Maybe there is something to “man-flu” after all. The hormone progesterone improves the lung function of influenza-infected mice and speeds their healing afterward. If humans respond the same way, women may be gaining partial protection against respiratory infections either while pregnant, for half their menstrual cycle, or full-time when taking the pill.

All oral contraceptives include progesterone, although some use estrogen as well. Although the pill is sometimes taken for medical reasons unrelated to pregnancy prevention, lung protection isn't one of these. That might change, however, with the publication in PLOS Pathogens of a paper reporting the response of female mice given progesterone pellets.

Dr Sabra Klein of Johns Hopkins University removed the ovaries from eight to 12-week-old mice and gave them either slow-release pellets of progesterone or a placebo. She found that progesterone didn't change levels of the flu virus, but reduced the inflammation and tissue damage in mice's lungs. Rodents given progesterone also recovered faster from their encounter with the virus, as measured by body temperature and lung function.

The doses Klein gave the mice were, scaled for size, equivalent to taking oral contraceptives, which in turn are based on normal levels after ovulation.

The progesterone-recipients had higher levels of T helper 17 (Th17) cells and the protein amphiregulin. Th17 cells are known to clear pathogens from the surfaces of mucosal cells, some of which are affected by respiratory tract infections.

Klein also tried injecting amphiregulin into the mice that had not been given progesterone, and saw similar benefits. The role of amphiregulin was confirmed when mice lacking the capacity to produce it failed to gain any benefit from progesterone doses.

The benefits may not be restricted to a specific virus. The paper notes that progesterone has been shown to decrease inflammation in both animal studies and cell cultures, yet few studies have explored how this affects disease response, other than for sexually transmitted infections. The authors consider this extraordinary given that more than 100 million women are taking progesterone-based contraceptives and this is expected to rise dramatically as access increases in many countries where availability is now limited.

However, the relationship between the flu and progesterone goes both ways. Klein has previously found that influenza A viruses suppress the production of reproductive hormones in young female mice that aren't getting artificial supplementation, making infections worse.

The inclusion of oral contraceptives in the health insurance plans caused a political storm in 2012, with conservatives outraged at the idea their availability should be part of mandated coverage. It remains to be seen whether this view will change if doctors start advising everyone, men included, to take progesterone to speed recovery from the flu.