Serologic evidence of human infection with swine influenza virus.

Serum specimens from 1,844 persons were tested for hemagglutination-inhibition antibodies against the A/Swine/Illinois/1/63 strain of influenza virus. Antibody was found in 28 per cent, with the highest prevalence in persons born before 1921. Serologic evidence of occupation-related infections was found in hog producers, veterinarians, and abattoir workers. An increase in the prevalence of antibody in one community between June 1965 and July 1966 was unexplained.