Multiple Influenza Virus Infections in 4 Consecutive Epidemiological Seasons: A Retrospective Study in Children and Adolescents

BACKGROUND: Recent observations provide evidence for group-specific immunity toward influenza A infections and raise the question of how often we can get the flu. METHODS: We retrospectively analyzed 2308 cases of children and adolescents with clinically manifested influenza and a positive PCR-test during the last 4 epidemiological seasons (2014–15 through 2017–18). RESULTS: In the 2015–16 epidemiological season, almost 12% of patients had experienced an influenza infection during the previous season; in the 2016–17 season, more than 14% had at least 1 infection during the previous 2 seasons, and in 2017–18 season, over 18% had 1 or more infections during the previous 3 seasons. The majority of these repetitive infections occurred in children between 3–8 years of age. 29 patients experienced 3 or 4 infections during these seasons, whereas 38 children had 2 influenza episodes within the same season. Epidemiological pattern of circulating viral strains changed yearly; however, we identified 5 patients with confirmed influenza B infections during the 2014–15 and 2017–18 seasons, when only subtype Yamagata was circulating in Austria. CONCLUSIONS: Repetitive influenza infections in consecutive epidemiological seasons occurred quite frequently in children and adolescents. Observations like ours contribute to a better understanding of the immunity against influenza virus infections and could have implications for future vaccination strategies.