Seasonal influenza vaccination status among children with laboratory evidence of pandemic H1N1 infection.

BACKGROUND The 2009 pandemic H1N1 influenza virus emerged in March 2009 and spread rapidly, causing many thousands of deaths worldwide. A case-control study of 60 Mexican adults with H1N1 suggested that the seasonal influenza vaccine protected against H1N1 infection (odds ratio [OR], 0.27; 95% confidence interval [CI], 0.11-0.66), but subsequent studies have had varied results and few have addressed this question in children. The objective of this study was to evaluate the effect of 2008-2009 seasonal influenza vaccination on pandemic H1N1 infection in children. METHODS Cases (n = 165) were Kaiser Permanente Colorado inpatients and outpatients aged between 18 months and 18 years, with laboratory-confirmed pandemic H1N1 infection from May to November 2009. Controls (n = 660) were pediatric Kaiser Permanente members without documented H1N1 infection who were matched by age and gender. Seasonal influenza vaccination status was recorded for all cases and controls; conditional logistic regression analyses were used to calculate matched odds ratios. RESULTS Cases were more likely than controls to have underlying chronic health conditions (45% vs. 21%, P < 0.0001). Pandemic H1N1 cases were neither more nor less likely to have received the 2008-2009 seasonal influenza vaccine (OR, 1.31; 95% CI, 0.92-1.88). After adjustment for chronic medical conditions and health-seeking behavior, H1N1 cases were as likely as controls to have received the 2008-2009 seasonal influenza vaccine (OR, 1.08; 95% CI, 0.75-1.57). CONCLUSIONS There was no overall association--either protection or risk--between seasonal influenza vaccination and medically attended pandemic H1N1 infection in children. These results have important implications for understanding influenza immunity and future public health efforts to prevent pandemic influenza.