Institutional risk factors for influenza outbreaks in Hong Kong elderly homes : a retrospective cohort study

Abstract

Background Physiological changes and comorbidities put elderly at risk of developing influenza and its complication. Clustering of high risk elderly at institution setting further increases the risk of influenza outbreak. Influenza and its complications among elderly pose a significant burden to healthcare expense. In Hong Kong, the government support for elderly homes by providing education and competency transfer on infection control, and epidemiological support during outbreak. However, not much is known on the effect of institutional factors on frequency and severity of influenza outbreak. Methods A cohort of 822 elderly homes was observed from January 2011 to December 2015. In the first part of the study, each elderly-home-year was treated as one observation unit. The outcome was number of influenza-like illness (ILI) outbreak in each year. In the second part of the study, each ILI outbreak was treated as one observation unit, the outcome was the severity of outbreak, defined by number of cases, attack rate and hospitalisation rate. Poisson regression models were applied for estimation of the rate ratio (RR) and 95% confidence interval (CI) for the potential risk factors. Results A total number of 571 ILI outbreaks were identified during the study period. The average outbreak rate was 15.1 per hundred home-years; non-profit nursing homes (RR = 0.59, 95% CI: 0.34-0.96) and for-profit homes (RR = 0.60, 95% CI: 0.47-0.76) and homes with staff with better gowning competency (RR = 0.72, 95% CI: 0.56-0.95) were found to be associated with reduced ILI outbreak rate. Elderly homes with higher nurse-to-resident-ratio (RR = 1.05, 95% CI: 1.01-1.08), larger capacity (RR = 1.01, 95% CI: 1.01-1.01), higher dementia prevalence (RR = 1.01, 95% CI: 1.01-1.02), better infection control officer (ICO) (RR = 3.04, 95% CI: 1.37-3.64) and other staff (RR = 1.52, 95% CI: 1.05-2.28) hand hygiene competency were at increased risk of ILI outbreak. Nursing homes (RR = 0.76, 95% CI: 0.62-0.93), homes with single room (RR = 0.74, 95% CI: 0.62-0.92), 3/4 height partition (RR = 0.72, 95% CI: 0.57-0.92), 1/2 height partition (RR = 0.44, 95% CI: 0.20-0.97) isolation, or keeping 1-meter distance between case and other residents (RR = 0.60, 95% CI: 0.39-0.91) were associated with fewer cases in each ILI outbreak; homes with better ICO hand hygiene competency (RR = 1.64, 95% CI: 1.09-2.61) had an elevated risk of large ILI outbreak. For-profit homes (RR = 1.11, 95% CI: 1.02-1.21), high nurse-to-resident ratio (RR = 17.2, 95% CI: 5.52-53.63), high staff-to-resident ratio (RR = 1.57, 95% CI: 1.20-2.07) were associated with higher attack rate. No factors were associated with low rate, and no factors were associated with hospitalisation rate during ILI outbreaks.

Conclusions In contrary to common belief, non-profit care-and-attention elderly homes are at higher risk of ILI outbreak occurrence than for-profit homes. In contrary, if an ILI outbreak occurred, higher attack rate was associated with for-profit homes, and homes with increased nursing manpower. Most types of isolation facilities were effective in preventing spread of ILI outbreak, except mobile screen. These results shall be considered for review on risk assessment of ILI outbreaks in Hong Kong elderly homes, and in the implementation of infection control programmes.