As the respiratory illness season begins, cases of respiratory syncytial virus (RSV) cases are rising in Canada. But a new tool — nirsevimab, a protective antibody for infants — offers hope for prevention, though it is currently available only in some provinces.

The latest data from Health Canada, covering the week of Nov. 24 to Nov. 30, shows RSV cases are already climbing, with infants under one year old making up the largest proportion of infections.

RSV is a common and highly contagious respiratory virus that typically causes mild symptoms, but in infants, it can lead to serious complications such as bronchiolitis and pneumonia, which may require hospitalization or even result in death, according to Health Canada.

Early infection has also been linked to long-term health issues, including persistent wheezing later in childhood.

“Respiratory syncytial virus can affect everyone, but more consistently, it severely affects children less than five years of age,” explained Dr. Darine El-Chaar, a maternal-fetal medicine physician at the Ottawa Hospital.

“A little baby has an immunity gap and doesn’t have the strongest immunity system to protect themselves, so they are more vulnerable and sensitive to get really sick,” she said.

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Parents may be more aware of the dangers of RSV following the surge in infections two years ago, which occurred after COVID-19 lockdowns left some children unexposed to the virus and more vulnerable.

Nearly half (49.8 per cent) of RSV-related hospitalizations recorded in Canada from 2021 to 2022 were in patients under six months of age.

While last year’s respiratory illness season in Canada was less severe than the year before, RSV is still a serious threat, especially for infants.

That’s why health professionals, like el-Chaar, are relieved to have two treatment options available to protect infants this respiratory illness season — nirsevimab and Abrysvo (a vaccine given to pregnant people).

“It’s really an exciting time for RSV treatment,” el-Chaar said. “So we certainly did have strategies for RSV beforehand, but they were limited to high-risk infants.”

The drug previously used to protect high-risk infants during the RSV season was palivizumab, a monoclonal antibody given monthly.

“Nirsevimab, it’s a monoclonal antibody. Let’s call it the cousin of palivizumab. The difference is that it’s a one-time injection instead of monthly for our babies and it can last up to eight months and ten months so it can protect them during an RSV season,” el-Chaar said.

Approved by Health Canada in April 2023, nirsevimab is a long-acting monoclonal antibody that provides a single-dose solution for preventing RSV in newborns and infants up to 24 months old.

It works by blocking the virus’s ability to enter cells, offering 50 times more neutralizing activity than palivizumab, according to Health Canada.

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Nirsevimab starts working right away and offers longer-lasting protection, especially in the first few months of life when infants are most at risk for RSV, Health Canada said.

Palivizumab offers shorter protection.

And then there’s the vaccine, Abrysvo (also called RSVpreF), which was approved by Health Canada in December 2023, for use on pregnant people in the third trimester (between 32 and 36 weeks gestation).

The vaccine, introduced in May 2024, provides infants with passive protection through transplacental antibody transfer from the mother to the fetus.

“So we vaccinate the mom to possibly transfer antibodies to the newborn who is born with that protection that can last for six months,” el-Chaar said.

Although there are three forms of treatment for RSV, nirsevimab is preferred over palivizumab and the Abrysvo vaccine, Health Canada said.

In May 2024, the National Advisory Committee on Immunization (NACI) began recommending nirsevimab as the preferred option for preventing severe RSV disease in infants.

NACI’s goal is to eventually establish a universal RSV prevention program using nirsevimab for all infants, starting with high-risk groups.

“Program introduction could occur in stages depending on access, cost-effectiveness, and affordability of available options,” NACI said in its media release.

Although NACI is recommending the antibody, not all provinces can provide it this season.

“There are differences across different provinces because of the way our health-care systems are,” el-Chaar said. “Quebec was very early to have a strategy to use nirsevimab for all of its province, for all its babies born this season”

“And Ontario has a clear guideline that was published in late summer clearly delineating our strategy of using nirsevimab mainly, but also actually offering the vaccine for patients who choose to make that their preferred option and that has been going since late October.”

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In British Columbia, monoclonal antibody medications (palivizumab and nirsevimab) will not be available for most infants in the province during the 2024 to 2025 RSV season, the province said.

Currently, these medications are available only to certain infants and young children who meet specific criteria, such as young, premature babies with chronic medical conditions.

Alberta is working to secure nirsevimab, but the manufacturer has been unable to guarantee supply, a spokesperson from the Alberta Minister of Health’s office told Global News Wednesday. In the meantime, the province said it will continue to offer palivizumab to high-risk infants during the 2024 to 2025 RSV season.

Nirsevimab is not available this RSV season in Saskatchewan, but palivizumab is eligible for infants and children who meet the criteria.

Manitoba will not provide nirsevimab this season. However, as part of its high-risk infant program, the province will use palivizumab, a government spokesperson told Global News on Thursday.

Both Ontario and Quebec now have provincially-funded programs that will offer the newer monoclonal antibody, nirsevimab, to all babies experiencing their first RSV season.

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Coverage of nirsevimab is also now available in Nunavut, the Northwest Territories and Yukon.

In Nova Scotia, the province said a limited supply of nirsevimab will be available for high-risk children in the 2024 to 2025 season.

Nirsevimab is not currently publicly funded or available in New Brunswick or Newfoundland and Labrador.

Although the use of nirsevimab is still in its early stages, as more provinces begin rolling out strategies for its use, el-Chaar hopes to see RSV cases start to decline with these improved treatment options.  In fact, she said that numbers could start to decline this RSV season.

“We hope to see the effects of these products with good uptake… we should see a downgrade reduction of RSV infection. We won’t know now, we will know in a month or two, but I think we will see the effect,” she said.