Getting the flu when vaccinated
Q: Why do people who get the influenza vaccine sometimes also get sick with the flu?
A: The short answer is because there are hundreds of flu viruses and only 2-3 in each annual vaccine. Often the vaccine formula is right on the money, and sometimes an unexpected strain that was not part of the vaccine formula creates havoc. There are so many flu viruses that it is possible to contract a different one from the flu-vaccine types. In that case, however, typically the symptoms are mild and do not require hospitalization among those who were vaccinated.
In the 2019-2020 flu season, only 51.8 percent of individuals 6 months and older in the US received the flu vaccine—leaving 48.2 percent without any vaccine protection. The CDC estimated that about 38 million individuals became ill with the flu during the 2019-2020 season with 225,000 hospitalizations and ultimately more than 20,000 deaths. (There is no requirement for reporting influenza or flu, so the estimated numbers may be lower than actual.)