January 2022 | Natural antibody wane reduces immunity from infection, reinforcing importance of vaccination
In this month’s analysis, we modelled the effect of waning antibodies, known as seroreversion, to account for the natural loss of antibodies that occurs through time. Our calculations, based on the most recent research on time to antibody wane, found that as of November 30, 2021, an estimated 1 in 21 Canadians (4.7%) still had detectable antibodies due to a previous infection. This proportion ranged from a low of 1 in 69 (1.5%) in the Atlantic provinces to a high of 1 in 13 (7.7%) in Alberta. The data were prior to widespread exposure to the Omicron variant.
Notably, this estimate is lower than our previous month’s estimate of seroprevalence (7.8%), which did not incorporate seroreversion rates, suggesting that antibody wane has a profound effect on the accumulation of people with detectable antibodies in the Canadian population. This being the case, it is evident that vaccination is crucial to increase the level of immunity throughout the population.
Double the number of reported COVID-19 infections across Canada
Accounting for seroreversion also made it possible to estimate the number of true infections in the population prior to Omicron. We assumed that seroprevalence due to infection results from infections 7 to 10 days before the serosurvey or earlier. Although 1.8 million cases of COVID-19 had been reported by November 21, 2021, our modelling estimates that there had been 3.1 million infections in Canada.
Our analysis combined seroprevalence results from 35 studies with data on confirmed cases of SARS-CoV-2 infection to determine the proportion of Canadians infected with COVID-19.