Epidemic mechanisms of type A influenza.

The antigenic varieties of influenza A virus isolated from 1968 to 1976 in a surveillance of a small, rather remote population were similar to those from England and Wales as a whole, despite frequent antigenic changes during the period. Household studies in the first two H3N2 influenza A epidemics found low attack rates within households, a high proportion (70%) of affected households with only one case of influenza, similar distributions of affected households in the two epidemics by the number of cases of influenza and similar distributions of the influenza cases by the day of their onset in the household outbreak. No serial interval could be demonstrated by cumulating household outbreaks. More than one minor variant was causing influenza contemporaneously in the same villages in several seasons, and different variants were on one occasion found on successive days in bedfellows. The regular occurrence of epidemics in winter was often accompanied by the disappearance of the epidemic variants and their replacement, after a virus-free interval, by new variants. These epidemiological findings seem best interpreted on the following tentative hypothesis. Influenza A sufferers do not transmit the virus during their illness; instead it rapidly becomes latent in their tissues so that they become symptomless carrier-hosts and develop specific immunity. Next season an extraneous seasonally mediated stimulus reactivates the latent virus residues so that the carrier-host becomes briefly infectious, though symptomless. Antigenic drift occurs because particles reconstituted to be identical with the progenitor virus cannot escape the specific immunity it has provoked in the carrier host. He can shed only mutants also determined by the progenitor virus. From the assortment of mutants shed by the carrier-host, his non-immune companions select that (those) which is best fitted to survive, and it rapidly causes influenzal illness. Epidemics consist largely or entirely of such persons sick with influenza caused by reactivated virus caught from symptomless carrier-hosts.