Administration of varicella vaccine is recommended for children 12 to 18 months of age and for older children who have not already had chickenpox. However, varicella outbreaks in vaccinated children have occurred. In this case-control study, Vázquez and colleagues attempted to determine the extent to which age at vaccination and time since vaccination influence the effectiveness of the varicella vaccine.

The case group consisted of children 13 months to 16 years of age who developed chickenpox. In an interview conducted on day 3 to 5 of the illness, information regarding risk factors and severity of illness was collected. Polymerase chain reaction (PCR) assay was used to confirm the diagnosis of chickenpox. For each case subject, two control patients who had not had chicken-pox were selected. Medical and vaccination records were reviewed for case and control patients. The effectiveness of the varicella vaccine was calculated using a standard equation.

Of the 530 case subjects, PCR assay was positive in 69 percent, negative in 23 percent, and indeterminate in 8 percent. The overall effectiveness of the vaccine was 87 percent. Chickenpox was significantly more severe in unvaccinated children than in vaccinated children. The protection of the vaccine against moderate to severe disease was 98 percent, and this overall effectiveness was not changed significantly if the child was vaccinated at 15 months or younger, although in the first year after administration, the vaccine was only 73 percent effective in children who were immunized before 15 months of age, compared with 99 percent efficacy in children immunized at 15 months or older. The effectiveness of the vaccine declined from 97 percent in the first year after vaccination to 86 percent in the second year and 81 percent after seven to eight years.

The authors conclude that at least through the first eight years after immunization, the varicella vaccine is effective, despite occasional breakthrough varicella disease, which is usually mild. A substantial, statistically significant decrease in effectiveness is evident in the second year after vaccination, with no further statistically significant decrease up to year 8 after vaccination. Effectiveness is substantially lower in the first year after immunization in children vaccinated before 15 months of age. Delaying vaccination until 15 months of age or adding a booster vaccination might solve this problem. The efficacy of the vaccine beyond eight years has not been assessed.