Breakthrough disease rates similar in children who received one or two doses of varicella vaccine

Issue: October 2009

Data collected during a varicella outbreak at an Arkansas elementary school were the first to document the disease in both one- and two-dose vaccine recipients since the Advisory Committee on Immunization Practices expanded varicella vaccine recommendations in June 2006 to include a second, school-entry dose.

“As of the 2008-2009 school season, 12 states require a second dose of varicella vaccine, and nine additional states are actively considering adding the requirement in the near future,” the researchers wrote.

Officials from the Arkansas Department of Health and the CDC investigated 84 cases of varicella that occurred between Sept. 1, 2006 and Dec. 18, 2006 and found similar vaccine effectiveness rates among children who received one dose and those who received two, with both groups experiencing mild courses of the disease.

Differences in varicella disease rates between the two groups were not significant at 10.4% among the 25 children who received two doses of vaccine and 14.6% among the 53 one-dose recipients. Most children experienced 50 or fewer skin lesions, including all of the two-dose recipients and 80% of one-dose recipients.

Vaccination coverage among the 871 students who had vaccination records available was high at 97%, and 39% had received a second dose during a vaccination campaign launched in February 2006 in response to an earlier varicella outbreak.

Using unvaccinated rates from a postlicensure trial involving a varicella outbreak at a child care center, the researchers determined an 85.4% vaccine effectiveness rate for one dose of vaccine and an 89.1% effectiveness rate for two doses among the Arkansas children, both of which were significantly lower than the 94.4% and 98.3% effectiveness rates previously published in a 10-year follow-up study.

“It is possible that higher two-dose coverage at the school would have prevented the second outbreak from occurring or that high coverage with two doses of varicella vaccine will reduce the number of outbreaks in the population,” the researchers wrote.

They called for additional studies to determine the effect of routine two-dose vaccination, including studies to monitor the number and size of varicella outbreaks in the United States. – by Nicole Blazek

Gould PL. Pediatr Infect Dis J. 2009;28:678-681.

This study raises questions about the additional benefits derived from providing a second dose of varicella vaccine. This observational study documented a clinical varicella attack rate of 10.4% in two-dose recipients vs. 14.6% in one-dose recipients — a relative reduction of 38% due to the second dose — although, because of small numbers this difference is not statistically significant. However, the study should not be over interpreted.

The numbers of study participants were small and the 95% confidence intervals are very wide. Second, regarding any single study, there is the potential for outliers in the results and repeat studies would be important to see if these findings can be confirmed. Third, it would be important to determine whether there were risk factors for vaccine failure such as time since the second dose, age at second dose, interval between doses etc.

Regardless, all of the cases in the study among two dose recipients were mild. Should we abandon the two dose recommendation? This would be premature. This study did show benefit of second dose, although not as much as suggested in prelicensure studies.

– Walter A. Orenstein, MD

Infectious Diseases in Children Editorial Board

I think the most important point is that these results are from one study and that we wouldn’t expect one study to tell the whole story. Our one-dose estimates ranged from 42% to 100% with a median vaccine effectiveness of 85%. It took many studies to get a good understanding of one-dose vaccine effectiveness and this is likely to be the same for two doses. I’m aware of a second study that shows a much greater benefit from the second dose. Also interestingly, we’re seeing the lowest number of reported cases of varicella ever. So it looks to me, preliminarily, as though we might be starting to see a benefit from the two-dose policy.

– Jane Seward, MPH

Centers for Disease Control and Prevention