| naturally acquired immunity to influenza type a. lessons from two coexisting subtypes. | when the h1n1 subtype of influenza reappeared in the northern hemisphere during 1977, after a 20-year absence, it behaved very differently from the h3n2 subtype still in circulation. in sydney, we studied the incidence of both subtypes of laboratory-proven influenza type a in 287 unvaccinated volunteers whose serum antibody titres were measured before and after each winter, to facilitate the detection of subclinical as well as clinical infection. during a 1977 epidemic, the a/victoria/3/75 strai ... | 1985 | 3965918 |