Fatigue and weakness among the side effects. | Efe (Nacho Gallego)

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A study by the School of Public Health at the University of Haifa in Israel has found that almost twice as many women than men have reported side effects after having had the Pfizer vaccine, whether this has been the first, second or booster jab.

Professor Manfred Green, principal researcher of the study that appears in the journal Vaccines, says: "We don't know what mechanism is involved, but it may be related to differences between the sexes in terms of the immune system or in the perception of side effects." One possibility is that women's immune systems "respond more strongly than men's to foreign antigens".

The Pfizer vaccine is based on the injection of a nucleic acid (mRNA) that codes for one of the virus's proteins. The goal is to stimulate the production of antibodies against the virus and protect the recipient against the disease. Vaccination is sometimes accompanied by side effects, such as pain in the entire arm, fever, weakness, and fatigue.

The study was based on the collection of data from four different sources, such as reports sent to Israel's ministry of health on side effects in people over 16 between December 2019 and June 2021. The results show that the reporting of side effects is approximately 1.9 times higher among women than among men.

The highest frequency of reported side effects was after the second vaccination. However, and for specific side effects, the differences between women and men were far greater after the first jab. For example, the proportion of women who reported weakness after the first jab was 30 times higher than that of men.