Naturally acquired immunity to influenza type A: a clinical and laboratory study.

After type A influenza virus had undergone major antigenic change in mid 1968, it was noted that individuals previously infected by strains of the old subtype (Asian), especially late strains, appeared to be unexpectedly resistant to clinical attack by the new subtype (Hong Kong). Prospective studies have since shown that, during the A/England/42/72 influenza epidemic of 1972, in which the incidence was approximately 7% in the community, clinical influenza due to this virus was not found in 229 subjects previously confirmed as having had A/Hong Kong/1/68 influenza, even though vaccine which had been effective against A/Hong Kong/1/68 was ineffective against A/England/42/72. During the A/Port Chalmers/1/73 influenza epidemic of 1974, clinical influenza resulting from Port Chalmers virus was not found in a closely monitored group of 176 unvaccinated subjects previously infected by A/Hong Kong/1/68 or A/England/42/72, although laboratory studies demonstrated Port Chalmers infection in five of these (2-8%). By contrast, among 99 subjects who had no such history of earlier infection, 22 developed laboratory-proven Port Chalmers influenza and most of them had typical illness.