Change of hepatitis B surface antigen in patients with hepatitis during a five-year period.

Between January 1970 and December 1974, 122 cases of acute type B hepatitis were subtyped; 66 (54%) were of the ad type and 56 (46%) were of the ay type. "Cluster" cases (from a dialysis unit) were not considered. During the first period, subtype ad predominated, whereas in the second year there was a clear predominance of the ay subtype; the difference was statistically significant (P less than 0.02). In yearly periods the differences were significant between the years 1970 and 1974 (P less than 0.05), 1972 and 1974 (P less than 0.05), and 1972 and 1973 (P less than 0.05). If only patients without parenteral exposure are considered, there was clearly a shift between 1970-1972 and 1973-1974 in favor of the ay subtype (P less than 0.01). Since epidemiological factors such as injections and transfusions seem not to be responsible, it is suggested that a change of virus strain may be responsible for the different distribution of subtypes of hepatitis B surface antigen in the last year.