The Influence of Staff and Resident Immunization Rates on Influenza-Like Illness Outbreaks in Nursing Homes

OBJECTIVE: To evaluate the influence of immunization rates on the likelihood of influenza-like illness (ILI) clusters in nursing facilities. DESIGN: Retrospective cross-sectional study. SETTING: Nursing facilities in a single for-profit chain (N = 301). PARTICIPANTS: Nursing home residents and staff in each facility. MEASUREMENTS: Resident and staff influenza immunization rates during the 2004-2005 influenza season, indicator of ILI cluster in facility defined as 3+ ILI cases reported within 72 hours in close proximity within the facility, hospitalization and mortality rates for facilities reporting ILI clusters, indicator of confirmatory laboratory testing for ILI cases in facility. RESULTS: Staff (median = 38%) and resident (median = 85%) rates of immunization did not independently predict the likelihood of an outbreak but jointly were strong predictors. For example, facilities having greater than 55% of staff and greater than 89% of residents immunized were almost 60% less likely to have an ILI cluster (odds ratio [OR]: 0.410; 95% CI: 0.19, 0.89) compared to all others. Facilities with higher proportions of Medicaid-funded residents were less likely to have an outbreak. Each 1% increase in the proportion of residents with Medicaid was associated with a 2.5% decrease in the risk of a cluster (OR: 0.975; 95% CI: 0.956, 0.995). Bed size and staff size did not significantly influence the likelihood of an outbreak. Among facilities with outbreaks, higher vaccination rates did not predict lower rates of hospitalizations or deaths. Approximately two thirds of all ILI clusters had laboratory testing to confirm the diagnosis of influenza. Three quarters of the facilities in which outbreaks occurred and for which confirmatory tests were performed (50/67, 74.6%) had 1+ cases positively identified as influenza. CONCLUSION: Both staff and residents must have high rates of vaccination to substantially alter the rate and impact of influenza outbreaks in nursing facilities.