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

Brown PE, Fu SH, Bansal A, Newcombe L, Colwill K, Mailhot G, Delgado-Brand M, Gingras AC, Slutsky AS, Pasic M, Companion J, Bogoch II, Morawski E, Lam T, Reid A, Jha P; Ab-C Study Collaborators. Omicron BA.1/1.1 SARS-CoV-2 Infection among Vaccinated Canadian Adults. N Engl J Med. 2022 May 18. doi: 10.1056/NEJMc2202879.

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

In a letter published in the New England Journal of Medicine, the CITF-funded Ab-C Study led by Dr. Prabhat Jha, from the University of Toronto, quantified SARS-CoV-2 incidence during the Omicron (BA.1/1.1) wave among Canadian adult members of the Angus Reid Forum. They found that infection-acquired antibodies rose from 11.2% pre-Omicron to 36.9% during the Omicron era. The study used self-collected dried blot spot samples (DBS) submitted from January 24 to March 15, 2022.

The study quantified SARS-CoV-2 incidence and the contribution of prior infection and concurrent vaccination towards protection in 5,031 Canadian adults. The increase noted was in the nucleocapsid (NP) positivity, indicative of infection-induced antibodies.

Key findings

  • Among 3,468 adults testing negative for infection (NP antibodies) before the Omicron wave (phase 3, August 15 – October 15, 2021), 1,040 turned NP positive in phase 4 (January 24 – March 15, 2022). Among 91 unvaccinated and uninfected in phase 3, 36 turned positive in phase 4.
  • Applying the vaccinated/unvaccinated between-phase incidence to Canada’s 29.7 million adults yielded an estimate of 9.0 (7.9-10.2) million adults newly infected during Omicron, including 0.9 (0.6-1.2) million infections among 2.3 million unvaccinated adults, representing 40% of all unvaccinated adults.
  • Omicron infection rose less among older (60+ years old) than among younger adults (18-59 years old).
  • Anti-Spike antibody levels were highest from three doses of vaccine plus infection, whereas these ratios were negligible among those who were uninfected or unvaccinated, and low in those who had only one vaccine dose (excluding those vaccinated more than one month before DBS collection).
  • The cumulative incidence of NP positivity prior to Omicron was 11.2% (10.4-12.0%) but rose to 36.9% (34.8-38.9%) during the Omicron era. NP positivity may have been under-estimated during actual Omicron infection because mild cases among vaccinated adults may not mount a strong antibody response or because people had not yet seroconverted when the DBS was taken.

Canada had kept natural infection levels low – perhaps less than 10 per cent of the adult population prior to Omicron – thus relying on vaccination for population protection.

The population sampled with Dried Blood Spots (DBS) was stratified by age, sex (59.3% females vs 40.1% males), vaccination status (95.5% vaccinated vs 4.5% unvaccinated) education (30.6% with some college education or less; 31.5% college graduate; and 37.9% university graduate), region (ONT 41.5%; BC and Yukon 20.9%; QC 12.2%; Prairie provinces and NWT 19.6%;  and Atlantic provinces  5.9%), health conditions (obesity 28.1%; diabetes 11.0%; and hypertension 30.4%) and smoking status (39.8% current smokers; 9.3% former smokers; and 49.7% non smokers). Of note, the Ab-C study has had fewer racial or ethnic minority adults (15.8%) but more Indigenous Canadian adults (10.2%) than the census population.

DBS samples were tested using highly sensitive and specific chemiluminescence-based enzyme-linked immunosorbent assays targeting the spike protein, receptor binding domain, and nucleocapsid. Laboratory analyses were conducted by the Lunenfeld-Tanenbaum Research Institute at Sinai Health.

Overall, the study highlights the importance of an ongoing assessment of the incidence of Omicron infections and future SARS-CoV-2 variants.