Why did i get the flu after getting a flu shot?

Flu: Different viral strain than vaccinated for.

About flu shots...: Getting a flu shot 2-6 weeks before first local cases of influenza begin appearing would be a good time. However, we can't have an entire country go to clinics all in a 4 week time span. Thus, for flu seasons that start in November, it's fine to get a shot between late-August and November. The shots may fully protect some people and partially protect others. Life is just that way, somewhat random.

Response/strain: Vaccine strains may not be matched to the flu vaccine in any given year. Alternatively, your response to the vaccine may be suboptimal.

Strain not in shot ?: In a given flu season about 6-8 strains of flu circle the globe at any given time. Air travel spreads them easily. In the preseason they make the vaccine & pick the top 3-4 strains based on world activity. More strains would weaken the protection for the main strains. It takes a minimum of 2 wks after the shot for you to be protected from the strains in the shot. ? too soon or new strain?

Often ineffective: As my colleagues have mentioned, there are many strains of flu and people often get strains not covered by the shot. Quoting the respected Cochrane Collaboration “71 people would need vaccination to prevent one case of influenza...Vaccination shows no appreciable effect on working days lost or hospitalisation.” For a summary of the research on flu vaccine see https://tinyurl.com/zeccbso