A pregnant woman wears a mask during the beginning of the COVID-19 pandemic. Charles Krupa/AP Photo Pregnant women carrying baby boys have fewer coronavirus antibodies than those carrying girls, a study found. Pregnant women also transferred fewer antibodies to male fetuses than females, the findings showed. The research offers a hint about how the male immune system responds to COVID-19. It's one of the pandemic's most persistent mysteries: Why are men and boys more vulnerable to severe COVID-19 than women and girls?A new study from Boston-based researchers suggests it may have to do with innate differences in their immune responses.The study looked at 38 women who were infected with the coronavirus during pregnancy, half of whom were carrying baby boys.