased the IAHA antibody level in subjects who failed to seroconvert after a single dose vaccination. According to another clinical study, additional varicella vaccination at 3–5 years after the initial vaccination achieved stronger immunogenicity. Because it is administered as part of the voluntary vaccination program, the varicella vaccination coverage rate has remained low in Japan, with no sign of a decrease in the number of varicella patients. We consider that implementation of routine varicella vaccination program based on the Preventive Vaccination Law would be the most effective approach for improvement of the coverage rate. Along with this, introduction of a two-dose schedule would also be desirable. In addition to decreasing the prevalence of characteristic breakthrough varicella infection, the vaccination coverage rate would also be expected to improve with a two-dose schedule due to an increase in opportunities for vaccination.