NEW YORK (NYTIMES) - When the coronavirus vaccines were first authorised in December last year, scientists knew little about how well they might work in pregnant women, who had been excluded from the clinical trials.

Since then, scientists have accumulated a small but steadily growing body of evidence that the vaccines are safe and effective during pregnancy. Preliminary results from two continuing studies provide additional encouraging news.

The Pfizer-BioNTech and Moderna vaccines produce robust immune responses in pregnant and lactating women, and are likely to provide at least some protection against two dangerous Covid-19 variants, according to a study published in Jama on Thursday (May 13).

Vaccinated women can also pass protective antibodies to their foetuses through the bloodstream and to their infants through breast milk, the research suggests.

In a second study, published in the journal Obstetrics & Gynecology on Tuesday, researchers found no evidence that either the Pfizer or Moderna vaccines damaged the placenta during pregnancy.

"We can shift our framework from, 'Let's protect pregnant people from the vaccine,' to 'Let's protect pregnant people and their infants through the vaccine,'" said Dr Emily S. Miller, an expert in maternal-fetal medicine at Northwestern University and co-author of the placenta study.

"I think that's really powerful."

Covid-19 presents serious risks during pregnancy. Research has shown, for instance, that pregnant women with coronavirus symptoms are more likely to be admitted to the intensive care unit, require mechanical ventilation and to die from the virus than are symptomatic women of a similar age who are not pregnant.

Because of these risks, the Centres for Disease Control and Prevention has recommended that the vaccines at least be made available to pregnant people, many of whom have opted to receive the shots.

In the Jama study, scientists at the Beth Israel Deaconess Medical Center in Boston and Harvard Medical School studied blood samples from 103 women who had received the Pfizer or Moderna vaccine between December 2020 and March 2021. Of these women, 30 had received the vaccine while pregnant, 16 while lactating and 57 while neither pregnant nor lactating.

The researchers analysed the blood samples for signs that the shots had conferred some protection against the coronavirus. Immune responses are complex, and may involve both antibodies - proteins that can bind to and block the virus - and T cells, which help the body recognise the virus and destroy infected cells.

The vaccines produced similar responses in all three groups of women, eliciting both antibody and T-cell responses against the coronavirus, the scientists found. Of particular note, experts said, was the fact that the shots produced high levels of neutralising antibodies, which can prevent the virus from entering cells, in both pregnant and non-pregnant women.

"Clearly, the vaccines were working in these people," said Dr Akiko Iwasaki, an immunologist at Yale University who was not involved in the research. "These levels are expected to be quite protective."

The researchers also found neutralising antibodies in the breast milk of vaccinated mothers and in umbilical cord blood collected from infants at delivery.

"Vaccination of pregnant people and lactating people actually leads to transfer of some immunity to their newborns and lactating infants," said Dr Ai-ris Y. Collier, a physician-scientist at Beth Israel who is the first author of the paper.

The results are "really encouraging", Dr Iwasaki said. "There is this added benefit of conferring protective antibodies to the newborn and the foetus, which is all the more reason to get vaccinated."

The scientists also measured the women's immune responses to two variants of concern: B117, which was first identified in Britain, and B1351, which was first identified in South Africa.

All three groups of women produced antibody and T-cell responses to both variants after vaccination, although their antibody responses were weaker against the variants, especially B1351, than against the original strain of the virus, according to the study.

"These women developed immune responses to the variants, although the asterisk is that the antibody responses were reduced several-fold," said Dr Dan Barouch, a study author and virologist at Beth Israel. (Dr Barouch and his colleagues developed the Johnson & Johnson vaccine, which was not included in this study.)

"Overall, it's good news," he added. "And it increases the data that suggests that there is a substantial benefit for pregnant women to be vaccinated."

The researchers also found that 14 per cent of pregnant women reported a fever after their second vaccine dose, compared with 52 per cent of non-pregnant women. They did not observe any severe complications or side effects.

The study will continue, with researchers monitoring women's longer-term immune responses. And larger epidemiological studies are still needed to confirm these lab-based results, Dr Collier noted.

In the second study, a research team from Northwestern University and the Ann and Robert H. Lurie Children's Hospital of Chicago examined the placentas from 200 women who gave birth between April 2020 and April 2021.

Eighty-four of the women had received either the Pfizer or Moderna vaccine during pregnancy, the remainder had not received any Covid-19 vaccine.

The placentas from vaccinated women were not any more likely to show signs of injury or abnormality than those from unvaccinated women, the researchers found.

"These data build upon the emerging data that's come out about these vaccines and their safety in pregnant people," Dr Miller said.

"These are translational data that suggests the placenta doesn't see any injurious impact of the vaccine. And that's really fantastic."

The findings have limitations, she acknowledged. Because the vaccines were only authorised recently, most of the women in the study were vaccinated in the third trimester of pregnancy, and many of them were healthcare workers, who were among the first people eligible for the shots.

Dr Miller and her colleagues are continuing to collect more data, including from patients who were vaccinated earlier in their pregnancies and who received the one-shot Johnson & Johnson vaccine.

"This is ongoing work," Dr Miller said. But they wanted to publish their preliminary data as soon as they had it, she added, "to help people make the best decision they can".