Effectiveness of influenza vaccine when given during an outbreak of influenza A/H3N2 in a nursing home.

An explosive outbreak of influenza A/H3N2 began in a nursing home in the Denver area early in November, 1987. Residents had not been vaccinated at that time. Vaccine was administered on November 11th. Two weeks after the vaccine was given, the attack rate differed between the vaccinated and unvaccinated individuals. There were 40 cases of influenza among the 98 elderly residents, 12 cases of pneumonia, and seven deaths. Vaccine efficacy two weeks after it was given was estimated to be 65.4% in preventing clinical illness. There were no cases of pneumonia and no deaths among people who were vaccinated more than two weeks before. The virus was identified as influenza A by complement fixation tests with convalescent sera from 16 individuals who had been ill. Many also had hemagglutination inhibition titers for A/Colorado/1/87, which were as high as titers for A/Leningrad/87. A/Colorado/1/87 closely resembled A/Sichuan/87.