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.