End-of-season outbreaks of nosocomial influenza caused by waning vaccine immunity.

The waning of vaccine protection may be responsible for outbreaks toward the end of the influenza season. Three of five outbreaks occurred at the beginning of April following an interval of >100 days from the date of vaccination; the reported index case was a nurse or office worker, and >50% of those affected were healthcare workers. The results are consistent with intra-seasonal waning of vaccine immunity that resulted in outbreaks at the end of season.