A mid-term estimate of 2018/2019 vaccine effectiveness to prevent laboratory confirmed A(H1N1)pdm09 and A(H3N2) influenza cases in Sicily (Italy).

Influenza season started in Italy during the month of October 2018, approaching the epidemic peak in January 2019. This report aim to explore the mid-term virologic surveillance data of the 2018-2019 influenza season in Sicily and to estimate the effectiveness of seasonal influenza vaccine (VE) against A(H1N1)pdm09 and A(H3N2) influenza viruses. A test-negative design was used to evaluate influenza VE. In Sicily, almost all influenza infections were sustained by influenza type A viruses, of which 62.3% were A(H3N2) and 36.3% A(H1N1)pdm09. A reduction of laboratory confirmed influenza cases in Sicilian population immunized against influenza were observed. In particular, an overall significant protective values were observed for any influenza A viruses (Adj-VE = 44.0%; 95%CI: 11.2-64.7%), especially among 15-64 years old age group (Adj-VE = 59.5%; 95%CI: 0.03-83.1) and among the elderly (Adj-VE = 73.6%; 95% CI: 29.4-90.2).