Incidence, risk factors and impact of seasonal influenza in pregnancy: A national cohort study

<jats:title>Abstract</jats:title><jats:sec><jats:title>Background</jats:title><jats:p>Pregnant women are particularly vulnerable to severe infection from influenza resulting in poor neonatal outcomes. The majority of evidence relates to pandemic 2009 A/H1N1 influenza. The objective of this study was to describe the characteristics and outcomes of pregnant women hospitalised with seasonal influenza.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>This national, prospective, observational cohort study used the UK Obstetric Surveillance System (UKOSS) to identify all pregnant women admitted to hospital between 01/11/2016 and 31/10/2018 with laboratory confirmed influenza together with a comparison group of pregnant women. Baseline characteristics, immunisation status, maternal and perinatal outcomes were compared.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>There were 405 women admitted to hospital with laboratory confirmed influenza in pregnancy: 2.7 per 10,000 maternities. Compared to 694 comparison women, women with influenza were less likely to be professionally employed (aOR 0.59, 95%CI 0.39-0.89) or immunised in the relevant season (aOR 0·59, 0·39-0·89) and more likely to have asthma (aOR 2.42, 1.30-4.49) or have had a previous pregnancy complication (aOR 2·47, 1·33-4·61). They were more likely to be admitted to intensive care (aOR 21.3, 2.78-163.1) and to have a caesarean birth (aOR 1·42, 1·02-1.98). Their babies were more likely to be admitted to neonatal intensive care (aOR 1.86, 1·01-3·42).</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>Immunisation reduces the risk of hospitalisation with influenza in pregnancy which is associated with increased risk of morbidity for both the mother and baby. There is a continued need to increase awareness of safety and effectiveness of immunisation in pregnancy and provision within antenatal care settings, especially for high risk groups.</jats:p></jats:sec><jats:sec><jats:title>Key points</jats:title><jats:p>The incidence of hospital admission with seasonal influenza in pregnancy in the UK is 2.7 per 10,000 maternities. Lack of immunisation increases risk of hospitalisation. Influenza in pregnancy increases risk of caesarean birth and neonatal admission to intensive care.</jats:p></jats:sec>