However, a few observational studies have failed to demonstrate an association between influenza infection and birth outcomes [26, 27] . The lack of observed effect in some studies could be due to a true lack of association, small or difficult to measure effect size, challenges related to the study population (e.g., administrative datasets), or non-differential misclassification due to challenges in retrospectively identifying influenza infection. Although less than ideal, modeling receipt of influenza vaccine as the exposure/ independent variable reduces the likelihood and the intensity of non-differential misclassification bias.