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

Breznik JA, Rahim A, Kajaks T, Hagerman M, Bilaver L, Colwill K, Dayam RM, Gingras AC, Verschoor CP, McElhaney JE, Bramson JL, Bowdish DME, Costa AP. Protection from Omicron infection in residents of nursing and retirement homes in Ontario, Canada. medRxiv 2022.06.28.22277016; doi:

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

A CITF-funded study in preprint, not yet peer-reviewed, led by Drs. Dawn Bowdish and Andrew Costa from McMaster University, showed that among residents of long-term care facilities, there was a lower risk of an Omicron infection when the person had:

  • Received three doses of Moderna or a combination including Moderna (vs. three doses of Pfizer);
  • Any fourth mRNA vaccine dose; and
  • Hybrid immunity induced by 3 vaccine doses and a SARS-CoV-2 infection in the 3 months prior to the beginning of the Omicron wave.

Moreover, neither age nor gender was a determining factor in the risk of Omicron infection.

Key Findings

  • 17.2% (171/997) of nursing and retirement home residents had a breakthrough infection between December 15, 2021, and May 3, 2022. Sex and age did not seem to make a difference (median age 87 years; 66% male vs 67% female).
  • Infection was less frequent in individuals with three Moderna vaccine doses (infection incidence – 27.5%, 47 of 171) or any combination of three mRNA vaccine doses including Moderna (infection incidence – 7%, 8/171) compared to three Pfizer doses (infection incidence – 68%, 116/171).
  • Individuals who had a SARS-CoV-2 infection in the three months prior to the emergence of the Omicron variants had a lower incidence of Omicron infection: those with a prior infection had a 6.5% (11/171) incidence of infection vs 94% (106/171) in those with no prior infection in the three months before Omicron.
  • In sum, the risk of Omicron infection was significantly lower in individuals:
    • who received three doses of Moderna when compared to three doses of mixed mRNA vaccines regimen (hazard ratio (HR)Hazard ratio (HR) measures how often a particular event happens in one group compared to how often it happens in another group over time. For example – HR – 0.53 means that 53% of the individuals that received three doses of Moderna had decreased risk of Omicron infection. – 0.54) or three Pfizer doses (HR – 0.53)
    • with any fourth mRNA vaccine doses (HR – 0.19)
    • with COVID-19 infection in the three months prior to December 15, 2021 (HR – 0.52)

The team recruited 997 participants who had received three mRNA vaccine doses as of December 15, 2021, within 17 nursing homes and 8 retirement homes in Ontario. Participants received two doses of Moderna or Pfizer and a third dose of an mRNA vaccine in the fall of 2021, at least 6 months after their second dose. Fourth doses were available as of early 2022. COVID-19 infections between December 15, 2021, and May 23, 2022, were determined by confirmed PCR test positivity and/or dried blood spot seropositivity for the anti-N IgG antibodies by validated ELISA assay.

The findings from this study back continued public health efforts to encourage vaccination programs and monitor vaccine immunogenicity in older adults.