Safety of MF-59 adjuvanted vaccine for pandemic influenza: results of the vaccination campaign in an Italian health district

Background: The occurrence of pandemic H1N1 influenza in 2009 led health authorities to promptly start massive vaccination campaigns. Due to the need of shortening time for development and approval, the pandemic vaccine was prepared by mock-up strategy, and limited safety data were available upon starting vaccine administration. Aims: To determine the frequencies and clinical features of adverse events (AEs) observed in a population of subjects of an Italian health district receiving pandemic vaccination. The risk for the development of at least one AE was calculated for patients receiving H1N1 vaccine only or H1N1+seasonal vaccine. Methods: This prospective observational study was performed on the population of La Spezia Health District scheduled to receive pandemic MF-59-adjuvanted H1N1 vaccine. Subjects were enrolled at the time of vaccine administration and followed up at least for 6 months. The study population was stratified in two subgroups defined by administration of H1N1 vaccine only or H1N1+seasonal vaccine. AEs were recorded during the first hour following vaccine administration (early AEs) and through periodic telephonic interviews (delayed AEs). Results: An overall number of 506 subjects receiving pandemic vaccine were enrolled (H1N1 only: 131 subjects; H1N1+seasonal: 375 subjects). 233 AEs occurred throughout the study period. Among them, 211 were resolved, while 22 were ongoing at the last available follow-up. Adjusted incidence rate ratio (IRR) for the development of at least one AE was 1.14 (0.77-1.69) for H1N1 only group vs H1N1+seasonal group. Patients with a nervous system AE were more frequent in the H1N1+seasonal group (adjusted IRR: 2.33; 95%CI: 1.04-5.23). Only two cases of ischemic stroke, both in the H1N1+seasonal vaccine group, occurred in patients with cardiovascular risk factors, were classified as serious AEs. Conclusions: In the present study, the MF59-adjuvanted H1N1 vaccine was generally well tolerated. AEs were usually transient and mild to moderate in intensity. Our findings support the validity of mock-up strategy for rapid development of a safe vaccine in the emergency of a pandemic influenza.