2009 A/H1N1 influenza vaccination in pregnancy: uptake and pregnancy outcomes – a historical cohort study

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

“…29,37,41,45,48 In some studies a higher prevalence of underlying risk factors was detected among vaccinated women, which is not surprising as vaccination was primarily recommended in women with comorbidities. There are also numerous confounding characteristics that may increase the risk of preterm births and poor birth outcomes in general including maternal age, smoking, alcohol or drug use, previous history of preterm birth, socio-economic status and chronic diseases.…”

“…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. 30,37,38,40,43,48 The A/H1N1 MF56-adjuvanted formulation was evaluated in Argentina, Italy and the Netherlands.…”

“…10,31,36,41 In Germany and Ireland more than one vaccine formulation was commercially available and recommended for use in the pandemic campaigns. 32,45 The emergence of A/H1N1pdm09 in 2009/10 led to concurrent availability of monovalent vaccine and the seasonal influenza vaccines; with four reports from North America evaluating the birth outcomes when both type of vaccines were administered during the same pregnancy. 37,43,46,47 In the observational studies, as uptake of influenza vaccination could have been biased, adjusting for potential confounders is imperative.…”

“…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.…”

“…43 In contrast, 4 other studies found that mothers vaccinated with A/H1N1pdm09 had lower odds of preterm births, varying from 0.63 in the USA and 0.86 in Sweden. 29,30,41,45 In the USA Richards et al restricted their observation period to the timing of pandemic influenza season and limited their analysis to births to mothers who started their third trimester of pregnancy on or after the start of viral circulation. In this way only mothers who had the opportunity for third trimester exposure to the virus were included.…”