Check this out:

https://thebulletin.org/2014/03/threatened-pandemics-and-laboratory-escapes-self-fulfilling-prophecies/

Human H1N1 influenza virus appeared with the 1918 global pandemic, and persisted, slowly accumulating small genetic changes, until 1957, when it appeared to go extinct after the H2N2 pandemic virus appeared. In 1976, H1N1 swine influenza virus struck Fort Dix, causing 13 hospitalizations and one death. The specter of a reprise of the deadly 1918 pandemic triggered an unprecedented effort to immunize all Americans. No swine H1N1 pandemic materialized, however, and complications of immunization truncated the program after 48 million immunizations, which eventually caused 25 deaths.

Human H1N1 virus reappeared in 1977, in the Soviet Union and China. Virologists, using serologic and early genetic tests soon began to suggest the cause of the reappearance was a laboratory escape of a 1949-1950 virus, and as genomic techniques advanced, it became clear that this was true. By 2010, researchers published it as fact: “The most famous case of a released laboratory strain is the re-emergent H1N1 influenza-A virus which was first observed in China in May of 1977 and in Russia shortly thereafter.” The virus may have escaped from a lab attempting to prepare an attenuated H1N1 vaccine in response to the US swine flu pandemic alert.

The 1977 pandemic spread rapidly worldwide but was limited to those under 20 years of age: Older persons were immune from exposures before 1957. Its attack rate was high (20 to 70 percent) in schools and military camps, but mercifully it caused mild disease, and fatalities were few. It continued to circulate until 2009, when the pH1N1 virus replaced it. There has been virtually no public awareness of the 1977 H1N1 pandemic and its laboratory origins, despite the clear analogy to current concern about a potential H5N1 or H7N9 avian influenza pandemic and “gain of function” experiments. The consequences of escape of a highly lethal avian virus with enhanced transmissibility would almost certainly be much graver than the 1977 escape of a “seasonal,” possibly attenuated strain to a population with substantial existing immunity.