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

Banerjee A, Lew J, Kroeker A, Baid K, Aftanas P, Nirmalarajah K, Maguire F, Kozak R, McDonald R, Lang A, Gerdts V, Straus SE, Gilbert L, Li AX, Mozafarihasjin M, Walmsley S, Gingras AC, Wrana JL, Mazzulli T, Colwill K, McGeer AJ, Mubareka S, Falzarano D. Immunogenicity of convalescent and vaccinated sera against clinical isolates of ancestral SARS-CoV-2, beta, delta, and omicron variants. bioRxiv. 2022 Jan 13. doi: 10.1101/2022.01.13.475409.

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

CITF-funded researchers Drs. Sharon Straus, Allison McGeer, and Anne-Claude Gingras, all at the University of Toronto, are among those who contributed to this manuscript characterizing the ability of antibodies acquired via immunization, infection, or both, to neutralize Omicron. The preprint, which has not yet been peer reviewed, shows that previous infection alone does not create sufficient levels of neutralizing antibodies to protect against Omicron. Triple vaccination, however, was associated with greater levels of neutralizing antibodies against several variants, including Omicron.

The study used blood samples collected from long-term care residents in Ontario recruited as part of the CITF-funded Wellness Hub study. Samples were assessed for the level of virus-blocking (i.e., neutralizing) antibodies against the original strain of SARS-CoV-2 and the variants of concern (VOCs) Beta, Delta, and Omicron.

Key findings:

  • Previous infection with the original strain or the Delta variant of SARS-CoV-2 was shown to induce high levels of neutralizing antibodies, which blocked reinfection by the same strain (i.e., Delta against Delta and original SARS-CoV-2 against the original) in vitro. However, antibodies from these blood samples had a reduced ability to neutralize Omicron. This suggests that prior infection alone may not be sufficient to neutralize Omicron.
  • Receipt of two doses of Pfizer-BioNTech’s Comirnaty vaccine following infection with the original strain of SARS-CoV-2 was associated with high levels of neutralizing antibodies against that initial strain, as well as the Beta, Delta, and Omicron VOCs. These levels were dramatically higher than those conferred by infection alone, highlighting the benefit of vaccination.
  • Vaccination with one dose of Comirnaty resulted in little neutralization of the original strain of SARS-CoV-2 and none against the three VOCs assessed. This underlines the importance of receiving the prescribed second dose, which is critical for elevating the levels of neutralizing antibodies and, thus, the effectiveness of the vaccine.
  • Triple vaccination, with either Comirnaty or Moderna’s Spikevax, was shown to neutralize the original strain of SARS-CoV-2 as well as the Beta, Delta, and Omicron variants, though neutralization of Omicron was notably lower than the rest. This suggests that third (booster) doses of vaccine aid in improving the neutralization of VOCs.

It should be noted that the study focuses on the antibody response, which is only one part of the immune response. Other facets, such as — cell-mediated immunity — may also contribute to immunity against Omicron and other VOCs.