The authors were interested in influenza vaccine effectiveness (IVE) against laboratory-confirmed influenza-associated hospitalisations during pregnancy. They used data from the Pregnancy Influenza Vaccine Effectiveness Network (PREVENT), covering Australia, Canada (Alberta and Ontario), Israel and the United States (California, Oregon, and Washington). They included pregnant women aged 18–50 years whose pregnancies overlapped with the local influenza seasons between 2010–2016. Among 19,450 hospitalisations with a discharge diagnosis of acute respiratory or febrile illness (ARFI), only 6% of these pregnant women had been tested for influenza viruses using rRT-PCR. Half of these women had pneumonia or influenza discharge diagnoses (54%). Influenza A or B virus infections were detected in 58% of the ARFI hospitalisations that were tested. Across sites and seasons, 13% of rRT-PCR-confirmed influenza-positive pregnant women had been vaccinated compared with 22% of influenza-negative pregnant women, giving an adjusted overall IVE of 40% (95%CI 12-59%) against influenza-associated hospitalisation during pregnancy. Therefore between 2010–2016, influenza vaccines only offered moderate protection against laboratory-confirmed influenza-associated hospitalisations during pregnancy.
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