Jonas Salk and Thomas Francis created the first vaccines against the flu virus in 1938. The first flu vaccines were used on soldiers in the U.S military before World War II to prevent them from getting the disease.
Salk developed an effective polio vaccine in 1952 by using his experience with the flu vaccine. Modern vaccines are purified, but vaccines created from 1940 to 1960 were not. As a result, side effects such as aches, fatigue and fever often developed. People who developed these side effects mistakenly thought they contracted the flu from the vaccines. However, the vaccines killed the virus.