Doctors dismissed rumours spreading online that suggest having a Covid-19 vaccine could affect fertility as nonsense.
The false claims are widely seen by experts as the latest round of misinformation from anti-vaxxers, who believe unfounded suggestions that vaccinations cause harm.
Despite being untrue, such rumours are blamed in part for "vaccine hesitancy" – a reluctance to be vaccinated – which could hamper efforts to control the pandemic.
Prof Paul Hunter, an infectious diseases specialist and professor of medicine at the University of East Anglia in the UK, said "somebody is putting out a lie" suggesting vaccination reduces fertility.
“I cannot see how it would feasibly do that, knowing how these vaccines work,” he said.
"If people are going to make these statements, they have to produce hard evidence, otherwise we have to take them as made up.
"It's anti-vaxxers spreading more lies and misinformation ... it's nonsense and it should be treated as nonsense."
Writing in The New York Times this week, Prof Akiko Iwasaki, a professor of immunobiology at Yale School of Medicine, and one of her postgraduate students, Alice Lu-Culligan, branded a claim that the vaccine generates antibodies that affect a protein in the placenta as "completely false".
One suggestion, they said, is that the protein in question, syncytin-1, is similar to the coronavirus spike protein, leading to the idea that antibodies produced by vaccination could harm a developing child.
But in laboratory tests the researchers found no reaction between antibodies collected from women with Covid-19 and syncytin-1, nor did they identify any significant similarities between this protein and the coronavirus spike protein.
They also pointed out that there had been no reports of infertility in women who have recovered after being infected with the coronavirus.
Dr Andrew Freedman, an infectious diseases specialist at Cardiff University in the UK, said there were "so many false rumours around", and dismissed suggestions that vaccination harmed fertility.
"There's no evidence and there's no conceivable mechanism I can think of why that would be the case," he said.
"I presume it comes from anti-vaxxers with no evidence at all."
Similarly, there is no evidence that vaccination causes male infertility, and a study being conducted by the University of Miami that analyses the effect of an mRNA Covid-19 vaccine is expected to confirm this.
Researchers behind that study have, however, found that the virus can harm sperm production and that it can linger inside the testes, raising the prospect – as yet unconfirmed – that infection could potentially affect male fertility.
While there is no evidence Covid-19 vaccines harm fertility, there is caution about giving the jabs to pregnant women because – as is normal practice – they were not included in clinical trials.
Typically, it is recommended that pregnant women are not given vaccines made from live attenuated pathogens (which can replicate but not lead to disease), because there are concerns the pathogen could multiply in the foetus. However, none of the Covid-19 vaccines being administered contain live attenuated pathogens, so the risk of harm from vaccination during pregnancy is thought to be very low.
Tests on animals during pregnancy have not highlighted potential problems with coronavirus vaccines – another indication that there are unlikely to be issues if pregnant women are vaccinated.
Regulators suggest pregnant women be vaccinated only if she has specific medical conditions, such as heart disease, chronic kidney disease, or severe respiratory conditions, that put her at greater risk should she become infected with Covid-19.
Officials suggest that women who are at greater risk of catching the coronavirus, perhaps because of their work, may want to discuss getting vaccinated.
"In general terms, a woman who's pregnant [and who works] in an office or something like that, you probably wouldn't give them a vaccine, but if it was a pregnant nurse who works in a Covid ward, probably, after discussing it with the nurse herself, you would decide to go ahead with it," Prof Hunter said.
Pregnant women are more at risk of severe symptoms if infected, leading some countries, such as Israel, to include them in a vaccine programme.
Compared with other women of the same age, pregnant women with Covid-19 are more likely to be admitted to an intensive care unit, to be put on a ventilator and to die, although the risks are not thought to be as great as those faced by, for example, the elderly.
In other advice from national regulators, the Centres for Disease Control and Prevention in the US said a pregnancy test before being given a coronavirus vaccination is not necessary, and it is safe to become pregnant after being inoculated.
Meanwhile, there are no known risks from giving coronavirus vaccines to breastfeeding women, according to the UK's Joint Committee on Vaccination and Immunisation, although officials stress there is limited data.