Canada’s path to hybrid immunity during pandemic charted in new study

A large majority of Canadians now bear the signs of COVID-19 in their blood in two ways: through vaccination and through infection, a study of antibody levels across the country has found. The findings dictate Canada’s progression to hybrid immunity.

The study, supported by the COVID-19 Immunity Task Force and published Monday in the Canadian Medical Association Journal, combines data from seven separate studies including some 700,000 blood samples. It provides an overview of how the pandemic unfolded across the country during a three-year period starting in the spring of 2020.

With the EG.5 version of the virus now designated as a variant of interest by the World Health Organization, the study carries lessons for the next phase of the COVID-19 response as well as for pandemics yet to come – namely, the need for vaccination, particularly for vulnerable groups who haven’t yet had COVID-19.

“The most important take home message is that the vaccine compliance of Canadians was amazing,” said Bruce Mazner, a senior scientist at the McGill University Health Centre in Montreal and associate scientific director for strategy with the task force.

That compliance, he said, helped to keep the infection rate relatively low, even after vaccines were introduced in 2021 and public-heath measures began to ease.

However, that situation changed after the first wave of infections from the Omicron variant in early 2022. The variant frequently broke through vaccine-induced immunity. But among vaccinated individuals, cases were milder and hospital stays were less likely, preventing what might otherwise have been a tragically high number of excess deaths during the second half of the pandemic in Canada.

Dr. Mazner and other co-authors on the study estimate that by March of this year, some 80 per cent of individuals under 25 had already been infected with COVID-19. The proportion is lower for older age groups. Only about 60 per cent of Canadians aged 60 and older had been infected by spring 2023.

All of the estimates are based on antibodies that are generated by the immune system in response to proteins from SARS-CoV-2, the virus that causes COVID-19, and spotted in donated blood samples. Among them are antibodies that attach to the spike protein of the virus. Because all the of the vaccines that were approved in Canada target the spike, the presence of those antibodies could be caused by exposure to through infection or through vaccination.

Also measured were antibodies that stick to the nucleocapsid proteins found inside the virus. Their presence can only be the result of a COVID-19 infection.

Hybrid immunity

Most Canadians now carry two kinds of antibodies related to the COVID-19 pandemic. Antibodies that target the virus spike protein were (mostly) first acquired through vaccination. Those that target the nucleocapsid protein inside the virus are products of infection.

Percentage of individuals sampled

Pre-vaccine

Vaccine roll-out

Omicron waves

Spike protein antibodies

Nucleocapsid antibodies

the globe and mail, Source: CMAJ/Aug. 14, 2023

Hybrid immunity

Most Canadians now carry two kinds of antibodies related to the COVID-19 pandemic. Antibodies that target the virus spike protein were (mostly) first acquired through vaccination. Those that target the nucleocapsid protein inside the virus are products of infection.

Percentage of individuals sampled

Pre-vaccine

Vaccine roll-out

Omicron waves

Spike protein antibodies

Nucleocapsid antibodies

the globe and mail, Source: CMAJ/Aug. 14, 2023

Hybrid immunity

Most Canadians now carry two kinds of antibodies related to the COVID-19 pandemic. Antibodies that target the virus spike protein were (mostly) first acquired through vaccination. Those that target the nucleocapsid protein inside the virus are products of infection.

Percentage of individuals sampled

Pre-vaccine

Vaccine roll-out

Omicron waves

Spike protein antibodies

Nucleocapsid antibodies

the globe and mail, Source: CMAJ/Aug. 14, 2023

The team combined data from seven separate task force-funded studies to arrive at a broader picture of how immunity levels change in Canada. Like a series of snapshots, it records the story of the pandemic in three acts: The initial period before vaccines were available, the rollout period and the rise of Omicron.

During the first phase, infections were quite low, with only 0.3 per cent of the population exposed to the virus by July of 2020. A year later, once governments had enough doses to distribute vaccines widely, spike protein antibodies climbed steeply in blood samples.

In contrast, nucleocapsid antibodies only began to show up strongly with the rise of the variants, and then dramatically as Omicron took over.

By standardizing how the data are represented, the study also allows for comparison across region and age groups. For example, it suggests there was a significant lag in vaccine rates in western provinces as well as steeper infection rates from Omicron in the region a few months later.

“We probably need to think about our interventions and responses that account for that variation because it’s not one size fits all,” said David Buckeridge, a professor at McGill’s School of Population and Global Health who leads data management and analysis for the task force.

Despite its size, the study also comes with limitations, Dr. Buckeridge said. For example, it cannot disentangle the effects of multiple infections or of varying numbers of vaccine boosts among individuals.

Prabhat Jha, a co-author and global health researcher at St. Michael’s Hospital in Toronto who contributed some of the earliest data to the study said the results highlight how closely the vaccination campaign was followed by the Omicron wave. The combination of the two may have helped Canada to weather the latter part of the pandemic better than many countries, he said.

Dr. Mazner said that the prevalence of nucleocapsid antibodies continued to flatten into June, which suggests there is a minority of individuals, mainly among older and more vulnerable groups, who have managed to avoid getting COVID-19 for the first time. He said that a new round of vaccination would help to keep those groups protected.

“I think there needs to be a fall campaign,” he said. “It could be aimed at the general population, but it’s really got to target those that are vulnerable – individuals who are over 60, who are in long term care, or individuals with chronic diseases.”