Greenberg, S. B. (Department of Microbiology and Immunology, Baylor College of Medicine. 1200 Moursund Avenue. Houston, Texas 77025), R. B. Couch and J. A. Kasel. An outbreak of an influenza type A variant in a closed population: the effect of homologous and heterologous antibody on infection and illness. Am J Epidemiol 100:209–215. 1974.—An outbreak of influenza caused by the England variant of type A influenza virus was studied in a Texas prison population during January 1973. The effect of pre-existing serum Hong Kong and England antihemagglutinin (AHAb) and antineuraminidase (ANAb) antibody on the occurrence of infection and illness was evaluated. One hundred forty-seven (69%) of 230 men studied experienced influenza virus infection and 97 of 147 (66%) exhibited illness. Of 147 men who were infected, 55% or 81 men had both virus isolations and fourfold antibody rises by HI tests; 27 had virus isolations only and 39 had antibody rise in HI tests only. Increasing Hong Kong AHAb was associated with decreasing frequencies of infection, of overall illness and of illness among those infected. Men with prior Hong Kong ANAb had less infection and illness than those lacking such antibody but the frequencies of these did not decrease with increasing levels of antibody. Men who had both pre-existing Hong Kong AHAb and ANAb in their sera had significantly lower illness and infection rates than those with only Hong Kong AHAb or ANAb in their pre-outbreak sera. Men with pre-existing England AHAb and/or ANAb in their sera had significantly lower illness and infection rates than those without England antibody.