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Birth outcomes not affected by flu vaccine in any trimester

Birth outcomes, specifically birth weight and weeks' gestation of the infant at birth, are not affected by the influenza vaccine during any trimester of pregnancy, according to an Australian study.

According to an Infectious Disease Advisor report, Dr Lisa McHugh from the National Centre for Epidemiology and Population Health at the Australian National University in Canberra, and colleagues conducted a nested retrospective cohort study of "FluMum" participants to investigate the effects of influenza vaccination during any trimester of pregnancy with respect to 2 critical birth outcomes: infant birth weight and weeks' gestation at birth of the infant.

Participants were recruited between 1 April 2012, and 31 December 2014, from 6 cities in Australia: Sydney, Melbourne, Perth, Darwin, Adelaide and Brisbane. There was an even distribution between the 6 participating Australian sites.

In accordance with World Health Organisation guidelines, the study classified infants born before 37 completed weeks' gestation as preterm, 35 weeks as very preterm, and 30 weeks as extremely preterm. Infants with a birth weight less than 2500 g were classified as low birth weight, less than 1,500 g as very low birth weight, and less than 1000 g as extremely low birth weight.

Of the 7,126 women who gave birth to a live singleton infant, 34% received the influenza vaccine during pregnancy. McHugh and colleagues found no statistically significant differences between infants born to vaccinated vs unvaccinated women. Infants born to vaccinated women had a mean gestation of 38.7 weeks vs 38.8 weeks in the unvaccinated group (P =.051). Mean birth weight of infants born to vaccinated women was 3337 g vs 3352 g in the unvaccinated group (P =.29).

Most of the infants (83%) were born during the Australian influenza season, and no relationship between season of birth and either low birth weights (relative risk [RR], 1.04; 95% CI 0.87-1.24; P =.67) or preterm births (RR, 0.94; 95% CI, 0.84-1.06, P =.34) were observed.

Of the 1,705 women with an identified influenza vaccination date, 14% received the vaccine in the first trimester, 51% received the vaccine in the second trimester, and 35% received the vaccine in the third trimester.

Compared with the unvaccinated group, there were statistically significant differences in mean gestational ages of the infant at birth for each trimester of pregnancy a vaccine was given: vaccinated in first trimester: 38.4 weeks (95% CI, 38.1-38.7; P =.003); vaccinated in second trimester: 38.6 weeks (95% CI, 38.4-38.7; P =.005); and vaccinated in third trimester: 38.9 weeks (95% CI, 38.8-39.1; P =.016)

Compared with the unvaccinated group, there were no statistically significant differences in mean birth weights of infants by trimester of pregnancy: vaccinated in first trimester: 3293 g (95% CI, 3215-3372; P =.11); vaccinated in second trimester: 3345 g (95% CI, 3307-3384; P =.74); and vaccinated in third trimester: 3383 g (95% CI, 3342-3425; P =.20)

"Physicians can reassure women who will be pregnant during the flu season that there is no increased risk of having a preterm baby or low birth weight baby if they get an influenza vaccine during any stage of their pregnancy," said McHugh in the report. "This is especially important for pregnant women who have comorbidities or other risk factors which may affect their immunity and capacity to cope with an infectious disease in pregnancy such as influenza," she explained.

"There are still a wide range of birth outcomes that need to be analysed, particularly now with the introduction of pertussis-containing vaccines recommended in pregnancy, and these are planned as part of our 'next steps' with ongoing research," concluded McHugh. "We will be analysing our data with respect to congenital anomalies, small for gestational age, and preterm and low birth weight babies who have had both pertussis and influenza vaccines."

Abstract
Introduction: In Australia, influenza vaccination is recommended for all women who will be pregnant during the influenza season. Vaccine safety and effectiveness are key concerns and influencers of uptake for both vaccine providers and families. We assessed the safety of receiving an influenza vaccination during any trimester of pregnancy with respect to preterm births and infant birthweight.
Methods: We conducted a nested retrospective cohort study of ‘FluMum’ participants (2012–2014). Our primary exposure of interest was influenza vaccination during pregnancy. The primary outcomes of interest were infant birthweight and weeks’ gestation at birth for live singleton infants. Analyses included comparisons of these birth outcomes by vaccination status and trimester of pregnancy an influenza vaccine was given. We calculated means, proportions, and relative risks and performed multivariable logistic regression for potential confounding factors.
Results: In the 7126 mother-infant pairs enrolled in this study, mean maternal age at infant birth was 31.7 years. Influenza vaccine uptake in pregnancy was 34%. Most mothers with a known date of vaccination received a vaccine in the second trimester (51%). Those mothers with a co-morbidity or risk factor were 13% more likely to have influenza vaccine during pregnancy compared to other mothers (RR 1.13, 95% CI 1.04–1.24, p = 0.007). Mean weeks’ gestation at birth was 38.7 for the vaccinated and 38.8 for the unvaccinated group (p = 0.051). Infants in the vaccinated group weighed 15 g less in birthweight compared to the unvaccinated infants (95% CI −12.8 to 42.2, p = 0.29).
Conclusion: Results arising from this large Australian cohort study are reassuring with respect to two critical safety outcomes; preterm births and low infant birthweights. Studies examining a broader range of birth outcomes following influenza vaccination during pregnancy are required, particularly now that maternal vaccination in pregnancy has expanded to include pertussis as well as influenza.

Authors
Lisa McHugh, Ross M Andrews, Stephen B Lambert, Kerri A Viney, Nicholas Wood, Kirsten P Perrett, Helen S Marshall, Peter Richmond, Kerry-Ann F O'Grady

[link url="http://www.infectiousdiseaseadvisor.com/influenza/influenza-vaccine-safe-during-pregnancy/article/646008/"]Infectious Disease Advisor report[/link]
[link url="http://www.sciencedirect.com/science/article/pii/S0264410X1730155X"]Vaccine abstract[/link]

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