This is a summary, written by members of the CITF Secretariat, of:

Brown PE, Fu SH, Newcombe L, Tang X, Nagelkerke N, Birnboim H C, Bansal A, Colwill K, Mailhot G, Delgado-Brand M, Tursun T, Qi F, Gingras AC, Slutsky AS, Pasic M D, Companion J, Bogoch II, Morawski E, Lam T, Reid A, Jha P, Ab-C Study Collaborators. Hybrid immunity from SARS-CoV-2 infection and vaccination in Canadian adults: cohort study. medRxiv 2023.12.27.23300588; doi:

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

A CITF-supported study, published in medRxiv and undergoing peer review at eLife Sciences, highlighted that to maintain population-level hybrid immunity up-to-date vaccination coverage is required, including among those recovering from SARS-CoV-2 infection. The study was led by Dr. Prabhat Jha (University of Toronto) in collaboration with other Action to beat Coronavirus (Ab-C) study collaborators. These results remain relevant to the current JN.1 viral wave that accounts for most new infections.

Key findings:

  • After receiving at least three doses of vaccine, adults who were last infected more than six months prior to testing showed a notable and continuous decrease in anti-spike antibody levels for nine months post-vaccination. In contrast, those who had been infected within the previous six months experienced a more gradual decline in anti-spike antibody levels.
  • Declines in anti-spike antibody levels were similar by sex, age group (15-59 years or 60+ years), and ethnicity (including visible minorities and Indigenous populations). Recent vaccination within the last six months attenuated declines in spike levels from older infections.
  • Near the end of 2022, about 35% of adults above age 60 had had their last vaccine dose more than six months prior and about 25% remained uninfected. The cumulative incidence of SARS-CoV-2 infection rose from about 11% before Omicron to 78% by December 2022. However, the COVID-19 weekly death rate during the BA.2/5 waves was less than half of that during the BA.1/1.1 wave, implying a protective role for hybrid immunity.
  • There were marked increases in infection among younger (18-59 years) and older (60+ years) mostly vaccinated adults, rising from about 11% in each age group by August 2021 to about 86% and 75%, respectively, by December 2022.
  • Anti-spike antibody levels were higher in infected than in uninfected adults, regardless of vaccination doses.
  • In a convenience sample of 39 adults, all 32 vaccinated adults had positive spike T-cell responses. The T-cell titers and spike antibody levels were correlated.

From May 2020 to December 2022, serial assessments (each of ∼4000-9000 adults) were conducted examining SARS-CoV-2 antibodies within a mostly representative Canadian cohort drawn from a national online polling platform. These covered the periods from March 2020 to December 2021 when Canada faced waves of ancestral, Alpha, and Delta variants of SARS-CoV-2; January-March 2022 during the Omicron BA.1/1.1 wave; and April-December 2022 during the Omicron BA.2 and BA.5 waves. Adults, most of whom were vaccinated, reported viral test–confirmed infections and mailed self-collected dried blood spots to a central lab. Samples underwent highly sensitive and specific antibody assays to spike and nucleocapsid protein antigens, the latter antibody response triggered only by infection. Cumulative SARS-CoV-2 incidence prior to the Omicron period and during the BA.1/1.1 and BA.2/5 waves was estimated and compared to the overall COVID-19 death rates reported by national agencies.