Title: Contagiousness of varicella in vaccinated cases: A household contact study
Abstract: Seward JF, Zhang JX, Maupin TJ, Mascola L, Jumaan AO. JAMA 2004;292:704-8. Context Some data suggest that cases of varicella in vaccinated patients may be less contagious than those who are unvaccinated; however, these data are limited. Objectives To describe secondary attack rates within households according to disease history and vaccination status of the primary case and household contacts and to estimate varicella vaccine effectiveness. Design Population-based, active varicella surveillance project. Setting A community of approximately 320,000 in Los Angeles County, California, from 1997 to 2001. Participants Cases were reported by child care centers, private and public schools, and health care clinicians. Information on household contacts' age, varicella history, and vaccination status was collected. Main outcome measures Varicella secondary attack rate among household contacts; vaccine effectiveness using secondary attack rates in unvaccinated and vaccinated children and adolescents. Results A total of 6316 varicella cases were reported. Among children and adolescents aged 1 to 14 years, secondary attack rates varied according to age and by disease and vaccination status of the primary case and exposed household contacts. Among contacts aged 1 to 14 years exposed to unvaccinated cases, the secondary attack rate was 71.5% if they were unvaccinated and 15.1% if they were vaccinated (risk ratio [RR], 0.21; 95% confidence interval [CI], 0.15-0.30). Overall, vaccinated cases were half as contagious as unvaccinated cases. However, vaccinated cases with 50 lesions or more were similarly contagious as unvaccinated cases whereas those with fewer than 50 lesions were only one third as contagious (secondary attack rate, 23.4%; RR, 0.32 [95% CI, 0.19-0.53]). Vaccine effectiveness for prevention of all disease was 78.9% (95% CI, 69.7%-85.3%); moderate disease, 92% (50-500 lesions) and 100% (clinician visit); and severe disease, 100%. Conclusions Under conditions of intense exposure, varicella vaccine was highly effective in preventing moderate and severe disease and about 80% effective in preventing all disease. Breakthrough varicella cases in household settings were half as contagious as unvaccinated persons with varicella, although contagiousness varied with numbers of lesions. Comment This article indicates several important phenomena concerning varicella and varicella vaccine. It is now well known that “breakthrough” varicella sometimes occurs when a vaccinated person is exposed to the wild type virus. How frequently this occurs is still being evaluated. In this 5-year population-based, active-surveillance study, vaccination provided 80% complete protection against the disease after household exposure and prevented severe breakthrough varicella almost entirely. Household exposures to varicella provide the strongest test for the vaccine because reported attack rates range from 61% to 100% among susceptible individuals following this intense form of challenge. Reassuringly, transmission of varicella from children with breakthrough disease was only about one third that expected, compared to transmission by unvaccinated children. These data are in keeping with other published information and provide important confirmation about the efficacy of routine immunization against chickenpox. Interestingly, even among individuals with past natural infection, there were some cases of varicella and therefore one must question therefore whether protection approaching 100% by vaccination can ever be achieved against chickenpox. On the other hand, given the numerous outbreaks of varicella in schools and day care centers reported since 1995, when the vaccine was licensed in the US, it seems worthwhile to explore whether a second dose of varicella vaccine, given routinely to all children, would improve protection and possibly result in more lasting immunity due to an anamnestic response. The CDC is currently considering whether or not to recommend a second routine dose for children aged 1 to 12 years.