This claim was false, but it compounded real uncertainty. Pfizer and Moderna hadn’t yet specifically tested their vaccines on pregnant or breastfeeding women, and the FDA’s emergency use authorization doesn’t cover pregnancy. The guidance from the American College of Obstetricians and Gynecologists only goes as far as saying that “vaccines should not be withheld from pregnant or lactating individuals.” Even so, by February, more than 30,000 pregnant women had signed up for a U.S. government monitoring program after getting Covid shots, and so far there have been no red flags. More recent studies have found the vaccines are not only effective on pregnant women, but they also pass antibodies on to their newborns. And because pregnant women are at greater risk of dying of Covid, many doctors are recommending they get the shot anyway. “Women are confused,” says Lori Metz, a licensed clinical social worker in New York who specializes in fertility. “Your doctor may say one thing, and then you read a blog that starts to pull on all these other fears.”

This gray area was fertile terrain for anti-vaccine activists. In December, Del Bigtree, founder of the Informed Consent Action Network, shared the false post about sterilization with hundreds of thousands of his followers on Facebook and Instagram. The blog was subsequently shared on Facebook more than 25,000 times. “I am seeing this EVERYWHERE!” a woman named Emily wrote along with a screenshot of the fake Pfizer blog, which can still be found circulating on Facebook in multiple languages. “I am starting to believe this.” Commenters flooded her post with more (completely false) “evidence” backing up the claim. Some said it proved another debunked conspiracy: that the Covid vaccine is part of the worldwide Bill Gates-funded depopulation effort.

The effects of this disinformation are already showing up in survey data. Of people who say they are not likely to get a vaccine, more than half of women in the U.S. are concerned about side effects, compared with 44% of men, according to a U.S. Census Bureau survey from Mar. 3 to Mar. 15. Many of the already eligible women are turning down the shot, according to surveys and interviews with over a dozen people.