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Objectives The aim of the study was to identify care home characteristics associated with reported care home influenza outbreaks and factors associated with increased transmission of influenza-like illness (ILI) in care homes in Cheshire and Merseyside during the 2017–2018 influenza season. Study design This is a matched case-control study comparing characteristics between care homes with and without a declared influenza outbreak and a retrospective risk factor analysis of care home residents with ILI. Methods Routinely collected outbreak surveillance data on symptomatic residents and staff, antiviral prophylaxis and influenza vaccination history, which were reported to Public Health England, were extracted from health protection electronic records. Further care home characteristics were extracted from the Care Quality Commission (CQC) website. Care homes with declared influenza outbreaks were matched with care homes without outbreaks. Chi-squared tests and logistic regression were used to examine associations between care home factors and ILI. Results There were no significant differences in characteristics between 77 care homes with declared influenza outbreaks and 77 matched care homes without outbreaks. Of 2,744 residents from the homes with a declared outbreak, 644 (24%) developed an ILI. The care home risk factors were having a low CQC score and activation of antiviral prophylaxis and the protective factors were having higher numbers of residents, specializing in dementia care and having the highest CQC score. Significantly more cases occurred in residential homes than in nursing homes, in homes with lower CQC scores and in homes where eligible residents were given antiviral prophylaxis. Conclusions In homes with declared outbreaks, certain characteristics including activation of antiviral prophylaxis were associated with an increased risk of ILI. Further research is needed, particularly focussing on temporality between provision of prophylactic antivirals and the onset of ILI.

To assess the association of antiviral prophylaxis and care home characteristics with the spread and severity of influenza-like illness in care homes with influenza outbreaks in North West England in the 2017/2018 influenza season.This is a retrospective observational study.Routinely collected outbreak surveillance data reported to Public Health England were extracted from health protection electronic records. Data included use of antiviral prophylaxis, influenza-like illness or confirmed influenza, hospital admissions and deaths. Care home characteristics were obtained from the Care Quality Commission website. Single variable analysis and multivariable logistic regression were used to examine associations between care home characteristics, antiviral prophylaxis and influenza-related outcomes.In the 109 homes, there were 3498 residents; of whom, 855 (24%) developed an influenza-like illness. Antiviral prophylaxis was given to residents of 67 of the 109 care homes with outbreaks (61%). A significantly higher attack rate was observed among residents of homes given antiviral prophylaxis (27%) than among residents of homes not given antivirals (20%) (P 0.001). Significantly more deaths occurred in homes for people with learning disabilities and homes that received antiviral prophylaxis (P 0.001).In homes given antiviral prophylaxis, there were a higher number of residents with influenza-like illness and deaths. To improve our understanding of the impact of antiviral prophylaxis use in real life, enhanced and timely data collection is needed for identification of temporal associations between exposure and administration of antiviral prophylaxis. Consideration needs also to be given to ensure people with learning disabilities are protected through the seasonal influenza vaccine and timely antiviral prophylaxis when appropriate.