Pregnant women are expected to be excluded from the Food and Drug Administration’s authorization of COVID-19 vaccines, at least at first, an adviser to the agency says.

Clinical trials have not yet included pregnant or breastfeeding women or young children, giving scientists insufficient data to determine if the shots are safe for them. 

The FDA is expected to approve the first U.S. vaccine, developed by Pfizer and BioNTech, for emergency use, with the first shots being administered early next week. An independent panel of advisers voted Thursday to recommend that the FDA authorize the vaccine, which clinical trial data show is 95% effective.

A second vaccine candidate, from Moderna Inc. and the National Institutes of Health, is a week behind Pfizer’s in the approval process.

Dr. Paul Offitt, a member of the FDA’s advisory panel, the Vaccines and Related Biological Products Advisory Committee, predicted in an interview with CNN on Thursday that the agency’s “recommendation will be that pregnant women not receive this vaccine until we know more.”

He added that some women who were unknowingly pregnant were likely part of clinical trials and that it’s inevitable that more pregnant women, including health care workers, will be innoculated. 

The United Kingdom began administering Pfizer’s shots this week. Its guidance says “those who are pregnant should not have this vaccine.” It also recommends that women planning to get pregnant in the next three months avoid the vaccine.

Dr. Anthony Fauci, the U.S. government’s top infectious disease expert, said Thursday that drugmakers and federal regulators will likely launch clinical coronavirus vaccine trials on pregnant women and young children next month. Those groups were left out of initial trials because scientists wanted to ensure the vaccine was safe for healthy adults before testing it on more vulnerable populations.

“That will not necessarily be looking at efficacy, but we’ll be looking at safety and immunogenicity to bridge to the efficacy in the adult non-pregnant population,” Fauci said at a virtual Columbia University event. “The same holds true for the pediatric population. Those studies will probably start in mid-to late-January.”

According to the World Health Organization, pregnant or recently pregnant women who are older, overweight and have preexisting medical conditions such as hypertension or diabetes appear to have an increased risk of developing severe cases of COVID-19. In general, pregnant women can be badly affected by respiratory infections, the WHO says.