H1N1 Influenza Vaccination During Pregnancy and Fetal and Neonatal Outcomes

Objectives. We evaluated the relationship between maternal H1N1 vaccination and fetal and neonatal outcomes among singleton births during the 2009–2010 H1N1 pandemic.Methods. We used a population-based perinatal database in Ontario, Canada, to examine preterm birth (PTB), small-for-gestational-age (SGA) births, 5-minute Apgar score below 7, and fetal death via multivariable regression. We compared outcomes between women who did and did not receive an H1N1 vaccination during pregnancy.Results. Of the 55 570 mothers with a singleton birth, 23 340 (42.0%) received an H1N1 vaccination during pregnancy. Vaccinated mothers were less likely to have an SGA infant based on the 10th (adjusted risk ratio RR] = 0.90; 95% confidence interval CI] = 0.85, 0.96) and 3rd (adjusted RR = 0.81; 95% CI = 0.72, 0.92) growth percentiles; PTB at less than 32 weeks’ gestation (adjusted RR = 0.73; 95% CI = 0.58, 0.91) and fetal death (adjusted RR = 0.66; 95% CI = 0.47, 0.91) were also less likely among these women.Conclusions. Our results suggest that second- or third-trimester H1N1 vaccination was associated with improved fetal and neonatal outcomes during the recent pandemic. Our findings need to be confirmed in future studies with designs that can better overcome concerns regarding biased estimates of vaccine efficacy.During the 2009–2010 H1N1 influenza pandemic, early case reports documented more severe illness among pregnant women than among the general population, as well as higher rates of hospitalization and intensive care unit admissions.1,2 Later reports confirmed the disproportionately severe clinical course among pregnant women infected with H1N1 influenza.3–6 Public health organizations7–9 and professional associations10,11 strongly encouraged pregnant women to receive an H1N1 vaccination, and recent evidence suggests that the intensive vaccination campaign resulted in higher maternal vaccination rates during the pandemic than had been documented in previous influenza seasons.12–14Recommendations for routine vaccination of all pregnant women with inactivated influenza vaccine have been in place in Canada and the United States for a number of years.15–18 Nevertheless, seasonal vaccination rates prior to the 2009–2010 pandemic year were low in the United States,19–22 ranging from 0.7% to 20% (estimates were not available for Canada). In both countries, misconceptions about the risk of complications from influenza infection during pregnancy23 and concerns about safety12,23 are commonly cited reasons for not receiving an influenza vaccination, whereas care provider recommendations have been shown to increase vaccination rates.12,14,24Despite ongoing maternal concerns about vaccine safety, no evidence of serious harmful effects following influenza vaccination during pregnancy has been reported in the available studies on this topic19–21,25–32; thus, vaccination is promoted as the best way of preventing maternal morbidity from influenza infection.31 Theoretically, maternal influenza vaccination should also benefit the fetus by averting maternal illness and associated hyperthermia and other morbidity.28,31 Recent studies have reported a lower risk of preterm birth (PTB)33 and small-for-gestational-age (SGA) infants33,34 among women receiving an influenza vaccination during their pregnancy. However, the impact of maternal influenza vaccination on fetal and neonatal outcomes has not been extensively evaluated, possibly as a result of low immunization rates and limited sample sizes that preclude assessment of rare outcomes.In Ontario, Canada, the 2009 pandemic H1N1 vaccination campaign started on October 26, 2009; high-priority groups, including pregnant women, were targeted. During the pandemic, Better Outcomes Registry & Network (BORN) Ontario collected influenza immunization information from all pregnant women who gave birth in the province. Using this large, population-based birth cohort, we examined the association between maternal H1N1 influenza vaccination and fetal and neonatal outcomes.