Seroepidemiology of H1N1 influenza: striking differences in the attack rate among young people.

The H1N1 influenza outbreak in winter 1977-78 covered the whole of Finland in a uniform fashion, whereas striking differences emerged between various population segments in the susceptible age group. The attack rate was 68 per cent among young servicemen who had been in the armed forces during the epidemic and 33 per cent among those who had entered the service after the epidemic, but only 6 per cent among pregnant women of the same age. Based on routine specimens sent to the laboratory for reasons unrelated to influenza, the attack rate was 4 per cent among children born in 1973-76 and increased by age up to 45 per cent among subjects born in 1957-62. The findings on pregnant women and small children suggest a novel mechanism of innate resistance operating specifically against H1N1 influenza.