Effectiveness over time of varicella vaccine

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Vazquez M, LaRussa PS, Gershon AA, Niccolai LM, Muehlenbein CE, Steinberg SP, et al. JAMA 2004;291:851-5

Context Varicella outbreaks in groups with high immunization rates have increased concern about the effectiveness of varicella vaccine.

Objectives To assess whether the effectiveness of varicella vaccine is affected either by time since vaccination or by age at the time of vaccination.

Design Case-control study conducted from March 1997 through June 2003.

Setting Twenty separate group practices in southern Connecticut.

Participants 339 case subjects and 2 controls per-case subject, matched for age and practice, were identified by active surveillance. Case subjects were defined as children 13 months or older with a clinical diagnosis of chickenpox and a polymerase chain reaction test positive for varicella-zoster virus DNA.

Main outcome measures The effectiveness of the vaccine, especially the effects of time since vaccination and age at the time of vaccination, adjusted for possible confounders.

Results The overall effectiveness of the vaccine was 87% (95% CI, 81-91). The effectiveness in the first year after vaccination was significantly higher than in years 2 to 8 after vaccination (97% vs 84%, P=.003). However, there was not a statistically significant difference in effectiveness between year 2 and any subsequent year. When compared with vaccination after 15 months of age, vaccine effectiveness in year 1 was lower if vaccine was administered before 15 months of age (73% vs 99%, P=.01), but there was no statistically significant difference in effectiveness between these groups in years 2 to 8. Significantly lower severity scores were reported in cases among vaccinees compared with unvaccinated participants (P < .001).

Conclusions The effectiveness of varicella vaccination decreases significantly after year 1, although most breakthrough cases are mild. If administered before 15 months of age, vaccination was less effective in year 1. There was, however, no statistically significant difference in effectiveness for subsequent years.

Comment The live attenuated varicella vaccine is an effective vaccine that has been in widespread use in the United States for nearly 10 years. Recently, however, a number of reports have described outbreaks of chickenpox in groups with high vaccination rules. Retrospective analyses of these outbreaks have suggested that age at vaccination and time since vaccination may be risk factors for breakthrough infections.^1., ^2., ^3. This ongoing case-control study of the effectiveness of varicella vaccine finds that vaccination before 15 months of age is less effective than vaccination after that age but that the difference is limited to the first year after vaccination. Overall, the vaccine is most effective in the first year compared with postvaccination years 2 through 8. There is, however, no statistically significant difference between years 2 to 8. These data have implications for two important questions. First, should the current practice of administering the vaccine after 12 months of age be changed to 15 months? As the authors note, such a change must weigh the significant, albeit temporary, improvement in effectiveness with the later vaccination age against the risks of a longer period of unvaccinated exposure.

Second, should two doses of vaccine be administered to all children (currently, two doses are recommended over the age of 13 years) to offset potential waning immunity? Previous retrospective outbreak-based studies indicated that children vaccinated 4 to 5 years before exposure were at increased risk for breakthrough infection during the outbreaks, suggesting a waning of immunity. However, the vaccine effectiveness in 2 of 3 recently reported outbreaks was similar to that reported by Vazquez for years 2 through 8. Although direct comparisons are difficult, these observations are not entirely consistent with a significant waning of immunity (vaccine effectiveness) and may be due to differences in clinical setting (outbreak vs general practice). Because these data are far from definitive, and because decreased disease prevalence is likely to diminish exposure-based boosting of immunity, continued monitoring of vaccine effectiveness will be required to determine if a second dose is indicated.

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References 

1. Outbreak of varicella among vaccinated children—Michigan 2003. MMWR Morb Mortal Wkly Rep. 2004;53:389–392
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2. Tugwell BD, Lee LE, Gillette H, Lorber EM, Hedberg K, Cieslak PR. Chickenpox outbreak in a highly vaccinated school population. Pediatrics. 2004;113:455–459
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3. Galil K, Lee B, Strine T, Carraher C, Baughman AL, Eaton M, et al. Outbreak of varicella at a day-care center despite vaccination. N Engl J Med. 2002;347:1909–1915
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