This is part a continuing series of recommendations put out by CoVaRR-Net. These recommendations are directed to public health officials.


The incidence of vaccine breakthrough infections, where a person who has been fully vaccinated tests positive for the SARS-CoV-2 virus, is low. Breakthrough infections were noted to be rare with the Pfizer, Moderna, and AstraZeneca vaccines.

The Government of Canada recommends that all vaccine recipients who develop symptoms should be tested for COVID-19 to help document breakthrough illnesses, especially in the context of variants of concern (VOCs).

  • In Ontario, of 6.5 million vaccinated people, 0.15% (9,703) became infected when they were partially vaccinated and 0.02% (1,292) became infected when they were fully vaccinated and were considered breakthrough cases as of May 15, 2021. The highest proportion of breakthrough cases in Ontario were among individuals 80 years of age and older.
  • As of April 30th, 2021, the United States reported 10,262 breakthrough infections (0.001%) among the 101 million Americans who had been fully vaccinated for 14 days or more. A total of 160 breakthrough cases were fatal (2%) and the median age of patients who died was 82 years

No vaccine is 100% effective. Breakthrough cases are expected. As SARS-CoV-2 mutates and new variants emerge, the concern is whether current vaccines will remain effective in the future.

The current situation of vaccine breakthrough in Canada is difficult to contextualize due to the lack of available data. Vaccination campaign schedules have also been changed, making it difficult to directly compare data from other jurisdictions.

As provinces start their re-opening plans based primarily on vaccine uptake, breakthrough rate data are important. The information can shed light on the potential of future booster shots and on populations that were under-represented in clinical trials.

CoVaRR-Net Recommendations

  • Set up a pan-Canadian surveillance system to track breakthrough rates, with a particular focus on Indigenous, racialized, and low-income populations.
  • Seek out high-quality studies on VOCs and vaccine effectiveness.
  • Increase testing and sequencing of viral DNA in infected persons to understand the transmissibility, virulence, and ability of variants to evade current vaccines.
  • Profile and then develop guidelines for groups who are showing weaker immune responses.
  • Until more data is available, encourage people to continue wearing a mask, maintaining appropriate distance from others, avoiding crowds, avoiding poorly ventilated spaces, and washing hands.

CoVaRR-Net’s full Situation, Background, Assessment and Recommendation (SBAR) document on breakthrough infections

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