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Vials of Pfizer's new respiratory syncytial virus (RSV) vaccine Abrysvo during manufacture.PFIZER/Reuters

Health Canada has approved the first prenatal vaccine against respiratory syncytial virus, a milestone that should mitigate the toll RSV takes on babies and Canadian pediatric hospitals every winter.

The new shot, which the regulator approved Thursday, is designed to be given to expectant mothers late in pregnancy so they can pass antibodies to RSV on to their newborns.

Protecting infants is critical because RSV, a common pathogen that usually causes nothing more than a mild cold, can lead to complications such as bronchiolitis and pneumonia in infants. The virus is the top cause of hospital admissions for babies younger than 1 in Canada.

Health Canada also approved the shot for adults aged 60 and older – the other demographic group vulnerable to severe illness caused by RSV. The new injection, called Abrysvo, is the second to be greenlit for the prevention of serious RSV disease in seniors.

Jesse Papenburg, a pediatric infectious diseases specialist and medical microbiologist at Montreal Children’s Hospital, predicted there would be a “tangible difference” in the pressure RSV places on pediatric hospitals if the maternal vaccine is widely available alongside nirsevimab, another new shot to shield babies against RSV.

Nirsevimab, which Health Canada authorized last April, is a once-a-season monoclonal antibody injection given to babies shortly after birth. Abrysvo and nirsevimab both offer passive immunity to RSV in the form of antibodies babies don’t make themselves. Typically, if an expectant mother received Abrysvo, her baby would not need nirsevimab.

“RSV is such a big burden that, if we were able to reduce the burden by 60 to 80 per cent, that would be big for pediatric hospitals,” Dr. Papenburg said. “That’s not hyperbole – [RSV] is the leading cause of hospitalizations in kids under 1 and the cause of so many emergency department visits.”

The clinical trial that led Health Canada and other international regulators to approve Abrysvo found it to be 81 per cent efficacious in preventing RSV-caused severe lower-respiratory tract illness that required medical attention in infants three months or younger. Efficacy waned as the babies got older.

The trial, which saw participants injected with the real vaccine or a placebo between 24 and 36 weeks of pregnancy, detected a possible link between the vaccine and elevated risk of preterm birth, although it wasn’t statistically significant. To lessen the possible risk, Health Canada followed the U.S. Food and Drug Administration’s lead and approved the shot for use between 32 and 36 weeks of pregnancy, when babies are closer to full term.

The challenge now, Dr. Papenburg said, will be getting adequate supply of both the maternal shot and the monoclonal antibody in Canada for a reasonable price, and then persuading parents to use them.

Despite being approved last spring, nirsevimab is not widely available in Canada because of supply shortages caused in part by high demand in the United States, where the Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices recommended the injection for all babies younger than eight months of age entering their first RSV season and for high-risk babies entering their second season.

The same U.S. panel recommended the new maternal RSV vaccine be given to all expectant mothers between 32 and 36 weeks of pregnancy. Canada’s National Advisory Committee on Immunization has not yet made recommendations on either new method of protecting infants from RSV.

Abrysvo-maker Pfizer said Thursday that it was still determining when the new shot would be available to Canadians. The company is “committed to bringing the supply of this medicine to Canadians as quickly as possible, likely later in the spring,” Andréa Mueller, Pfizer Canada’s primary care portfolio lead, said by e-mail.

She declined to reveal how Pfizer would price the shot in Canada, other than to say the company intended to work with policy makers to make it accessible.

When it comes to future uptake of the prenatal RSV shot, Darine El-Chaâr, a maternal-fetal medicine specialist at The Ottawa Hospital, believes it could be high because it’s designed to protect expectant mothers’ babies. (Dr. El-Chaâr has received payments from Pfizer for sitting on an advisory board for Abrysvo.)

Mothers-to-be “will take the one that protects their baby, but not the one that protects them,” Dr. El-Chaâr said. “All you have to look at is the [pertussis] vaccine uptake in Canada. It’s way higher” than the flu shot.

In 2021, the most recent year for which survey data are available from Statistics Canada and the Public Health Agency of Canada, 65 per cent of pregnant women received a shot to protect their babies from pertussis, or whooping cough.

By contrast, only 53 per cent of mothers-to-be in 2021 got a flu shot, which is recommended as a means of protecting women, who are at high risk of influenza complications during pregnancy, although the shot provides some protection to babies as well.

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