Researchers studying COVID-19 vaccines failed to get much of any data on pregnant people. That disservice was compounded last week when the WHO recommended against giving pregnant women the Moderna or Pfizer vaccines, appearing to conflict with earlier advice offered by the CDC and the American College of Obstetricians and Gynecologists. It later backtracked, though too late to avoid raising a round of new questions.

Experts who study sex differences, pregnancy and the immune system say the limited evidence available suggests pregnant people are more at risk from the virus than from the vaccine.

"Shame on everybody for not including pregnant females at every stage along the pipeline," says Sabra Klein, an immunologist at Johns Hopkins University. That should have included testing vaccines on pregnant animals as well as enrolling pregnant women in clinical trials, she says.

The mixed messages are stirring undue fear and confusion at a time when people are being bombarded by anti-vaccine misinformation, some of it focused on women and fertility. And pregnant people deserve better advice than the vague "ask your doctor."

Klein says the CDC has reported that people who are pregnant are more likely to experience more severe COVID-19 and be hospitalized than the general public. And another CDC report showed an association between getting infected during pregnancy and having adverse outcomes, including preterm birth.

She says COVID-19 doesn't pose as much danger as the Zika virus, which caused a rash of severe birth defects, but it's bad enough that for most pregnant women, the risk of getting the disease outweighs any potential dangers from the vaccine.

There are biological that the immune systems of pregnant women might react differently to the virus, and possibly to the vaccine. When a woman is pregnant, some parts of her immune system are damped down to prevent any reaction that might hurt the growing mass of foreign cells that make up her fetus.

"While it's important to be able to fight infection, from an evolutionary standpoint, the most important thing going on in your body is your pregnancy and keeping your baby alive," said Klein. That means a vaccine might not work as well at generating an immune response. And on the other side, she says, it's possible an exuberant response would harm the pregnancy.

But that risk can't be considered out of context. The fact is, it's hard to avoid infection with SARS-CoV-2, especially for women in the high-risk professions likely to be offered the vaccine in these early weeks.

Tara Smith, an epidemiologist at Kent State University, considers the WHO's wavering to be a blunder. The initial recommendation was more nuanced than what people heard, however. It ultimately left the vaccine decision up to women and their doctors — which is not that different from the CDC's guidance. And it did recommend the vaccine for pregnant health care workers at high risk of exposure, who are pretty much the only pregnant people currently eligible for vaccination.

Vaccine-related fears aimed at women were already circulating from an earlier rumor that the vaccines might induce antibodies that would attack the placenta and cause infertility. That, Smith says, was based on an observation that the proteins produced by the vaccine to elicit antibodies were slightly similar to a protein in the placenta called syncytin-1.

She says getting COVID-19 would induce even more varied antibodies — increasing the risk of an adverse effect. And the vaccines didn't cause infertility in the animal or clinical studies. "It was like a game of telephone," she says, the initial biochemical findings being a far cry from the resulting infertility rumors.

Yale immunology professor Akiko Iwasaki and student Alice Lu-Culligan debunked the infertility claim in the New York Times last week, basing their argument on studies they did in their own laboratory. The authors also discussed the lack of pregnancy data and fears the information void might cause.

Klein, the Johns Hopkins professor, says it's unfair to put the vaccine decision burden on women without adequate information and amid too many negative stories. Already, she says, unwarranted vaccine fears have led to a very low acceptance of flu vaccines among pregnant women, though it's known that flu can be more severe during pregnancy and can lead to complications — including miscarriage.

She says what's critical now is that pregnant women getting the vaccines are closely monitored so that any side effects or lack of efficacy might be revealed before the vaccines are offered to a broader swath of younger people. And she says with lots of additional pre-clinical work going on to study how the vaccine works against new variants of the virus, researchers could start including pregnant animals. It isn't too late to make up for past mistakes.

Faye Flam is a Bloomberg Opinion columnist and host of the podcast "Follow the Science." She has written for the Economist, the New York Times, the Washington Post, Psychology Today, Science and other publications.