Vaccination against H1N1 influenza with Pandemrix^® during pregnancy and delivery outcome: a Swedish register study

Abstract

Objective To describe a large study on pregnancy outcome after vaccination against H1N1 during the 2009/10 pandemic.Design A cohort study of women vaccinated with Pandemrix Ò during pregnancy.Setting The Swedish Medical Birth Register was used for the analysis. Information on vaccination and pregnancy week when vaccination was made was obtained from antenatal care documents.Population All women who gave birth during 2009 and 2010 in Sweden.Methods Characteristics of the vaccinated women and their delivery outcome were compared with two groups of women: women without a known vaccination who gave birth in 2009/10 after 1 October 2009, and women who gave birth during 2009 before 1 October. Adjustment was made for year of delivery, maternal age, parity, smoking habits and body mass index.Outcome measures Stillbirth, congenital malformations, preterm birth, low birthweight, small for gestational age.Results A total of 18 612 vaccinated women having 18 844 infants were studied. The risk for stillbirth, preterm birth and low birthweight was lower than in the comparison groups whereas the risk for small for gestational age and a congenital malformation (after vaccination during the first trimester) did not differ from the comparison groups. No clear-cut explanation to the 'protective' effect of vaccination was found.Conclusions Vaccination during pregnancy with Pandemrix Ò appeared to have no ill effects on the pregnancy. On the contrary, the rate of preterm birth and low birthweight was lower than expected, which agrees with some previous results.

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

“…Influenza vaccination is not associated with adverse pregnancy outcomes including fetal death, preterm birth and congenital anomalies. (12)(13)(14)(15)(16)(17) Vaccination may have reduced the risk of influenza-related fetal death during the pandemic. (12) It is essential that extensive data are available on pregnancy outcomes after gestational vaccine exposure in order to inform decisions on vaccination by pregnant women and health professionals, particularly during a pandemic situation.…”

Risks associated with viral infections during pregnancy

“…Data collected from the 2009 influenza H1N1 pandemic revealed that women with H1N1 were more likely to have adverse pregnancy outcomes, such as spontaneous miscarriage and preterm birth (62)(63)(64). The rate of PTB correlated with maternal disease severity (65), and, moreover, vaccination against H1N1 reduced the rates of PTB and low birth weight (66)(67)(68)(69). Maternal hepatitis B virus (HBV) infection was also associated with PTB, although the underlying mechanism was not determined (70).…”

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

“…27,29,42 Four studies in Scandinavia described the birth outcomes after vaccination with an AS03-adjuvanted influenza vaccine. 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.…”

“…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.…”

“…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.…”