Hospitalizations and mortality associated with norovirus outbreaks in nursing homes, 2009-2010

Norovirus outbreaks are common among vulnerable, elderly populations in US nursing homes.

To assess whether all-cause hospitalization and mortality rates are increased during norovirus outbreak vs nonoutbreak periods in nursing homes, and to identify factors associated with increased risk.

A retrospective cohort study of Medicare-certified nursing homes in Oregon, Wisconsin, and Pennsylvania that reported at least 1 confirmed or suspected norovirus outbreak to the Centers for Disease Control and Prevention's National Outbreak Reporting System (NORS), January 2009 to December 2010. Deaths and hospitalizations occurring among residents of these nursing homes were identified through the Medicare Minimum Data Set (MDS).

Rates of all-cause hospitalization and mortality during outbreak compared with nonoutbreak periods were estimated using a random-effects Poisson regression model controlling for background seasonality in both outcomes.

The cohort consisted of 308 nursing homes that reported 407 norovirus outbreaks to NORS. Per MDS, 67 730 hospitalizations and 26 055 deaths occurred in these homes during the 2-year study. Hospitalization rates were 124.0 (95% CI, 119.4-129.1) vs 109.5 (95% CI, 108.6-110.3) hospitalizations per nursing home−year during outbreak vs nonoutbreak periods, yielding a seasonally adjusted rate ratio (RR) of 1.09 (95% CI, 1.05-1.14). Similarly, mortality rates were 53.7 (95% CI, 50.6-57.0) vs 41.9 (95% CI, 41.4-42.4) deaths per nursing home−year in outbreak vs nonoutbreak periods (seasonally adjusted RR, 1.11; 95% CI, 1.05-1.18). The increases in hospitalizations and mortality were concentrated in the first 2 weeks (week 0 and 1) and the initial week (week 0) of the outbreak, respectively. Homes with lower daily registered nurse (RN) hours per resident (<0.75) had increased mortality rates during norovirus outbreaks compared with baseline (RR, 1.26; 95% CI, 1.14-1.40), while no increased risk (RR, 1.03; 95% CI, 0.96-1.12) was observed in homes with higher daily RN hours per resident (P = .007 by likelihood ratio test); the increase in hospitalization rates did not show a similar pattern.

Norovirus outbreaks were associated with significant concurrent increases in all-cause hospitalization and mortality in nursing homes.