Influenza antibodies in the population of the USA; an epidemiological investigation.

Sera taken from persons of various ages in 1951 and collected from adults yearly from 1943 to 1951, inclusive, were tested by the haemagglutination-inhibition method with influenza viruses selected to represent the subgroups of each antigenic type. These were influenza A-WS (1933) and PR8 (1934); influenza A-prime-FM1 (1947) and FW-1-50 (1950); influenza B-Lee (1940) and IB1 (1950); influenza C-1233 (1947). The sera tested with influenza A and B viruses were treated with cholera filtrate to remove non-specific inhibitor. Since influenza C virus was not affected by the non-specific substance, the sera tested against this agent were not so treated.Children's sera showed high antibody level, attained at an early age, for FM1 and FW-1-50 viruses, and essentially no antibody for WS or PR8. By contrast, adult sera revealed high antibody content for PR8 and moderate titres for WS and the A-prime viruses. In adult sera, antibody against the PR8 virus increased significantly in 1944, after the 1943-4 epidemic, and remained at a relatively constant level for the eight succeeding years. The antibody pattern for WS was similar to that for PR8, but the values for its titres were only half as great. Antibody against the A-prime strains rose steadily from inconsequential levels in 1943 to high mean titres in 1951. These findings were consistent with virus isolation studies which suggested that the A-prime viruses, such as FM1, were introduced about 1946 and have been continuously prevalent since that time, while the WS and PR8 agents have been recovered only occasionally in recent years. Sera tested with Lee and IB1 viruses showed essentially identical titres. Antibody to these strains was low in the sera of children, indicating that there had been little previous exposure to the B agents. Adult sera showed marked elevation in titre between 1944 and 1946, and the titres have remained at a high level since that time. The increase in 1946 followed the influenza B outbreak of 1945-6; the increase in the previous year occurred in spite of the fact that there was no epidemic. Children developed significant antibody to influenza C in early life, and high antibody levels were observed in adult sera collected over the nine-year period. These observations indicate that the virus was widespread in the population and was prevalent before 1943.