Getting vaccinated against COVID-19 during pregnancy passes along protection against infection and hospitalization to newborns, a Canadian study says. A pregnant woman wearing a face mask and gloves holds her belly as she waits in line for groceries, at St. Mary's Church in Waltham, Mass., Thursday, May 7, 2020. THE CANADIAN PRESS/AP-Charles Krupa

Getting vaccinated against COVID-19 during pregnancy passes along protection against infection and hospitalization to newborns, a Canadian study says.

The research, published in the BMJ (British Medical Journal) on Wednesday evening, found that protection against COVID-19 for infants was most effective when mothers got their second or third dose of mRNA vaccine during pregnancy.

A booster shot during pregnancy bolstered protection against the Omicron variant in particular, said the study authors from the Canadian Immunization Research Network.

Canadian infectious diseases specialists, obstetrician-gynecologists and immunologists have long urged pregnant people to stay up to date on their COVID-19 vaccinations — including boosters — because they are at higher risk of serious illness if they become infected. That in turn can harm the fetus, they say.

This study shows that getting vaccinated during pregnancy is also "of great benefit to the infant" after they're born, said Dr. Deborah Money, a professor of obstetrics and gynecology who specializes in infectious diseases at the University of British Columbia.

Money, who was not involved in the study, said the Canadian findings are consistent with existing international research.

The researchers found that maternal vaccination protected the baby against COVID-19 infection for the first eight weeks after they were born.

Dawn Bowdish, an immunology professor at Hamilton's McMaster University who was not involved in the study, noted those are important weeks with "major developmental stuff going on."

"One of Mom's best gifts for that baby (has) always been her antibodies," said Bowdish, noting that booster shots for pertussis (whooping cough) and influenza are routinely recommended for pregnant women so that their babies are born with some immunity to those diseases.

There is currently no COVID-19 vaccine available for babies until they reach six months of age.

The researchers looked at data for 8,809 infants born in Ontario who received COVID-19 PCR tests between May 31, 2021, and Sept. 5, 2022, — a time period when the Delta variant was widely circulating at first, followed by the Omicron variant later.

They linked that information to the number of mRNA COVID-19 vaccinations the mothers had during pregnancy — one, two or three doses — as well as unvaccinated mothers.

The data came from the independent non-profit health data organization ICES.

The researchers found that a second vaccine dose during pregnancy provided strong protection against the Delta variant for the baby. The study showed 95 per cent effectiveness against Delta infection and 97 per cent effectiveness against hospital admission due to Delta infection.

For the Omicron variant, the mother's second dose provided newborns with moderate protection: 45 per cent protection against Omicron infection and 53 per cent protection against hospitalization due to the Omicron variant.

But protection for newborns against Omicron increased substantially when mothers had a third dose of vaccine during pregnancy, with 73 per cent effectiveness against infection and 80 per cent effectiveness against hospital admission.

According to ICES, fewer than half of pregnant women in Ontario have had three doses of COVID-19 vaccine.

A spokeswoman for the Public Health Agency of Canada said it does not have information on COVID-19 vaccine coverage among pregnant women but that it plans to collect it in the future.

Lead author Sarah Jorgensen said that a lot of women are unsure if it's worth getting a booster while they're pregnant.

"I think our study gives them more certainty around the benefit to the infant," said Jorgensen, who is a pharmacist studying perinatal epidemiology.

The researchers also found that getting a COVID-19 vaccine dose during the last trimester of pregnancy could increase protection for the baby.

But both Money and Bowdish emphasized that the most important factor is how long it's been since the mother's last dose due to waning immunity.

"All the data, including research from my own group, shows that to really protect you from symptomatic (COVID-19) infections, you can't be more than six months out when you encounter this virus," said Bowdish.

If it's been more than six months since your last dose, "we're suggesting (you) just get vaccinated regardless of where you are in your pregnancy," Money said.

In some cases, two doses of vaccine could be warranted over the course of a nine-month pregnancy, she said.

"If you, say, got vaccinated very early in pregnancy, you could boost later because you can boost within as short as three months," Money said.

The study authors noted that their research has limitations.

One is that mothers also pass along antibodies to their infants through breastfeeding, so it's possible that played a role that couldn't be measured. Another is that they only had data on babies who were tested for COVID-19 with lab-confirmed PCR tests, so they couldn't include any home rapid antigen test results.

This report by The Canadian Press was first published Feb. 8, 2023.

Canadian Press health coverage receives support through a partnership with the Canadian Medical Association. CP is solely responsible for this content.

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