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

Grewal R, Nguyen L, Buchan SA, Wilson SE, Nasreen S, Austin PC, Brown KA, Fell DB, Gubbay JB, Schwartz KL, Tadrous M, Wilson K, Kwong JC. Effectiveness of mRNA COVID-19 vaccine booster doses against Omicron severe outcomes. Nature Communications. March 2023. DOI: 10.1038/s41467-023-36566-1

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

In this CITF-funded study, published in Nature Communications, Dr. Jeffrey Kwong (University of Toronto), along with Drs. Kumanan Wilson and Deshayne Fell (University of Ottawa) found that a third and fourth dose of mRNA vaccine sustained the protection conferred against severe Omicron-related outcomes for at least three months in people aged 50 years or older. A fourth dose yielded even greater and longer-lasting protection than a third dose.

Key findings:

  • The effectiveness of a third dose of mRNA vaccine against severe outcomes ranged between 91% and 98% up to 60 days post-dose, depending on the age group. It declined to 76%-87% after 240 days (8 months).
  • A fourth dose restored vaccine effectiveness to 92-97% for up to 60 days and to 86-89% after 120 days (4 months) post-dose, across all age groups.
  • Vaccine effectiveness was lower among those of more advanced age.
  • Vaccine effectiveness was lower and declined more rapidly when the subvariants BA.4 and BA.5 were predominant.

While these estimates are promising, it is important to continue monitoring vaccine effectiveness for longer periods post-vaccination, particularly with the recent introduction of bivalent vaccines and the potential for newly emerging sub-variants of concern.

Adults aged 50 years or older who underwent at least one SARS-CoV-2 RT-PCR test between January and October 2022 in Ontario and who were not immunocompromised were included in the study. A total of 11,160 Omicron cases (Delta cases were not integrated in the analysis) and 62,880 controls who tested negative were analyzed. Severe outcomes included COVID-19-associated hospitalization or death.

If you want to learn about vaccine effectiveness in children between the ages of 5 and 11, you can read our summary of another study conducted by Drs Kwong and Wilson published in Pediatrics in March 2023.