When some countries began rolling out coronavirus vaccines late last year, each put forward its own criteria for which populations to prioritize — a list that, for the most part, excluded or cautioned against the inclusion of women who were breastfeeding, pregnant or expected soon to be pregnant.

This precaution resulted from an absence of information rather than any directly negative data. Pregnant women were excluded from the first round of human trials, as is common for such studies.

But in some places, including Israel and Britain, the guidance has shifted as scientists learned more about the vaccines and fears mounted over highly transmissible variants of the virus.

Scientists have gleaned relevant information from ongoing studies, while comparable vaccines that have been around longer do not carry safety concerns for pregnant women, so the benefits increasingly outweigh the risks, said Marian Knight, a professor of maternal and child population health at Oxford University.

On Tuesday, Israel’s Health Ministry and council for gynecological health updated its guidance to recommend the coronavirus vaccine for pregnant women, especially those with preexisting health issues or who work in close contact with the public.

“Pregnant women who become infected and sick with the virus have a higher incidence of serious illness from the disease than their cohorts in the general population,” the ministry said in a statement, echoing findings from studies in other countries, including Canada, Sweden and the United States.

The change followed a recent rise in Israel of reported hospitalization among pregnant women with covid-19, some of whom developed complications with their pregnancies. Israel is providing its citizens with the Pfizer-BioNTech vaccine.

“In light of reports about women and young couples avoiding the vaccine for fear of it damaging fertility, it is important to stress that there are neither facts nor a scientific basis pointing to damage to fertility as a result of any of the recognized vaccines,” Israel’s fertility research association wrote in a recent position paper, according to the newspaper Ha’aretz.

In December, Britain, among others, did not at first include women who were pregnant, breastfeeding, or could soon be pregnant in its vaccine program, citing a lack of evidence due to the exclusion from Phase 3 human trials.

The policy drew criticism from some women’s health advocates and public health experts. In Britain and worldwide, women are in the majority in many front-line professions confronting the virus, such as health-care work and teaching.

In the weeks since, however, Britain has shifted its stance on the Moderna and Pfizer-BioNtech vaccines available there. British health officials now recommend that pregnant women consult with their doctor to weigh the risks. They no longer exclude lactating women or women considering a pregnancy, citing a lack of cause for concern.

“There is no known risk with giving inactivated virus or bacterial vaccines or toxoids during pregnancy or whilst breastfeeding,” Public Health England said in a statement Jan. 5. “However, the coronavirus vaccines have not yet been tested in pregnancy, so it has been advised that until more information is available, pregnant women should not routinely have these vaccines.”

It added, “If a coronavirus vaccine is given to a pregnant woman, she should be reassured that the vaccine does not contain live SARS-CoV-2 virus and therefore cannot cause covid-19 infection in her or in her baby.”

The U.S. Centers for Disease Control and Prevention and Health Canada have gone further, recommending that pregnant women who want to get the vaccine do so in consultation with their doctor. The American College of Obstetricians and Gynecologists has said that coronavirus vaccines should not be denied to pregnant women.

“This is a natural course of a vaccine going through the usual clinical trials,” said Alyson Kelvin, a researcher at the Canadian Center for Vaccinology and Dalhousie University. Some women who did not know they were pregnant at the time, for example, were part of various Phase 3 trials. Scientists have been able to glean information from these cases and update their guidance.

“As small amounts of data come up, then it can be reexamined and pregnant women can start to be included,” she said.

Still, there remains much to be learned about the short- and long-term effects of the coronavirus vaccines that have been rolled out, which is why public health experts remain cautious with their recommendations.

“It’s very scary when you’re pregnant,” Kelvin said. “You’re vulnerable. And there’s a lot of misinformation out there.”

An article published late last year in the Lancet, a leading British medical journal, concluded that “inclusion of pregnant women in clinical treatment trials is urgently needed to identify effective covid-19 treatment for this population” such as “effective therapies that might improve maternal health, pregnancy, and birth outcomes, and avoid the delay of developing treatment recommendations for pregnant women.”

The European Medicines Agency said in December that the Pfizer-BioNTech vaccine should be used by pregnant women only on a case-by-case basis depending on a patient’s other risks factors, citing the current lack of clinical trials.

Last week, India began unrolling a massive vaccination effort using a locally manufactured version of the Oxford-AstraZeneca vaccine and a vaccine called Covaxin developed by Bharat Biotech, an Indian pharmaceutical company. The country’s ministry of health has said that women who are pregnant or lactating cannot get the vaccine under the current emergency authorization.

Russia’s Sputnik V vaccine has also yet to cleared by the government for pregnant women, though a trial to test it on children and those who are pregnant will begin in the spring, according to Russia’s RIA Novosti news agency.

China has not provided any specific guidance regarding pregnancies and its vaccine. Coronavac, one of the Chinese-developed vaccines, will soon be tested on pregnant women and children in Brazil, according to China’s state-run Global Times, citing a Brazilian official. Brazil had been considering the inclusion pregnant women as part of its priority groups for early vaccination, according to Reuters, but now is allocating it on a case-by-case basis.

Other countries, such as South Korea, have not yet provided guidance as they prepare to begin inoculation programs in the coming weeks.

Fears over an increased risk of severe infection and hospitalization for pregnant women with covid-19 have mounted amid the spread of highly transmissible virus variants. Younger populations have been particularly hit hard in some countries. There is no evidence that these variants are more harmful to pregnant women, Knight said, but more people, including pregnant women, are likely to be exposed to the virus and its resulting risks.

Isabelle Khurshudyan in Moscow, Min Joo Kim in Seoul, Regina Cabato in Manila, Gerry Shih in Taipei, Taiwan, and Terence McCoy in São Paulo, Brazil, contributed to this report.