Outbreaks of influenza: a virus in vaccinated elderly residents

Annual influenza immunisation is the recommended practice for people such as the elderly, for whom influenza poses a substantial risk of serious illness and death. Influenza vaccines are effective in preventing, or ameliorating influenza, reducing complications and mortality (Department of Health, 2001, 2002). In long-stay residential accommodation, influenza immunisation should prevent rapid spread of infection causing outbreaks (Communicable Disease Centre, 2003). This paper reports on two outbreaks of influenza that occurred in vaccinated elderly in residential institutions. In both outbreaks the isolated virus strain was included in the composition of the influenza vaccine for that winter season. Suboptimal protection against influenza may be provided in the vaccinated elderly, due to declining competency of the ageing immune system. Furthermore, the occurrence in the late winter of these outbreaks may demonstrate waning immunity following immunisation early in the season.