Impact of repeated influenza immunization on respiratory illness in children with preexisting medical conditions

PURPOSE Annual influenza immunization in medical risk groups is recommended in many countries. Recent evidence suggests that repeated inactivated influenza vaccine (IIV) immunization throughout childhood may impair long-term immunity against influenza. We assessed whether prior immunization altered the effect of IIV in children with preexisting medical conditions on primary care–diagnosed respiratory illness (RI) episodes during the influenza season. METHODS Electronic records of IIV-immunized children who met the criteria for annual IIV immunization according to Dutch guidelines were extracted from a primary care database from 2004 to 2015. For each year, we collected information on IIV immunization status, primary care–attended RI episodes (including influenza-like illness, acute RI, and asthma exacerbation), and potential con-founders. Generalized estimating equations were used to model the association between prior IIV and occurrence of at least one RI episode during the influenza season, with “current year immunized but without IIV history” as reference group. RESULTS A total of 4,183 children (follow-up duration: 11,493 child-years) were IIV immunized at least once. Adjusted estimates showed lower odds for RI in current year–immunized children with prior IIV compared with those without (odds ratio [OR] = 0.61; 95% CI, 0.47-0.78 for “current year immunized and one IIV in previous 2 years”; OR = 0.85; 95% CI, 0.68-1.07 for “current year immunized and ≥2 IIVs in previous 3 years, including prior year”). CONCLUSION Repeated IIV immunization in children with preexisting medical conditions has no negative impact on, and may even increase, long-term protection against RI episodes diagnosed during the influenza season in primary care.