Early studies on the effectiveness of the COVID-19 vaccines show an interesting but worrisome trend: They might not be as effective for women as they are for men. Breakthrough infections — cases of COVID diagnosed after someone is fully vaccinated — are rare. Only 0.01% of vaccinated Americans developed breakthrough infections as of the end of April, according to the CDC. But women make up close to two-thirds of those breakthrough infections. This real-world data lines up with what we saw from the clinical trials. The Pfizer vaccine had a greater than 96 percent efficacy rate for men, but just under 94 percent for women. The Moderna and Johnson & Johnson trials showed similar trends. Theories of why women suffer more breakthrough infections include the vaccine not protecting against variants as well for women as men, or that immunity from the vaccines declines after menopause. Most of the breakthrough infections occur in people aged 40 to 74. There is still much we need to know, since more women than men have been vaccinated so far, and women could be more inclined to report COVID symptoms and get tested.