Recent data from the U.S. CDC found that more women than men reported vaccine side effects from their Pfizer or Moderna shots.

In the data 79 per cent of reported side effects came from women. This was the case even though women only received 61.2% of the vaccines. Severe, life-threatening allergic reactions to the vaccines happened almost entirely to women, not men.

The results are similar to what we know about other vaccines. The flu vaccine triggers much higher rates of allergic reactions in women than men. Overall, women are more likely to react to many types of vaccines, including hepatitis B, measles, mumps and rubella (MMR) and yellow fever.

We don't know exactly why women react worse than men to some vaccines. Different studies have evaluated different likely causes. Some possible causes include biology, behavior, genetics and hormones, all of which have strong links to immune reactions.

Some studies have looked at whether women report vaccine side effects more often than men, or seek healthcare for side effects more often than men.

Most clinical trials do not separate data between men and women to analyze whether or not the two groups experience differences in vaccine responses. This is part of why it's hard to know exactly why the differences exist.

What we do know is that broadly speaking, women's immune systems respond more strongly to vaccines, which is probably why they experience stronger side effects. Some experts have suggested that strong reactions to the injections might lead to greater protection against viruses and improve resistance to infections.

While women are reporting more side effects to COVID-19 vaccines, studies suggest that men are more likely to suffer severe COVID-19 infections and die from the virus. Some scientists believe that the reason women are reporting more reactions to vaccines may also be why they are less likely to suffer severe infections from COVID-1: their immune systems respond more strongly to the virus.