Swine influenza virus and the recycling of influenza-A viruses in man.

Sera collected in 1967 and 1972 from people in the 0-100 age-group showed haemagglutination-inhibition (H.I.) antibody to swine virus A/Iowa/15/30 (Hsw1N1) in greatest number and with highest titre in people born before 1918. A slight decrease was observed from 1967 to 1972 in the number of sera with antibody to swine virus and in the height of the titres. The recently isolated A/New Jersey/10/76 (Hsw1N1) virus showed a result comparable to that of the Swine/1930 virus in sera of 1972. On the analogy of the findings in 1968, when the Hong Kong virus became epidemic in human populations and antibody to this virus was found in sera of people over 70 years, the suggestion is made that the recurrence of swine virus as an epidemic agent of human influenza may be expected around 1986. Fourfold or greater increase of antibody to Swine/1930 virus was observed in about 4--5% of people infected by or immunised with H3N2 viruses. This response occurred in people who had been in touch with the epidemic influenza-A viruses Hsw1N1, H0N1, and H1N1 during the swine era of 1918 to 1956. Following immunisation with H3N2 viruses of persons showing no response to H3N2 viruses in their serum 5% did show a fourfold or greater heterotypic H.I. antibody rise to swine virus. This finding is of consequence for the diagnostic serology of influenza.