There are plausible mechanisms related to mechanical and immunological changes that may render women more vulnerable to respiratory infections during pregnancy [4] and [5]. The European Centre for Disease Prevention and Control (ECDC) has concluded that vaccination of pregnant women could reduce the number of influenza-related hospitalizations and deaths in this group and potentially the burden of influenza in children younger than six months [6]. The WHO SAGE committee has referred to “compelling evidence of substantial risk of severe disease in

this group…” [7], and WHO has subsequently recommended pregnant women as the highest priority group for vaccination against seasonal influenza. However, a recent systematic review [8] concluded that pregnancy as a risk factor for seasonal influenza, as opposed to pandemic influenza including A(H1N1)pdm09, is not sufficiently studied. Furthermore, S3I 201 ECDC has concluded that European studies of the disease

burden of seasonal influenza in pregnant women are needed [6]. Whereas an increased risk of influenza-associated Metabolism inhibitor deaths for pregnant women has been documented during pandemics [9], [10], [11], [12] and [13], deaths in pregnant women due to inter-pandemic influenza have only been described in occasional case reports [14], [15] and [16], suggesting that this outcome is unusual. Moreover, the evidence of an increased risk of severe disease for healthy pregnant women due to seasonal, inter-pandemic influenza mainly consists of observational studies of health Oxalosuccinic acid service utilization in USA and Canada [17] and [18]. Albeit healthcare utilization often being applied as an indicator of disease severity, it should be interpreted

with caution since healthcare utilization may be context dependent. For example, despite similar symptoms and severity, there may be differences in healthcare seeking behaviour, access to healthcare or medical recommendations. Furthermore, the relative risk does not inform on burden of hospitalization, and a sufficient absolute risk is needed to motivate vaccination. Hospitalization rates of 15 and 25 per 10,000 pregnant women or third trimester women have been found in Canada and USA, respectively [17] and [18], and in a study set in the UK the rate was estimated to 13 per 10,000 pregnant women [19]. Since these rates may be context dependent and estimates in a European setting are sparse, it was deemed that a national estimate for Sweden was necessary for policy purposes. Therefore we conducted a study of hospitalizations due to seasonal, inter-pandemic influenza or respiratory infection attributable to inter-pandemic influenza among pregnant women in Sweden and assessed the number needed to vaccinate (NNV) to prevent one such hospitalization. We conducted a retrospective, register-based study of inter-pandemic seasons, using ICD-10 codes that indicate influenza hospitalizations.