My first shot was AstraZeneca. Can I have Pfizer for another dose? A UK study can guide this week


With more Ontario receiving eligibility for their second COVID-19 vaccine based on the recommended 16-week interval, many people may be wondering: Can I take a different vaccine for a second time?

And will it matter?

The short answer is no, and probably not. But a study going on in the United Kingdom this week might answer questions about how vaccines are being administered.

The study at the University of Oxford is primarily researching the potential risks or unwanted side effects of mixing a variety of vaccines for the first and second doses, technically known as heterozygous growth. Some people are hoping that if using different types of vaccines for different doses can actually strengthen our immune response, the study may reveal this.

There are two main weapons of the immune system: neutralizing antibodies and T cells, and different vaccines promote them in different ways, Dr. Sunnybrook Health Sciences Center clinical microbiologist Dr. Rob Kozak explained.

“Maybe we’re going to find out that if you use Pfizer it’s a little bit better at making antibodies and a little bit better at making AstraZeneca T cells, so you put these two together and When you feel like you get even more good response, ”said Kojak.

An infectious disease specialist from Dalhousie University, Drs. Lisa Barrett first compared the dose of the COVID-19 vaccine to primary school education, which is then extended to post-secondary education.

“And yet there may be some other cells that were lazy or not quite busy at the time … You could get more new cells to potentially actually attach by providing different bits of the virus or by a slightly different formulation. Do, ”said Barrett. “So you can really broaden your response and not educate the original respondents.”

But this will only provide a better response, in theory, if you are mixing two vaccines with different methods of attack.

Vaccines work in different ways. Pfizer-BioNTech and Moderna are mRNA vaccines that work to make your cells a little coronavirus spike protein, which in turn teaches your body how to fight a major infection.

AstraZeneca and Johnson & Johnson vaccines are known as viral vector vaccines and work by injecting small amounts of coronavirus virus into your body to get it to make spike proteins.

Returning to his education analogy, Barrett stated that the combination of the two mRNA vaccines “doesn’t matter much because you’re basically taking the same course with only one lecture difference, as opposed to 50 percent separation of the course.” Does. “

But the option of combining different mRNA vaccines may still be helpful in filling the gaps in supply, Kozak said, for example if the province runs out of Pfizer but still has a plentiful modern dose.

Standing on this, the Ontario Ministry of Health states that there is no evidence on the efficacy of a mixture of different COVID-19 vaccines and the position that both doses should be given with the same vaccine. Health Canada provided a similar response and said it was monitoring emerging evidence.

In contrast, Quebec has expressed a desire to allow residents to receive two different types of vaccines.

Kozak said that while he understands that governments are hesitant to change public health policy in the absence of solid science, he said the unfortunate Canada is not overfunding funds or using its own research into the effectiveness of interchangeable vaccines is following.

“I think they appreciate having to make tough decisions … but at the same time I think that sitting on your hands and this whole vaccine rollout is not one of the problems,” Kozak said.

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Matthew Miller, an infectious disease specialist at McMaster University, says there are many scenarios where mixing different vaccine supplements can be beneficial.

The first group includes individuals who have received their first AstraZeneca dose, but are hesitant to receive each other due to safety concerns around blood clots. Some Canadians are hesitant to receive the vaccine and may join them after the National Advisory Committee vaccinations on Monday. He stated that mRNA vaccines are the “favorite” vaccine.

(All four vaccines are considered safe and effective by Health Canada based on clinical trial evidence.)

Those who may benefit are people who had an allergic reaction to their first type of vaccine, or individuals who may need a boost after receiving the Johnson & Johnson vaccine, which is administered only once. it occurs.

But the reality is that there may be limited opportunities to see if mixing vaccines leads to better results as Canada’s supply of vaccines is heavily skewed towards MNA vaccines both now and in the future. Ontario is expecting millions of Pfizer-BioNTech vaccines in the coming weeks, but the arrival of more AstraZeneca doses is uncertain.

“Not all combinations of mixing and matching can be equally effective, so it is important to have data to drive these decisions,” Miller said in an email. “More practically, Canada’s supply is incredibly dependent on mRNA vaccines relative to all other platforms (eg AZ, J & J) and is therefore, from a practical point of view, likely to have very limited opportunities for mix-and-match. is.”

He said that the issue of interchangeability will become more important if and when vaccines need to be updated.

While the Oxford study is expected to release an update this week and some results in the summer, it is not known whether the summer vaccination efforts released enough to inform a meaningful change in policy in time. Will go.

Ontario’s chief coroner and co-coordinator of the provincial outbreak response, Drs. Durk Huyer said it would soon be known whether the results of the Oxford study would change the province’s policy on interchangeable vaccines.

“We’re looking, we’re interested. We really want to know what real-world evidence from the UK shows.”

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