From the Yes This IS Your Grandfather's Influenza department...

CNN reports on the unwise distribution of 1957's Asian flu (A, H2N2) to 4,000 laboratories worldwide for testing.

The samples, part of a package of pathogens sent to laboratories to test their ability to identify them, were last seen in nature in the United States in 1968, Gerberding said. Anyone born since then would presumably have no immunity to the virus, she said.
Gerberding said authorities were still trying to determine how many laboratories got the samples of the virus, called Influenza A H2N2.

Thanks to an unnamed source, who also mentioned The Unsung Vaccinologist:

Joining Merck in 1957, he mobilized the production of 20 million doses of flu vaccine, protecting the country from one of the last great flu pandemics.