Scott Halperin

Dr. Scott Halperin, MD

Deputy, CoVaRR-Net Immunology and Vaccine Protection Pillar
Director of the Canadian Center for Vaccinology and Nominated Principal Investigator, Canadian Immunization Research Network
Professor, Dalhousie University

Denis Leclerc

Denis Leclerc, PhD

Deputy, CoVaRR-Net Virology Pillar
Co-Chair, Division of Infectious and Immune Disease, CHU de Québec Research Center-Université Laval
Professor, Université Laval

Many Canadians wonder whether third and fourth doses of vaccine truly make a difference in protecting them against COVID-19 infection and reducing the risk of transmitting the virus to others. Because the Omicron subvariant BA.2 is highly transmissible and people who received third and fourth doses can still get breakthrough infections, some think not.

The answer from CoVaRR-Net experts, however, is that higher vaccine uptake protects against infection and helps lower viral transmission in multiple ways, in combination with wearing masks in crowded indoor spaces. Lowering transmission is important for many reasons, including the fact that it reduces the risk of new, vaccine-resistant variants emerging.

While third and fourth doses are designed to — and do — offer increased protection against severe disease, hospitalization and death from COVID-19, these additional doses also play a key role in lowering the number of infections and transmission among both individuals and the Canadian population as a whole to help curtail the sixth wave we’re in now.

Vaccines lower transmission

“Vaccines do lower the viral load by boosting production of antibodies and the amount of transmission is directly related to viral load. If you have a lower viral load, then you are less likely to transmit the virus to other people,” says Dr. Leclerc, CoVaRR-Net’s Virology Pillar Deputy, and Professor at Université Laval.

Additional doses, therefore, help cut viral transmission significantly. “Healthy people are less likely to transmit the virus if they received a third dose. People who receive a fourth dose to boost waning or low immunity will also transmit less virus compared to if they hadn’t received an additional dose,” adds Dr. Leclerc.

Third and fourth doses reduce risks of infection and symptoms

Only 54 per cent of Canadians 12 and older have received at least three doses of vaccine. That’s unfortunate because the effectiveness of two doses of vaccine in protecting against infection from the Omicron variants drops dramatically within months after receiving the second dose. “As your immunity wanes, there is a greater likelihood you’ll get infected. At four or five months after a second dose, you aren’t protected in terms of reducing the likelihood of getting infected and reducing the chances of being symptomatic. In both cases, you are much more likely to transmit the virus to others,” says Dr. Scott Halperin, CoVaRR-Net’s Immunology and Vaccine Protection Pillar Deputy, Director of the Canadian Centre for Vaccinology and Professor at Dalhousie University.

For Canadians who haven’t received a third dose, getting an additional dose now matters in curbing infections and transmission amid the sixth wave. “A third dose reduces the risk of infection and of symptoms if you do get infected. If you don’t have symptomatic illness — you’re not coughing and sneezing – then, you’re much less likely to transmit the virus, as you’re spreading fewer secretions and have a lower viral load. If you have a lower viral load, this also shortens the length of time in which you can transmit the virus by 24 to 48 hours. By not being infected or not being symptomatic, you substantially reduce the risk of transmission,” says Dr. Halperin.

Curbing transmission reduces the risk of new, vaccine-resistant variants emerging

Uncontrolled spread of the virus is a recipe for the emergence of new variants that could be more dangerous, infectious, and more resistant to current vaccines. “We can’t afford to drop our guard against the threat of new variants. The vaccines are working now, but if we don’t slow transmission of the virus, this will favour a strain that’s resistant to the antibodies generated by the current vaccines. More people need to produce high levels of antibodies by getting third and fourth doses, which will help to slow transmission and reduce the risk of a new, more resistant variant emerging and becoming dominant,” says Dr. Leclerc, CoVaRR-Net’s Virology Pillar Deputy, and Professor at Université Laval.


  1. Public Health Agency of Canada. COVID-19 vaccination coverage in Canada –

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