Pregnant women are a World Health Organization (WHO) priority group for influenza vaccination, but evidence of effectiveness and safety for pregnant women comes from observational studies, which are notoriously prone to confounding by indication and healthy-vaccinee bias. The latter type of bias leads to an overestimation of the effectiveness and safety of the vaccine, which may be what occurs in pregnant women. Indeed, better educated women with healthier behaviors and who seek better medical care may be more adherent to vaccinations recommended by doctors, scientific societies and health authorities. Therefore, it is fundamental to obtain information about vaccine effectiveness and safety from randomized controlled trials (RCTs). Cochrane reviews have identified only one RCT with “low risk of bias”. Its results were unclear in terms of maternal, perinatal, and infant deaths and hospitalization, and showed a Number Needed to Vaccine (NNV) of 55 for mothers, with an excess of local adverse effects. A Cochrane review concluded that the inactivated influenza vaccine provides pregnant women with uncertain or very limited protection against influenza-like illnesses and influenza. Some observational studies have suggested possible adverse effects of the inflammation following the vaccination. Consistent with the Cochrane reviewers’ conclusions, further trials for influenza vaccines with appropriate study designs and comparison groups are required before promoting universal seasonal influenza vaccinations of pregnant women. Meanwhile, vaccination in second to third trimester should be offered while communicating the uncertainties that still exist, promoting informed choices. Vaccination in the first trimester is debatable and debated. This does not mean leaving women defenseless; many other useful behavioral and environmental measures can reduce infectious disease.
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