Laboratory and epidemiologic assessment of a recent influenza B outbreak.

A viral surveillance system in Nashville detected an outbreak of influenza B that occurred between January and March 1986. Paired sera from 32 individuals with culture-documented influenza B illness were tested using three serologic assays. Enzyme-linked immunosorbent assay (ELISA) using purified hemagglutinin-neuraminidase and plaque neutralization detected a seroresponse in 69% and 66% of these individuals, respectively. These assays were superior to hemagglutination inhibition, which detected a 41% seroresponse. ELISA was perferred because of cost and ease of performance. A group of 286 individuals, aged 1-65 years, was studied more extensively including serologic assessment before and after the influenza B outbreak. Historical information and viral throat cultures were obtained from those with influenza-like illness during the epidemic. An influenza B infection rate (seroresponse and/or positive culture) of 31% and illness rate (infection with flu-like symptoms during the epidemic period) of 13% was demonstrated using these methods. Pre-epidemic mean serum ELISA IgG titers were lower in those with, versus those without, evidence of subsequent influenza B illness (1,541 vs. 4,311, P = .0026). Children less than or equal to 15 years of age were infected more frequently than adults (44% vs. 28%, P = .04). Fever greater than or equal to 101 degrees F was reported more frequently with influenza B than non-B illness (43% vs. 18%, P = .03). These data are useful in preparing for future epidemiologic studies of influenza B and demonstrate the value of and need for standardization of ELISA as a serologic assay for influenza B.