Researchers from the Canadian Immunization Research Network (CIRN) assessed the effectiveness of mRNA vaccines against symptomatic COVID-19 infection and severe outcomes and showed promising results. In their pre-print, therefore not yet peer reviewed, researchers suggest that while one dose of the mRNA vaccine provides some protection that improves over time, two doses appear to be highly effective against severe outcomes. The study found that mRNA vaccines are effective against all circulating variants of concern at the time of analysis.


Due to shortages in vaccine supply and the attempt to maximize the number of people receiving their first doses of the COVID-19 vaccine, Canadian provinces have recommended that second doses be delayed by up to 16 weeks. Because this dosing regimen differs from the recommendations set by the manufacturers, Dr. Jeff Kwong and colleagues from the Canadian Immunization Research Network (CIRN) sought to explore the real-world effectiveness of the Pfizer-BioNTech and Moderna mRNA vaccines following a single dose, as compared to two doses.

Key Points:

  • Two doses of Pfizer-BioNTech’s BNT162b2 and Moderna’s mRNA-1273 COVID-19 vaccines are highly effective against both symptomatic infection and severe outcomes caused by all circulating variants of concern at the time of the study (B.1.1.7/Alpha, B.1.352/Beta, P.1/Gamma)
  • Vaccine effectiveness is lower after a single dose, particularly for older adults, 14 days following the first dose


Study participants included Ontarians aged 16 and older (not including those in long-term care homes or other facilities) who were tested for SARS-CoV-2 between December 2020 and April 2021. Recipients of the Oxford-AstraZeneca vaccine were excluded. Provincial testing and vaccination data were linked to data regarding variants collected by public health authorities.

Both mRNA vaccines showed moderate vaccine effectiveness (~50-70%) against symptomatic SARS-CoV-2 with a single dose. Comparatively, they showed very high vaccine effectiveness (>90%) with two doses. Similar to results observed in clinical trials, both vaccines were significantly effective against severe disease and death, with overall vaccine effectiveness improving over time. Notable differences in vaccine effectiveness against the variants B.1.1.7/Alpha, B.1.352/Beta, P.1/Gamma were not observed, suggesting that the mRNA vaccines are protective against the known variants of concern at the time of the study.

Vaccine effectiveness against symptomatic infection was observed to be lower for older adults aged 70 and up compared to younger participants for the first few weeks after the first dose. Protection nevertheless became comparable between these groups approximately four weeks after the first dose, as well as when the second dose was administered.  The lower initial vaccine effectiveness observed among older adults suggests that it takes longer for them to develop a protective immune response. The authors recommend that everyone, especially older individuals and those with co-morbidities, continue to exercise precautions until fully vaccinated with both doses.


Chung H, He S, Nasreen S, Sundaram ME, Buchan SA, Wilson SE, Chen B, Calzavara A, Fell DB, Austin PC, Wilson K, Schwartz KL, Brown KA, Gubbay JB, Basta NE, Mahmud SM, Righolt CH, Svenson LW, Macdonald SE, Janjua NZ, Tadrous M, Kwong JC. Effectiveness of BNT162b2 and mRNA-1273 COVID-19 vaccines against symptomatic SARS-CoV-2 infection and severe COVID-19 outcomes in Ontario, Canada. 2021 May 28. doi: 10.1101/2021.05.24.21257744.