Older men are up to twice as likely to become severely sick and to die from the coronavirus as women of the same age.

Why? The first study to look at immune response by sex has turned up a clue: Men produce a weaker immune response to the virus than women, the researchers concluded.

The findings, published on Wednesday in Nature, suggest that men, particularly those over 60, may need to depend more on vaccines to protect against the infection.

“Natural infection is clearly failing” to spark adequate immune responses in men, said Akiko Iwasaki, an immunologist at Yale who led the work.

The results are consistent with what’s known about sex differences following various challenges to the immune system. Women mount faster and stronger immune responses, perhaps because their bodies are rigged to fight pathogens that threaten unborn or newborn children.