Pandemic (H1N1) 2009 Virus in 3 Wildlife Species, San Diego, California, USA

When the fi rst patients with ILI in households were unvaccinated, ILI was observed in 59 of 136 children, yielding an SAR b0 of 43.4%.Among 6 exposures caused by vaccinated fi rst patients, ILI developed in 1 person, yielding an SAR b1 of 16.7%.Although not signifi cant (p = 0.40), the reduction in infectiousness by vaccination was estimated to be 1 -SAR b1 /SAR b0 = 61.6%(95% CI -132.3% to 93.6%).The SAR 01 was 0% (i.e., 1 exposure to an unvaccinated person caused by a vaccinated fi rst patient did not result in infl uenza).Limiting the defi nition of infl uenza to confi rmed cases, all 8 exposures to vaccinated persons did not result in infl uenza, and SAR 0b and SAR 1b were 10.8% and 0%, respectively.Similarly, all 5 exposures caused by vaccinated fi rst patients did not result in confi rmed cases, and SAR b0 and SAR b1 were 10.5% and 0%, respectively.Although the CIs of the estimates included zero because of the small sample size, the expected reductions in susceptibility and infectiousness were 75.0% and 61.6%, respectively, which is consistent with fi ndings from a meta-analysis of vaccine effi cacy against seasonal infl uenza (9).Two limitations must be noted, namely, estimates based on nonrandom samples and a case defi nition that relied on symptoms of case-patients.The former point cannot be explicitly addressed by a retrospective study design, but we enforced strict inclusion criteria for analyses and limited our study to healthy children.Accounting for the latter point (e.g., serologic diagnosis to capture symptomatic and asymptomatic cases) could yield slightly higher estimates than ours, provided that vaccination reduces the probability of clinical illness if infection occurs.Thus, despite these limitations and a critical need for further studies that include estimations of effectiveness (10), our results provide insight into the effects of vaccination in reducing risks for infection and clinical attack among children exposed to pandemic (H1N1) 2009 virus in their households.