Fall COVID-19 vaccine cut illness risk by half overall

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The fall COVID-19 vaccine cut the risk of COVID-19 illness by about half overall, and by 67 per cent for those with a previous infection, according to new research from the Canadian network that has long tracked the performance of the annual flu shot.

This is the first time the Canadian Sentinel Practitioner Surveillance Network, which is comprised of hundreds of primary care providers in British Columbia, Alberta, Ontario and Quebec, produced mid-season estimates of the effectiveness of immunization against COVID-19.

For people with hybrid immunity against COVID-19 – meaning they’ve had a jab and an infection – the shot targeting the Omicron XBB.1.5 variant performed slightly better against COVID-19 than the flu shot did against influenza.

The network estimated the influenza vaccine was 63 per cent effective against influenza A(H1N1), the dominant strain this season, and 40 per cent effective against the A(H3N2) subtype. That’s a “decent” mark by historical standards, said Danuta Skowronski, a physician epidemiologist with the British Columbia Centre for Disease Control and leader of the network’s research.

By comparison, in 2019-20, the last winter before the pandemic, the flu shot was estimated by the end of the season to have been 43 per cent effective against A(H1N1), 50 per cent effective against A(H3N2) and 65 per cent effective against influenza B. H1N1 was the most common strain that winter.

“Over all, the message is there’s good protection for those who received the autumn 2023-2024 influenza vaccine,” Dr. Skowronski said.

The Canadian Sentinel Practitioner Surveillance Network’s decision to track the effectiveness of both shots in tandem is another sign that physicians and epidemiologists are treating SARS-CoV-2, the virus that causes COVID-19, as a regular feature of winter virus season.

“Both influenza and SARS-CoV-2 are highly changeable viruses. They’re unique in that way, compared to other vaccine-preventable diseases,” Dr. Skowronski said. The network’s leaders felt a duty to gauge the effectiveness of both vaccines, given that public-health officials urged Canadians to receive them at the same time, she added.

The network’s leaders measure flu shot effectiveness by having several hundred family doctors and nurse practitioners in Canada’s four largest provinces test patients who come in with flu-like symptoms to determine if they have a lab-confirmed case of influenza. Researchers then look to see what share of each group – positive or negative for flu – was vaccinated, and compare the results to estimate vaccine effectiveness. The network used the same method to judge the COVID-19 shot.

The surveillance effort doesn’t capture mild cases of either virus that don’t require medical attention, nor does it measure how well the shots perform against severe illness that puts people to hospital.

The Canadian network’s estimates of flu shot effectiveness jibe with those published earlier in the season in Alberta, where researchers were able to link data on immunizations, lab tests for influenza, and visits to clinics and hospitals to produce estimates of flu-shot effectiveness in near real time.

That group estimated the flu shot in Alberta to be 61 per cent effective against the widely circulating A(H1N1), 49 per cent effective against the less-common A(H3N2) and 75 per cent effective against influenza B, which typically peaks later in the season.

Real-time estimates are useful because “there’s a lot of people who are just not sold on flu shots,” said Jeff Kwong, associate director of the Centre for Vaccine Preventable Diseases at the University of Toronto. He is one of the authors of a study based on the Alberta data.

Dr. Kwong hopes more people will get the flu shot if they hear early in the season that it provides solid protection. In seasons like 2014-15, when the overall effectiveness of the shot was an abysmal 9 per cent, early data would allow hospitals and public health to prepare, he added.

The 2023-24 influenza season has unfolded more typically than any season since COVID-19 struck, said Allison McGeer, an infectious disease consultant at Toronto’s Mount Sinai Hospital. “On balance, it’s doing what you would expect it to do, which is getting back to normal postpandemic.”

Last season, pediatric hospitals were overwhelmed as several viruses rebounded after COVID-19-control measures were lifted. In the fall and winter of 2022-23, there were 2,444 hospital admissions linked to influenza for children younger than 4, up from 33 such admissions the season before, according to a report the Canadian Institute for Health Information released Thursday. That figure was a return to prepandemic levels.

The institute also found that, among children younger than 4, hospital admissions doubled for lower respiratory infections last season, while hospital stays for COVID-19 reached 3,310, a 43-per-cent increase over the season before.

Editor’s note: A previous version of this article incorrectly stated that the fall COVID-19 vaccine cut the risk of COVID-19 illness by half for people who had not caught the virus before. The fall vaccine cut the risk of COVID-19 illness by half overall. This version has been updated.