Heart and lungs

Contraceptive pills not proven to protect against the flu

"How taking the pill could protect you from the flu," was the curious headline on a recent Mail Online article.

The equally curious animal study involved female mice who had their ovaries surgically removed – half were then given progesterone implants, half weren't.

Progesterone is one of the active ingredients in the combined pill and the main ingredient in the progestogen-only pill.

Researchers administered a lethal dose of flu virus directly into the mouse's nose. Those who had the progesterone implants survived about two days longer.

Examining the lung tissue suggested that progesterone hormone may be needed for cell repair processes after lung infection or damage.

Neither the species, the fatal flu dose given directly into the nose, or the hormone scenario are directly applicable to humans.

So despite the media latching onto contraception, this study provides no evidence that women taking progestogen-containing contraception have added protection against flu or other infections.

Practicing good hygiene, for example, frequent hand washing and always putting used tissues in a bin, can help prevent a flu infection. Those groups particularly vulnerable to the harmful effects of the flu should make sure they get their annual flu jab.

Where did the story come from?

The study was carried out by researchers from The Johns Hopkins Bloomberg School of Public Health and School of Medicine. The study was funded by grants from the National Institutes of Health, National Institute of Allergy and Infectious Diseases.

The study was published in the peer-reviewed scientific journal PLOS Pathogens.

The study is open-access so can be read online or downloaded for free.

The Mail and The Daily Telegraph's coverage didn't mention the study involved mice until quite far down in their articles. Both say that contraception may be protective – which hasn't been demonstrated at all as these mice had had their natural hormone depleted. If anything, this was more like a test of hormone replacement therapy (HRT) than contraception.

The Mail also reports: "Contraception helps body keep hold of virus-fighting hormones" which isn't correct. This wasn't contraception to help the body "keep hold of" anything; it was hormone replacement. Also, progesterone wasn't directly fighting the virus – it seemed to be needed for normal cell repair processes.

What kind of research was this?

This was an animal study that aimed to see whether artificial progesterone hormones confer protection against potentially lethal flu disease in mice.

All contraceptive hormones taken by women contain some form of artificial progesterone (progestogen) – either in combination with oestrogen as in the combined contraceptive pill ("the pill") or progestogen alone, such as in the "mini pill", injections or implants.

The researchers say previous research suggested progesterone may give some protection against infections of the reproductive tract. Their new study aimed to look at possible protection against respiratory tract infections.

Animal studies are useful for testing theories to see how things might work in humans, but we do not have identical biology.

What did the research involve?

The research involved young female mice (7-8 weeks old) who were housed in standard conditions. A couple of weeks later they had an operation to remove their ovaries. After recovery they were assigned to receive implants under their skin containing either inactive placebo or 15mg of progesterone – which was released at a steady dose over the course of a month.

They were then inoculated via the nose with either placebo or with an influenza virus (H1N1), at a dose lethal for these animals.

They examined whether the mice developed signs of illness – shortness of breath, hair standing on end, hunched posture or absence of an escape response. They also examined samples of blood and lung tissue to look at the make-up of inflammatory cells.

What were the basic results?

The researchers found that the mice with progesterone implants had some protection against flu infection and related lung damage. They died after 11.1 days compared to 9.5 days for the mice who didn't have progesterone implants.

However, progesterone had no effect on blood levels of the virus, suggesting it didn't make them more resistant to infection. Comparing body temperatures though, they found that those treated with progesterone were less likely to have hypothermia after getting flu.

They also had less lung inflammation and damage. Further analysis of lung tissue showed the progesterone mice had greater cell proliferation, reduced protein leakage into the airways, and other findings suggestive of a "repair environment" in the lungs.

The researchers further confirmed that giving a cellular growth factor to the mice lacking progesterone also improved their outcomes following flu infection. 

How did the researchers interpret the results?

The researchers conclude that the progesterone hormone is an essential factor in mediating the production of a cellular growth factor involved in the repair of lung tissue following infection.


These are interesting scientific findings but they have limited implications.

Animal studies are useful for giving an indication of how biological processes may work in humans but we're not identical. So the scenarios tested here – the progesterone, or the flu injection – can't be taken as representative of real-life in humans.

For one thing all the mice had surgery to remove their ovaries before being infected. It makes sense that the mice that had been given some additional recovery boost in the form of hormone replacement may have been in a better health state than those left hormone depleted.

They were also directly inoculated through the nose with a flu dose that has previously been demonstrated to be lethal in these animals, and the animals did die. It's just those with progesterone survived about an extra two days.

The findings do suggest the progesterone hormone may have various roles in female health – also here seeming to improve lung cellular repair. However, since most women have the progesterone hormone naturally in their bodies this doesn't mean a great deal.

We can't leap to saying that women who take progestogen-containing contraception hormones have added protection against infection, or are less likely to get flu. This certainly hasn't been tested.

To reduce your risk of getting flu or spreading it to other people, you should always:

  • make sure you wash your hands regularly with soap and warm water
  • clean surfaces such as your keyboard, telephone and door handles regularly to get rid of germs
  • use tissues to cover your mouth and nose when you cough or sneeze
  • put used tissues in a bin as soon as possible

A flu vaccine is available for free on the NHS for:

  • anyone aged 65 and over
  • pregnant women
  • anyone who is very overweight (with a body mass index over 40)
  • children and adults with an underlying health condition (particularly long-term heart or lung disease)
  • children and adults with weakened immune systems

Your GP can advise you about whether you require the annual flu jab. 

NHS Attribution