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It is with a remarkable level of confidence – and a conspicuous lack of complete clinical trial data – that the country’s health authorities have assured concerned Canadians who have received their first dose of AstraZeneca’s COVID-19 vaccine that they will likely get to choose which vaccine they receive for their second shot.

In the United Kingdom, a clinical trial is currently assessing the effects of mixing the Pfizer and AstraZeneca vaccines in 820 participants aged 50 and above (and it has recently been expanded to include an additional 1,050 volunteers). Preliminary data released mid-May indicated that mild to moderate reactions (such as fevers and headaches) appear to be more frequent among those with “mixed dosing schedules” (i.e. two different vaccines) compared with participants who received two shots of the same vaccine. There was no specific data reported about the effectiveness of mixed dosing. Early results from a similar study of 672 participants in Spain, however, suggested that mixing Pfizer and AstraZeneca doses is safe and possibly more effective than receiving two shots of AstraZeneca’s vaccine, but that trial is continuing, and there has not yet been preprint or peer review.

Still, provincial health authorities have all but confirmed that when these trials wrap up, Canadians will be able to pick the type of vaccine they want for their second dose.

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Tam expects AstraZeneca recipients will be able to choose second COVID-19 vaccine

Quebec’s National Director of Public Health, Horacio Arruda, suggested as much back in April, even before the province stopped giving first doses of AstraZeneca. In Saskatchewan, Chief Medical Officer of Health Saqib Shahab said Pfizer “will likely be what will be offered in Saskatchewan as a second dose.” Provincial Health Officer Bonnie Henry said B.C. residents will likely be given a choice; Canada’s Chief Public Health Officer, Theresa Tam, said the same on Tuesday. Indeed, the presumption seems to be that the clinical trial data will demonstrate that a mixed-dose schedule is both safe and effective, and that those who are understandably wary of receiving a second AstraZeneca shot will opt for a Pfizer dose instead.

When you dig into the science, there is good reason for this degree of confidence. Scientists and researchers will tell you there is precedent for administering different vaccines in a relatively short period of time – when babies get their inoculations, for example – and for situations where adults get one vaccine and receive a booster of another, as is the case with pneumococcal vaccines. It’s also believed that since the AstraZeneca and Pfizer vaccines teach the body to target SARS-CoV-2 in different ways, a combined dosing schedule could offer recipients better protection by bridging any gaps from a single vaccine type.

But this is complicated stuff, yet unproven as it relates specifically to COVID-19 vaccinations, which is why the assurances that Canadians will be able to mix and match might seem premature. Public-health authorities in Canada have expressed confidence in a myriad of positions before – the risk to Canadians remains low, border controls are not effective, putting on a mask is not beneficial, take the first vaccine you’re offered – only to change course not so long after. It would be utterly disastrous if the same thing were to happen here, however unlikely that outcome might be.

Levelling with Canadians about what we do and do not know at any given point, rather than jumping the gun on what we think the results of a clinical trial will show, has to be the better approach to preserving public trust. Obviously public-health authorities want to assure people who are now anxious about AstraZeneca that they will have another option for a second dose. But the reality is that clinical trial data from the U.K. is just a snapshot of safety and efficacy information on mixed dosing in fewer than 2,000 people who are age 50 and over.

Real-world data from the U.K., by comparison, shows that use of AstraZeneca’s vaccine (which accounts for the majority of doses administered in the country, followed by Pfizer’s) has been extremely effective in preventing serious illness and death after being administered more than 23 million times. And though there is still a risk of the rare blood clot associated with the AstraZeneca vaccine after the second dose, its incidence appears to be significantly reduced: There have been eight recorded cases after roughly 7.5 million second doses.

Canadian health authorities can thus say, confidently, that AstraZeneca’s vaccine has proven to be safe and effective in millions of people after the second dose. It’s peculiar that provincial and federal leaders have not made that the prevailing message about second doses, and instead have jumped to offer an alternative based on data that hasn’t been rigorously proven.

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