Rates and determinants of seasonal influenza vaccination in pregnancy and association with neonatal outcomes

Review on the effects of influenza vaccination during pregnancy on preterm births

“…The association of seasonal influenza vaccine and prematurity has been evaluated in the USA and in Canada in 6 observational studies that used data from years prior the A/H1N1pdm2009 28,33-35,44,49 and 3 studies that were conducted following the 2009 pandemic season. 37,39,46 Data on fetal safety of A/H1N1pdm09 monovalent vaccines administered to pregnant women during the 2009/10 pandemic are available from the USA, 30,37 Canada, 43 Argentina, 29 Taiwan 38 and from several European countries. 10,27,31,32,36,40,41,45,48 Unadjuvanted vaccines were used in the North American studies, in France and a study in Taiwan.…”

“…To adjust for group differences and reduce confounding bias in observational studies 2 main approaches were used: propensity scores 31,34,37 and multiple logistic regression analysis. 10,21,27,29,30,33,35,36,39,[41][42][43][44][45][46][47][48][49] In propensity score analyses probability of vaccination is estimated as a function of available variables and then that score is used as a single matching covariate. The propensity score matching allows comparing the risks after vaccination between cohorts that had the same likelihood of being vaccinated.…”

“…42 In this study a significant reduction in prematurity was seen in the vaccinated cohort in an analysis using proportional hazard models with gestational age as the time factor, whereas, when aOR was estimated using logistic regression models a similar however non-significant association between vaccination and preterm births was detected. 42 Preterm birth analyses after vaccination with seasonal IIV formulations containing pandemic strain Four studies in North America during and following the A/ H1N1 2009 pandemic assessed whether preterm birth rates differed between mothers who received seasonal influenza vaccine containing the pandemic strain and non-vaccinated women 37,39,46,47 (Table 2).…”

“…Using logistic regression analysis adjusted for potential confounding variables they found a 25% reduction on preterm births among infants whose mothers received the seasonal influenza vaccine during pregnancy. 39 …”

“…In some studies the results were essentially unchanged after adjusting for various maternal characteristics associated with vaccine uptake, which suggests that these variables were not actual confounders of the association between vaccine receipt and birth outcomes in those studies. 29,30,39,45 In most studies data on whether women were immunized were obtained from medical record based registries and women vaccinated at alternative sites might have been misclassified in these cohorts, also the partial completeness of these records was stressed in some studies. 34,36,41,48 Other studies relied on maternal self-reported vaccination status with 37 or without further confirmation from providers records.…”