Human influenza A (H1N1) was not detected again until 1977.

Reasons for the complete disappearance of this strain in 1957 are not clear, but it is likely that high levels of existing homologous immunity, coupled with a burst of heterologous immunity from the new H2N2 strain, were sufficient to eliminate the virus. -

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Even though human influenza A (H1N1) virus had not circulated since 1957 and the swine influenza A (H1N1) virus that had been identified at Fort Dix did not extend outside the base, in November 1977, the H1N1 strain reemerged in the former Soviet Union, Hong Kong, and northeastern China. This strain affected primarily young people in a relatively mild presentation. - Ibid Careful study of the genetic origin of the virus showed that it was closely related to a 1950 strain but dissimilar to influenza A (H1N1) strains from both 1947 and 1957.

This finding suggested that the 1977 outbreak strain had been preserved since 1950.

The reemergence was probably an accidental release from a laboratory source in the setting of waning population immunity to H1 and N1 antigens. http://www.nejm.org/doi/full/10.1056/NEJMra0904322