According to data tracked by the CDC, women accounted for 79% of all adverse reactions reported for the Moderna and Pfizer COVID-19 vaccines over a one-month period.

Among nearly 7,000 reports processed through the CDC’s Vaccine Adverse Event Reporting System from Dec 14 to Jan 13, more than 79% of them came from women. The most frequently reported side effects were headache, fatigue and dizziness.

Women also are more likely than men to experience some of the vaccine’s more unusual side effects, such as an itchy red rash that appears at the injection site commonly known as COVID arm or Moderna arm, because about 95% of the reactions occur with the Moderna vaccine. Overall, women account for 77% of the Moderna vaccine’s reported side effects.

Health experts say it may be the result of biological differences, inconsistent reporting by men and gender bias in clinical trials.