Modeling the seroprevalence and resurgence of COVID-19 in Canada

Despite increased vaccine distribution and a significant decrease in the number of COVID-19 cases in Canada, the future of the pandemic remains a major concern, based on the possibility of a short-term as well as long-term resurgence and understanding the future of immunity against Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). It is not clear whether the Canadian population will achieve herd immunity or whether there is a risk of a 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 calculations of the distribution of immunity.

Determination of COVID-19 Immunity in the Canadian Population Due to Infection and Vaccination

A recent study published on the medRxiv * preprint server by the COVID-19 Immunity Task Force (CITF) analyzing various seroprevalence studies determined the immunity of the Canadian population to COVID-19 as of May 31, 2021, as follows: 5, 4% due to infection and 44.9% due to infection and vaccination.

“CITF is the only working group to our knowledge that has attempted to quantify seroprevalence in the Canadian population. “

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

The seroprevalence as a percentage of the total population for the 10-year age groups is shown with a color intensity corresponding to the age group, assuming that there is no relaxation in. The red zone is the sum of the susceptible classes which have been exposed to the virus, either by natural infection or by reduction of the vaccinated classes. The blue and green regions show the populations of the classes vaccinated at the first and second dose, respectively. The total population with some immunity (the top of the red region) is equal to the vertical sum of the three regions blue, red, and green. The top row shows a decrease in immunity of one year between consecutive classes; the middle row is in decline of three-year immunity; the bottom row is not declining immunity. Columns from left to right represent vaccines 1 to 3, respectively.

Results show most Canadians will not have sufficient immunity against COVID-19 to prevent a resurgence of falls

The study estimated that coinciding with the end of the vaccination program, by the end of summer 2021, 60 to 80% of the Canadian population will achieve some immunity against COVID-19. However, model results show that this level of immunity is not sufficient to prevent a resurgence in the fall of 2021. In addition, the timing and severity of a COVID-19 resurgence may vary in magnitude due to several factors, including the rate of decline in immunity, the relaxation of non-pharmaceutical interventions such as social distancing, the transmissibility of the variants of concern of SARS-CoV-2, and the level of protection offered by vaccines against the disease. SARS-CoV-2 infection and disease severity.

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

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

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

To summarize the results, the researchers found that the magnitude of the COVID-19 resurgence in fall 2021 – winter 2022 depends on the relaxation of non-pharmaceutical interventions and social distancing measures, the rate of decline of the immunity, transmissibility of the virus and the effectiveness of vaccines against infections and diseases. In order to prevent a large-scale resurgence in Canada, a booster vaccination and / or reintroduction of public health measures may be necessary.

“As immunity is lost, a vaccine booster campaign or the reintroduction of public health mitigation will be necessary. “

*Important Notice

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