The Effectiveness of the Influenza Vaccine in Children

Young children bear a disproportionate burden of influenza-related disease, and since 2008 the Advisory Committee on Immunization Practices (ACIP) has recommended influenza vaccination for all children over the age of 6 months. Nevertheless, limited evidence exists on the effectiveness of the influenza vaccine in reducing illness for children under age 5, and many children go unvaccinated. To determine the effectiveness of the influenza vaccine in reducing outpatient visits and associated expenditures for children between 1 and 5 years of age for the 2008-2015 influenza seasons, we develop a cohort-based study using fluctuations in vaccination rates generated by variation in seasonal influenza vaccine availability and week of birth. Distinguishing between preventive (well-child) visits and other visits, we constructed exact age profiles of doctors’ visits for children covered by a large HMO in Southern California. Recognizing that most childhood vaccinations occur at preventive visits, we examined how variation in seasonal influenza vaccine availability and week of birth interact to affect influenza vaccination rates. The idiosyncratic nature of these factors created approximately random variation that provided a “natural experiment” for estimating the effectiveness of the influenza vaccine. After nonparametrically controlling for age using a cubic spline, the relative timing of influenza vaccine availability and preventive visits for children aged 1 to 5 was highly predictive of influenza vaccination status. There were concurrent reductions in doctors’ visits for influenza of 67.6% (95% CI, 48.9% to 86.3%) and of influenza or influenza like illness of 58.3% (95% CI, 33.1% to 83.5%). The results show that the influenza vaccine is highly effective in reducing illness for children ages one to five. Vaccine delivery to this group should be improved given their risk of influenza complications and role in transmitting the disease.