Second opinion: Two medical experts weigh in on vaccines and the Delta variant speaks with Zain Chagla and Ashleigh Tuite about AstraZeneca, degrees of protection, and weighing the risks
By Daniel Kitts - Published on Jun 16, 2021
Zain Chagla (left) and Ashley Tuite ( Iwanyshyn/U of T).



Just as many Ontarians line up for their second COVID-19 shots, the highly contagious Delta variant, first identified in India, threatens to create more vaccine uncertainty.

A report published this week finds that two shots of Pfizer are 79 per cent effective against the Delta variant, while two shots of AstraZeneca are less effective, at 60 per cent. This comes on top of some research suggesting that a first shot of AstraZeneca followed by a second shot of Pfizer can potentially generate a stronger immune response than two doses of the same vaccine.

But research also shows that all vaccines are very effective at doing their number one job: keeping people alive and out of the hospital. So do these gaps in vaccine effectiveness really matter when considering which shot to get?

To get a better handle on that, talked to two experts: Zain Chagla, a consultant physician at St. Joseph’s Hospital and Hamilton Health Sciences and an assistant professor of medicine at McMaster University; and Ashleigh Tuite, an epidemiologist, mathematical modeller, and assistant professor at the University of Toronto.

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You can count on TVO to cover the stories others don’t—to fill the gaps in the ever-changing media landscape. But we can’t do this without you. How should the gap in effectiveness against the Delta variant between different vaccines affect the decision of people figuring out whether to get a second AstraZeneca dose or to mix and match with mRNA vaccines, such as Pfizer and Moderna?

Zain Chagla: The data around Delta has to be caveated by the fact that a two-dose vaccine series still has incredible protection for people from severe outcomes. Considering the big goal of these vaccines is that we spare hospital utilization and [reduce] ICU deaths, two doses of any vaccine is going to be better than zero doses of the vaccine. Getting a full regimen, one way or another, is still important.

Ashleigh Tuite: The important thing to remember is that there’s actually no data for AstraZeneca as the first shot and Pfizer as second shot [when it comes to that combination’s effect on the Delta variant], which is the comparison that people would want to see. There’s no definitive answer.

If, over the coming months, we realize that the protection against infection provided by the AstraZeneca vaccine isn’t as strong as we would want it to be, that’s not the end of the road for people who have two doses of AstraZeneca. We’re already talking about if we might need to do booster shots because of emerging variants for which the vaccines don’t perform so well. With the understanding that each patient has to take different factors into consideration when making a decision, what’s the smarter move in Ontario right now — getting your second dose as quickly as reasonably possible or trying to get the most effective mix of vaccines against the Delta variant?

Chagla: Right now in Ontario, it’s probably easier to get an mRNA vaccine than an AstraZeneca vaccine, since there are only certain places that are offering AstraZeneca. The quickest option is probably Pfizer. I always give the advice to my family that I would give to patients. And looking at the data, particularly of that potential [efficacy] advantage against the variants and the relative [vaccine] safety, I’ve been recommending most individuals in my family go get mixed vaccines [i.e., mRNA as a second dose].

Tuite: I’m not a medical doctor, and I’m not comfortable providing medical advice for individuals about which vaccine they should take, but from a population-level perspective, a vaccine that reduces infections, and thereby likely reduces transmission to others, is preferable to one that doesn’t. And the data we have right now suggests that the mRNA vaccines are better at that. Quite a few people went in for a second AstraZeneca dose in the weeks before this conversation. How should they feel about the fact that the advice from many medical professionals now seems to be that, if you have the choice, you should go for a mixed dose?

Chagla: Certainly, there’s no reason to be concerned. We’re still seeing the vast majority of [Delta-variant] infections in the United Kingdom being in people that are under-vaccinated. A full series of AstraZeneca really does have profound protection against severe disease and hospitalization. That 60 per cent protection is not trivial. It’s not perfect, but it’s still enough to make those chances of getting sick pretty minimal, especially when things get better globally and everyone’s risk of COVID-19 goes down.

Tuite: As we’ve had the vaccine rollout, the communication has been really challenging. In Ontario, the approach has been that it’s up to an individual to decide. And, ultimately, that’s correct. But I also think that the amount of information coming out is overwhelming. And I think that there’s a role for the government to have provided some more clear guidance around this. A good example of a province that has done this well is Manitoba. They explicitly said: we recommend that if you got AstraZeneca for your first dose, get Pfizer or Moderna for your second dose, based on the data that we have. In Ontario, that was never made explicit. When it comes to the Delta variant, there appears to be 79 per cent protection with Pfizer versus 60 per cent protection with AstraZeneca. But what does that gap really mean to a person’s level of risk?

Chagla: It doesn’t mean that if you’re exposed after getting a vaccine with 60 per cent protection, you have a 40 per cent risk of getting COVID-19, as compared to 100 per cent risk when you weren’t exposed. There are lots of different factors at play. If you’re a family of four having some regular social interaction with a limited number of individuals, that’s different than someone who lives or works in long-term care.

Tuite: It’s complicated. If you’re exposed to somebody who’s infected, you may have increased likelihood of developing a symptomatic infection with the AstraZeneca vaccine relative to the Pfizer vaccine. But it’s not quite as binary as that. You may go on to develop an infection, but your infection will be less bad than it might have been had you not been vaccinated. It’s not like the vaccine either works or it doesn’t. It reduces your risk. And even if you do become infected, the risk that you have a severe outcome with any of the vaccines is dramatically reduced. Say I have 60 per cent protection through my vaccine, but I’m surrounded by people who also have 60 per cent protection through their vaccines. Does that mean that my protection is theoretically higher than 60 per cent?

Chagla: Yeah, absolutely. Vaccines have an additive effect on each other. In long-term care, where full populations were vaccinated in the context of the third wave, which is the worst we saw in Ontario, we saw very little encroachment. So that’s not only patients being vaccinated, but everyone around them being vaccinated to create these environments that are relatively safe.

Tuite: Being around vaccinated people dramatically reduces your risk. That’s the beauty of vaccines. Because everyone you encounter who is vaccinated, even if the vaccine is imperfect, has a dramatically lower risk of being infected if they’re exposed — and, similarly, a reduced risk of passing that infection on to others if they are infected. How concerned should we be that conversations about the varying effectiveness levels of vaccines are going to lead to indecision and slow down vaccination efforts?

Chagla: The expectation from the Angus Reid polling is that up to 88 per cent of Canadians want a vaccine. That’s probably a good indication that people want to get their second dose. So I don’t think that’s going to be a challenge for us.

Tuite: There is this sense of anxiety that we shouldn’t have to be dealing with right now. The vaccines are amazing. They’re a game changer. And, so, the fact that people are feeling anxious and uncertain is an unnecessary problem that I think we’ve inadvertently created by having vaccine choice. If we were in a situation where we had a single vaccine available, we wouldn’t be having this conversation. Having said that, looking at the lineups for second doses — I think there’s still a huge appetite to get vaccinated. This whole discussion about the vaccines being less effective against the Delta variant left me feeling deflated. I thought, “This is never going to end. As soon as we think we’re near the end of this thing, some new variant will come up, and my daughters will still be wearing masks and learning from home months from now.” How do you respond to that?

Chagla: There’s a lot of hope with this vaccine campaign, even in the context of emerging variants. We still have not seen a variant that is regularly able to evade vaccines and cause hospitalizations and deaths. So, again, the best decision is getting vaccinated now with whatever is available and whatever you’re comfortable with.

Tuite: Don’t feel deflated. These vaccines are amazing. The fact that we’re frustrated that they’re not as effective against some of these variants, is, to be perfectly honest, not a conversation that I thought we would be having in June 2021. I mean, I was not optimistic that we would even have a vaccine at this point.

This interview has been condensed and edited for length and clarity.

Zain Chagla, Maria Sundaram, and Asheigh Tuite on The Agenda, June 18, 2021.

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