Comparison of the characteristics of elderly influenza patients in two consecutive seasons

The seasonality of influenza infections can be affected by virus subtypes, climate, and social networking in populations. While these factors are well known, their relative influences in specific age groups have not been fully investigated. During 2010-2011, patients aged 65 years and above with influenza virus infections were recruited from a regional hospital in Hong Kong. They were either residents of homes for the elderly (n=60) or living with their family (n=75). Two seasons were distinguished, the summer season of 2010 dominated by H3N2 and the winter season of 2011 dominated by H1N1. The patients' clinical presentations and patterns of inter-personal connectivity were assessed. Overall, more elderly people living with their family were diagnosed with H1N1 compared to those in the homes for the elderly, and the former had visited a more diverse range of places 1 week prior to diagnosis. A higher proportion of patients living with family presented with lower respiratory tract symptoms, but these patients were less likely to have pre-existing chronic diseases. The results suggest that elderly patients infected during an influenza season could vary by virus subtype, which in turn is dependent on exposure locations and the pattern of social connectivity.