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COVID-19 breakthrough infections: A Q&A with Sunnybrook’s Dr. Rob Kozak

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Written by Lindsay Smith

More than 50 per cent of Canadians are now fully vaccinated against the COVID-19 virus, and in Ontario, we have begun to see the lifting of restrictions: restaurants, gyms and hair salons are all open for business again.

Along with rising vaccination rates, though, have been stories of vaccinated individuals who have experienced a “breakthrough infection,” which means they contracted COVID-19 at least 14 days after receiving their second dose of a COVID-19 vaccine.

We asked Dr. Rob Kozak, clinical microbiologist at Sunnybrook, to answer some questions about breakthrough infections: why they happen, what the risks are and how they can be prevented. You can read his answers below.

Dr. Rob Kozak is a scientist and clinical microbiologist who has been involved in COVID-19 research since the virus emerged in Canada. In February 2020, Dr. Kozak was part of a team that received a grant to develop genome sequencing of SARS-CoV-2. In March 2020, he was part of a team that isolated the virus. Learn about his vaccine development work in the University of Toronto Magazine.

Can you explain why breakthrough infections occur if the vaccines are considered effective?

It’s a great question. We are still studying the immune response, and are working to fully understand how the different arms of the immune system work to protect you from SARS-CoV-2.

We know the vaccines are very effective, but not perfect. Some individuals will mount a better immune response than others, and that means that a very small percentage of vaccinated individuals will develop an infection. Even in those cases, your immune response is still helping because it is lowering the viral load (amount of virus), helping make the infection much more mild. 

What are we seeing so far with regards to COVID-19 and breakthrough infections? Anything particularly noteworthy?

The number of infections in fully vaccinated people is extremely low. Data from Public Health Ontario has shown that, to date, only 0.02% of fully vaccinated people have become infected.

This means almost all the cases we are seeing of COVID-19 are in people who are partially vaccinated or unvaccinated.

What do you think is important for fully vaccinated people to know about breakthrough infections?

We don’t want anyone to believe that the vaccines don’t work. Even the term “breakthrough infection” is a bit misleading: the job of the vaccines is to train your immune system to be ready to fight the virus so that you don’t get a severe case of COVID-19, end up in hospital, or pass away from the infection.

We know we will see mild cases of COVID-19 in a small number of vaccinated people. So, it really isn’t a breakthrough; it is more your immune system preventing the infection from being very serious.

But even in these cases, a vaccinated person is much more likely to have milder illness, and is less likely to transmit the virus to those around them. This is good because it means you are helping to protect those close to you from COVID-19.

Can you talk about how increasing vaccination rates at home and globally could help minimize the risk of breakthrough infections from new variants?

Viruses need a susceptible host (that’s us) to replicate and spread to others. The antibodies and T-cells generated by vaccination will neutralize the virus, which lowers the viral load and helps prevent the virus from spreading to others. This is why it is important to drive up global vaccination rates — it reduces the number of susceptible hosts the virus can infect.

What, if anything, can fully vaccinated people do to further reduce their risk of experiencing a breakthrough infection?

The best way to prevent getting infected is to not get exposed to the virus. Being vaccinated is a layer of protection, but you can always do more. This means following the public health guidelines: wearing masks, maintaining distancing where required, getting tested if you think you were exposed, and staying away from others if you’re feeling sick.

Also, do what you can to convince friends and family to get vaccinated if they haven’t done so already. The more shots we have in arms, the more everyone is protected.

Why is it still important for people to get two doses of a COVID-19 vaccine if breakthrough infections are possible?

There are so many reasons to get both doses. The second dose of the vaccine is important for building immune memory, and robust antibody and T-cell response against the variants, and it increases the overall efficacy of the vaccine. This means your chance of having an infection is reduced when you have both doses.

Furthermore, if you’re fully vaccinated and you do get infected, you are more likely to have a milder case, and less likely to spread it to those close to you. Also, proof of full vaccination will be needed for travel and potentially other things too. By getting fully vaccinated it will mean you will have more things to enjoy this summer!

This interview has been edited for length and clarity.

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Lindsay Smith