Modeling of COVID-19 seroprevalence and resurgence in Canada

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Despite increased vaccine distribution and a significant decrease in the number of COVID-19 cases in Canada, the future of the pandemic is still a major concern, based on the possibility of a resurgence in the short term as well as long term and an understanding of the future of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) immunity. It is not clear if the Canadian population will reach herd immunity or if there is a risk of future resurgence of the virus in the Fall or Winter.

A crucial step that will help answer these questions is the determination and quantification of immunity against SARS-CoV-2 in the population. Seroprevalence studies can inform immunity distribution calculations.

Determining COVID-19 immunity in the Canadian population due to infection and vaccination

A recent study posted to the medRxiv* preprint server by the COVID-19 Immunity Task Force (CITF) analyzing various seroprevalence studies determined the Canadian population’s COVID-19 immunity as of May 31, 2021, as follows: 5.4% due to infection and 44.9% due to infection and vaccination.

“The CITF is the only working group that we know of that has attempted to quantify seroprevalence in the Canadian population.”

In this study, researchers used a mathematical model to determine immunity distributions by age, from infection, and from vaccination in the Canadian population. The model uses daily COVID-19 incidence data up to June 27, 2021, and incorporates actual coverage up to June 27, 2021, and projected coverage up to September 2021 of the first and second COVID-19 vaccine doses. The model was then used to quantify immunity distributions from January 2020 to March 2022, based on various assumed characteristics of the COVID-19 vaccines, such as protection from infection and protection from severe disease against different variants of concern and varying rates of waning immunity.

Seroprevalence as a percent of the total population for 10 year age classes are shown with colour intensity corresponding to age class, assuming no relaxation in . The red region is the sum of a susceptible classes that have been exposed to the virus, either from natural infection or through waning from the vaccinated classes. The blue and green regions show the populations of the first and second dose vaccinated classes respectively. The total population with some immunity (the top of the red region) is equal to the vertical sum of the three blue, red, and green regions. The top row is waning of immunity by one year between consecutive classes; the middle row is waning of immunity of three years; the bottom row is no waning of immunity. Columns left to right represent vaccines 1 to 3, respectively.

Seroprevalence as a percent of the total population for 10 year age classes are shown with colour intensity corresponding to age class, assuming no relaxation in . The red region is the sum of a susceptible classes that have been exposed to the virus, either from natural infection or through waning from the vaccinated classes. The blue and green regions show the populations of the first and second dose vaccinated classes respectively. The total population with some immunity (the top of the red region) is equal to the vertical sum of the three blue, red, and green regions. The top row is waning of immunity by one year between consecutive classes; the middle row is waning of immunity of three years; the bottom row is no waning of immunity. Columns left to right represent vaccines 1 to 3, respectively.

Results show that most Canadians will not have sufficient COVID-19 immunity to prevent a Fall resurgence

The study estimated that coinciding with the end of the vaccination program, by late Summer 2021, 60 − 80% of the Canadian population will gain some immunity to COVID-19. However, the findings from the model show that this level of immunity is not enough to prevent a Fall 2021 resurgence. Furthermore, the timing and severity of a COVID-19 resurgence may vary in magnitude due to multiple factors, including the rate of waning immunity, relaxation of non-pharmaceutical interventions like social distancing, the transmissibility of SARS-CoV-2 variants of concern, and the level of protection offered by vaccines against SARS-CoV-2 infection and severity of disease.

“Our model predicts that 60 to 80% of the Canadian population will have some immunity to SARS-CoV-2 by the end of the vaccination campaign in late Summer 2021.”

Booster vaccinations and/or public health measures needed to prevent major COVID-19 resurgence in Canada

Experts think that there is a threat of COVID-19 resurgence in Canada in Fall 2021. To better understand the possibility and severity of this threat, researchers quantified the level of protection in the population using an age-structured model that includes infection, vaccination, and waning immunity, which helps estimate the COVID-19 immunity distribution in the Canadian population.

To summarize the findings, the researchers found that the magnitude of COVID-19 resurgence in Fall 2021 – Winter 2022 is dependent on the relaxation of non-pharmaceutical interventions and social distancing measures, rate of waning immunity, the transmissibility of the virus, and the effectiveness of the vaccines against infection and disease. In order to prevent a large-scale resurgence in Canada, booster vaccination and/or re-introduction of public health measures may be needed.

“As immunity is lost, a vaccine booster campaign or the re-introduction of public health mitigation will be required.”

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:

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