Varicella vaccination of children in the United States: assessment after the first decade 1995-2005.

Live attenuated varicella vaccine (strain Oka) was approved for administration to healthy children in the United States in 1995. Over the past 10 years, varicella vaccine has been given to millions of children, usually at ages between 12 and 18 months. In states such as California, Michigan, and Texas, there has been a marked decline in the number of reported cases of varicella. Furthermore, there has been a 75% decrease in varicella-related hospitalizations across the United States, as well as a similar decrease in the number of deaths caused by complications of chickenpox. The main unanticipated result has been a growing number of outbreaks of varicella among immunized children ("breakthrough varicella"). The most cited risk factors for breakthrough varicella include the following: (1) 3-5-year interval since immunization and (2) immunization at the youngest ages, especially 12 months. Explanations for breakthrough varicella include a lessened immune response among the youngest recipients of the vaccine. Another possibility is genetic variation among circulating VZV strains. VZV strains can be separated into two geographic clades called European/North American and Asian, based on single nucleotide polymorphisms. Two mutant North American strains have been isolated from patients in the last 10 years. Several genomic differences between Oka vaccine strain and other strains have also been identified, including one site at the DNA origin of replication. Since breakthrough disease among vaccine recipients appears to be more common in the United States than in Japan, further comparisons between the varicella vaccination programs in Japan and the United States are warranted. In addition, data from varicella vaccination programs in Europe should provide further insight into the effectiveness of varicella vaccination in different geographic and ethnic populations.