Background
Hospital-acquired influenza virus infection (HAII) can cause severe morbidity and mortality. Identifying potential transmission routes can inform prevention strategies.
Methods
We identified all hospitalized patients testing positive for influenza A virus at a large, tertiary care hospital during the 2017-2018 and 2019-2020 influenza seasons. Hospital admission dates, locations of inpatient service, and clinical influenza testing information were retrieved from the electronic medical record. Time-location groups of epidemiologically linked influenza patients were defined and contained ≥1presumed HAII case (1st positive ≥48 hours after admission). Genetic relatedness within time-location groups was assessed by whole genome sequencing.
Results
During the 2017-2018 season, 230 patients tested positive for influenza A (H3N2) or unsubtyped influenza A including 26 HAII. There were 159 influenza A (H1N1)pdm09 or unsubtyped influenza A positive patients identified during the 2019-2020 season including 33 HAII. Consensus sequences were obtained for 177 (77%), and 57 (36%) of influenza A cases in 2017-2018 and 2019-2020, respectively. Among all influenza A cases, there were 10 time-location groups identified in 2017-2018, and 13 in 2019-2020; 19 of 23 groups included 4 or fewer patients. In 2017-2018, 6 of 10 groups had ≥2 patients with sequence data, including at ≥1 HAII case. Two of 13 groups met this criteria in 2019-2020. Two time-location groups from 2017-2018 each contained 3 genetically linked cases.
Conclusions
Our results suggest that HAII arise from both outbreak transmission from nosocomial sources as well as single infections from unique community introductions.