A new study suggests that 75 percent of Canadians had immunity to COVID-19 by March 2023.
The study said that during the first months of COVID, “despite the burden on Canadian society and health systems, rates of symptomatic infection remained low,” specifically 1.6 percent of the Canadian population had a confirmed case of the illness.
The arrival of Omicron caused an “unprecedented increase in the number of infections,” said the study, even with “high vaccine coverage.”
“The COVID-19 pandemic defied expectations about immunity arising from infection and vaccination.”
The study suggests that after the initial epidemic wave of COVID and before vaccines, less than 0.3 percent of the population had antibodies by July 2020, but that immunity due to infection increased at an average of 0.39 percent per month from January 2021 to Dec. 15, 2021.
By November 2021, according to this study, 9 percent of the population had antibodies to SARS-CoV-2.
“Despite high vaccine coverage in Canada, all previous increases in seroprevalence were far surpassed by the increase caused by the Omicron variant,” said the study.
After six months of the variant Omicron in circulation, the study suggests nearly half of the population had antibodies to the virus, 47 percent by mid-June 2022. By this time, the study data indicated immunity was increasing at a rate of 6.4 percent per month between Dec. 15, 2021, and July 2022.
“After a short interval of slower increases in the summer, infection-acquired seroprevalence continued to rise rapidly in the latter half of 2022 until it started to plateau at about 76% ... in March 2023,” said the study.
Estimates of seropositivity for anti-S IgG antibodies, reflecting previous vaccination or infection, were between 70 percent and 95 percent in summer 2021, depending on the source of the samples, added the study. It attributed the rise in these particular antibodies to vaccination.
Antibodies due to a previous infection ranged from 64 percent in November 2022 in Nova Scotia to 74 percent in Alberta. The study suggested that the Atlantic provinces were a “notable exception,” with infection-acquired immunity below other areas in Canada, which the study indicates was “possibly a result of the policies restricting travel into and out of the Atlantic provinces.”
The study indicated that the rate of immunity increased more quickly in younger age groups after the identification of the Omicron variant. By mid-June 2022, 57 percent of individuals younger than 25 had antibodies to COVID as a result of previous infection, while 40 percent of those aged 40 to 59 years had antibodies. Even among those aged 60 and older, 25 percent had infection-acquired antibodies.
“Except in adults older than 65 years, seroprevalence was between 70% and 80% by March 2023,” said the study. In other words, by September 2022, “most of the population had detectable antibodies from infection.”
“This rapid rise in seropositivity owing to infection occurred despite the roll-out of vaccines in 2021,” said the study.
The study suggested the rapid rise in infection seen in many countries with high vaccine coverage was “explainable, in part, by the limited effectiveness of the ancestral SARS-CoV-2 strain–based vaccines in preventing infection with the Omicron variant.”
The study’s authors said that the results suggest a “need for Canada to approach pandemic-related problems in a multidisciplinary manner, aligning expertise in public health and pandemic responses with input from the broader scientific community.”
While the study suggested there is a “need” in a pandemic to “rapidly develop vaccines that target a pathogen broadly,” as well as control spread and reduce the impacts on the population, “it is difficult to fully control pathogens that mutate rapidly through vaccination alone.”
The study maintained that vaccination “markedly decreased the severity of illness from those infected with SARS-CoV-2.
