Effectiveness of the trivalent MF59 adjuvated influenza vaccine in preventing hospitalization dueto influenza B and A(H1N1)pdm09 viruses in the elderly in Italy, 2017 – 2018 season

Background: Evidence on influenza vaccine effectiveness (VE) in preventing mortality and morbidity inthe elderly is weak. Our aim was to measure the VE against severe outcomes in the elderly.Methods: We conducted a multicentre hospital-based test-negative design (TND) case-control study,during the 2017/18 season, in four Italian hospitals. The study population included individuals aged≥65 years hospitalized with Severe Acute Respiratory Infections (SARI). Patients were classified as casesand controls based on the results of the PCR influenza testing. We estimated VE by virus subtypes andspecific VE for the trivalent adjuvanted vaccine (TIVadj).Results: 502 patients with SARI were enrolled: 118 (23.5%) tested positive (cases) and 384 (76.5%)tested negative (controls) for influenza. The adjusted VE of 48.5% for all vaccines was comparable to theadjusted VE for the TIVadj vaccine (48.3%). Adjusted VE for the TIVadj vaccine was 67.5% for A(H1N1)pdm09 and 44.5% for B viruses.Conclusion: We show a moderate adjusted VE of the TIVadj against all viruses, a good adjusted VEagainst A(H1N1)pdm09 strains and a moderate adjusted VE against B strains, despite a mismatchbetween the B circulating lineage and the lineage included in the vaccine. This is likely due to the cross-protection among B strains induced by the TIVadj in elderly patients.