Virus World

Published August 14, 2023 in CMAJ

More than half had antibodies showing they had been infected at some point during the pandemic, equivalent to 16.4 million Canadian adults. A large-scale survey of Canadian adults selected at random across all 10 provinces found that virtually the entire population had antibodies against the virus that causes COVID-19 by summer 2022. Some 98.1 per cent had detectable antibodies against SARS-CoV-2, the virus that causes COVID, either through vaccination, a past infection, or both, according to the survey from Statistics Canada and partners. Over half, 53.9 per cent, had antibodies indicating they had been infected at some point during the pandemic — which translates to 16.4 million adults having caught COVID, whether they realized it or not. And that number might actually be higher because antibodies usually wane over time and can become undetectable through blood testing. StatCan, the Public Health Agency of Canada and Canada’s COVID-19 Immunity Task Force partnered on the surveys, in which participants complete an online questionnaire and send a blood sample that is analyzed for antibodies against SARS-CoV-2. The survey conducted between April and August 2022 yielded information showing that antibodies against SARS-CoV-2 have become widespread across the country. Other revealing data include:

• While nearly half (47.9 per cent) of those who had a past infection had tested positive at some point, more than two-in-five Canadians (41.7 per cent) who had caught COVID were unaware they had ever been infected.

• Over one-in-10 (10.3 per cent) who suspected they had been infected had antibodies but had never tested positive, either through a rapid antigen or polymerase chain reaction (PCR) test.

It’s more than 20 times the rate of infection indicated by a similar survey conducted between late 2020 and April 2021. At that point, just 2.6 per cent of Canadian adults had a past infection.

• Over a third (36.9 per cent) of those who never tested positive nor suspected they had been infected showed the presence of infection-acquired antibodies.

• Nearly nine-in-10 (88.3 per cent) who had had a past infection still had antibodies in summer 2022. And 56.4 per cent who had never tested positive but thought they might have caught COVID had detectable antibodies from a past infection.

• By early 2022, rapid tests were more common than PCR tests: 59.8 per cent of those who had a past infection used the self-administered rapid test, while 40.2 per cent were given a PCR test.

• Younger Canadians were more likely than older adults to have antibodies against COVID, likely due to taking fewer precautions against contracting the virus than seniors and those with underlying conditions. 

• Antibodies against a SARS-CoV-2 infection were highest from 14 to 90 days after an infection (89.9 per cent) and this percentage gradually decreased over time. However, more than two-thirds (67.7 per cent) still had detectable antibodies more than 180 days later.

• Though rates were still fairly high (roughly 50 per cent), fewer people with a disability or chronic condition had antibodies indicating an infection than those who didn’t report any conditions (57.7 per cent).

• Seroprevalence of antibodies from infection differed somewhat from province to province. B.C. was in the middle of the pack (51.5 per cent), while Alberta and Saskatchewan (both 57.4 per cent) had the highest infection rates, and Nova Scotia the lowest (46.5 per cent).

• Racialized, non-Indigenous Canadians (60.1 per cent) had higher rates of infection than non-racialized Canadians (51.8 per cent). Infection rates varied widely among different population groups: 53.2 per cent who identified as Chinese had a past infection, while those who are Black (80.7 per cent), Arab (78.1 per cent), Filipino (69.8 per cent) or Latin American (64.6 per cent) had significantly higher rates. (Data on Indigenous Canadians wasn’t collected among those who live on a reserve or First Nation settlement for this survey.)

• Rates of past infection were virtually identical among men and women, 54.3 per cent and 53.4 per cent, respectively.

• Despite the fact they’re generally at higher risk of severe outcomes, over half of Canadians 65-and-older who had antibodies from past infection were unaware they had been exposed to the SARS-CoV-2 virus.

The data on SARS-CoV-2 antibodies is from dried blood spot (DBS) samples collected from a self-administered finger prick. The survey was conduacted from April to August 2022 and was sent out to 105,998 Canadian adults. (About half responded to the questionnaire and roughly 30 per cent sent blood samples for analysis.)

More about how the data was collected and analyzed can be found at the StatCan detailed information page.

jruttle@postmedia.com

The number of confirmed COVID-19 cases attributed to the highly contagious Delta variant jumped 66 per cent in Canada this week.

Canada’s chief public health officer Dr. Theresa Tam said as of Friday there are just over 2,000 confirmed cases of the variant. Just three days ago, the Public Health Agency told The Canadian Press there were 1,187 confirmed cases. “The Delta variant is now in all provinces and at least one of our territories,” Tam said at the House of Commons health committee. “Of course, as with all coronavirus cases, we may not know every single case that has occurred in Canada, hence my warning of precaution and the need to get two doses of vaccine into as many people as possible.” The variant was first identified in India but has now become the dominant strain in the United Kingdom where it is spreading rapidly, mostly among unvaccinated people. It is believed to be at least 1.5 times as contagious as the Alpha variant first identified in the U.K. Alpha is, for now, the dominant variant in Canada, with more than 216,000 cases confirmed. Public Health England reported Friday the number of confirmed Delta cases in the U.K. had grown 80 per cent in the last week, to almost 76,000. British Prime Minister Boris Johnson had to delay plans to fully lift all public health restrictions next week, with plans to keep the final limitations in place at least another month. 

Tam says the variant is a concern because one dose of vaccine isn’t as effective against it and it underscores the need for Canada to keep picking up the pace on second doses. Two doses of the vaccines Canada is using are believed to offer very good protection against Delta, and even one dose has been found to be good at preventing serious illness. All provinces are warning against the existence of Delta. It is believed to be the driver of a major outbreak at the Kashechewan First Nation in northern Ontario, is involved in an outbreak at a Calgary hospital, and is worrying health officials in Waterloo, Ont., which is suddenly the province’s COVID-19 hot spot. Data on the variant in Canada, however, is limited and it hasn’t yet been added to the Health Canada website reporting variant cases. Only Alpha, Beta (the B. 1.351 variant identified in South Africa, and Gamma (the P. 1 variant first identified in Brazil), are listed on Canada’s website.  That site also hasn’t adjusted the names to use the Greek alphabet now preferred by the World Health Organization. A spokeswoman for Health Canada told The Canadian Press in early June that Delta would be added to it in “the coming weeks.” When asked again this week, the same answer was provided. Dr. Howard Njoo, the deputy chief public health officer, said data gathering on the variant hasn’t been easy and there is a lag in reporting. “There are obviously some issues we need to work on,” he said, noting there “are probably more cases” than we have seen reported. He said, however, the data provinces and territories do have suggest most of the Delta cases are being found in people who aren’t vaccinated.

A dog in the Canadian province of Ontario has died of H5N1 bird flu after contact with wild birds, health officials say. It’s believed to be the first time that a dog has tested positive for the new strain of the virus. A statement from the Canadian Public Health Agency said a domestic dog in Oshawa, a city in Ontario, tested positive for avian influenza after chewing on a dead goose. The dog developed clinical signs of bird flu and died a few days later. “Both the dog and goose were tested for H5N1 highly pathogenic avian influenza (HPAI) virus, and both were positive,” said Dr. Scott Weese, the Director of the Centre for Public Health and Zoonoses at the University of Guelph. “Sequencing of the virus at the National Centre for Foreign Animal Diseases was performed and the virus from both the dog and goose were the same, and were consistent with the H5N1 strain that’s circulating in wild birds and domestic poultry,” he said. It’s believed to be the first time that a dog has been infected with the new strain of H5N1, which emerged in late 2021. In 2004, a dog in Thailand died of a previous strain of H5N1 after eating a duck which was infected with the virus. In December, a cat at a poultry farm in southern France also tested positive for the new strain of H5N1. The cat became sick and was euthanized on December 23. “Based on the current evidence in Canada, the risk to the general public remains low and current scientific evidence suggests that the risk of a human contracting avian influenza from a domestic pet is minor,” the government said in a statement. Nonetheless, pet owners have been advised to not give any raw meat from game birds or poultry to pets – such as dogs and cats – and to not let them eat or play with wild birds.

Dr. Weese called the case “concerning but not surprising” and “not a doomsday scenario.” “It’s concerning because any spillover into mammals raises concerns about continued adaptation of this virus to spread outside birds,” he said. “It’s not surprising because when you have millions of infected birds internationally, it’s inevitable that domestic and wild mammals will be exposed.” A wide range of animals in Canada have tested positive for bird flu during the past year, including foxes, seals, dolphins, black bears, wild mink, porpoises and skunks, according to the National Centre for Foreign Animal Diseases. The global spread of H5N1 clade 2.3.4.4b – and the recent spread to a growing number of mammals – has raised concern about the possibility of a future variant which could lead to human-to-human transmission. So far, only a few human cases have been found after contact with infected birds. “The global H5N1 situation is worrying given the wide spread of the virus in birds around the world and the increasing reports of cases in mammals, including in humans,” Dr. Sylvie Briand, a WHO official, said on February 24. “WHO takes the risk from this virus seriously and urges heightened vigilance from all countries.” Last week, Chile reported that more than 1,500 sea lions are believed to have died of H5N1 bird flu, which followed the deaths of at least 3,500 sea lions in neighboring Peru. Chile also reported its first human case of bird flu on March 29.

The BCCDC says this year's vaccine appears to cut the risk of medically attended influenza-like illness by more than half, with an estimated vaccine effectiveness of 55 per cent. The BC Centre for Disease Control says early evaluation shows this year’s influenza vaccine formula is providing “substantial” protection against the wave of flu so far. “The earlier-than-usual influenza epidemic this season has enabled us to assess vaccine effectiveness earlier than usual,” BC CDC epidemiologist and Canadian Sentinel Practitioner Surveillance Network lead Dr. Danuta Skowronski said. “We will update our analyses in the new year, but these interim findings show a substantial reduction in the risk of influenza illness for vaccinated people who seek medical care.” Analysis for the period from Nov. 1 to Dec. 14 with samples from B.C., Alberta, Ontario and Quebec had cut the risk of medically attended influenza-like illness by more than half, with an estimated vaccine effectiveness of 55 per cent. An update from the BCCDC on Thursday also showed there had been no additional flu-related deaths among children and youth this season. Six people under the age of 18 have died so far this year, several due to suspected influenza-linked bacterial infections. That same update showed that the epidemic wave appears to be waning. About 16 per cent of samples over the week ending Dec. 17 came back positive — down from a high of 27 per cent during the week ending Nov. 26.

The BCCDC’s latest analysis shows the number of tests that came back positive for RSV for the week ending Dec. 17 jumped to 13 per cent, up from six per cent in the week ending Nov. 26. Test-positivity for both RSV (37 per cent) and influenza A (21 per cent) remains higher among children and youth younger than 18 than in the general population, the BCCDC said. According to the BCCDC, the H3 subtype of influenza A remains the dominant strain of influenza this year, accounting for more than 90 per cent of tested samples. The H3N2 subtype is often linked to more severe outbreaks and lower vaccine effectiveness, however, this year’s vaccine appears to be as good as or better than in previous years the subtype was dominant, the agency said. “Vaccine protection is especially important for people at higher risk of severe complications and for a health-care system that is managing the circulation of multiple respiratory viruses at the same time as we enter the holiday period,” Skowronski said. The province continues to recommend vaccines for anyone aged six months and older, and is urging anyone who feels unwell to stay home.

US CDC historical data on influenza vaccine effectiveness in past seasons available at: 

https://www.cdc.gov/flu/vaccines-work/past-seasons-estimates.html