The rate of women who reported side effects after receiving the first, second or third shot of the Pfizer-BioNTech vaccine against Covid-19 was almost double that of men, according to recently published Israeli research.

In a study led by Prof. Manfred Green from the University of Haifa that was recently published in Vaccines, researchers found that the rate of women reporting side effects for all three doses of the vaccine was 1.9 higher than men, and that the most substantial side effects were reported after the second dose.

The reported side effects were mild, and included pain in the vaccinated area, numbness and weakness in the vaccinated arm, headaches and fever. The rate of women reporting arm pain was seven times higher after the first dose, 4.2 higher after the second dosage and 4.1 higher after the third dose.

Headaches were also reported to plague women more than men. The rate of women reporting headaches was nine times higher after the first dose, 3.2 times higher after the second dose and 2.45 times higher after the third.

The vaccine impacted women differently in terms of weakness, too. The rate of women reporting weakness was 30 times higher after the first dose, 2.6 times higher after the second and 1.6 times higher after the third.

“We don’t know what the mechanism involved is, but it might have to with differences between men and women in terms of the immune system or in the approach to side effects,” says Green. “One possibility is that women’s immune system reacts more to external stimuli than men’s.”

The data for the study was collected from reports submitted to the Health Ministry by people over the age of 16 between December 2019 and June 2021; from a survey carried out among 923 participants over the age of 30 in June 2021; and in two more surveys conducted among people aged 20 to 65 in September 2021.