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Charu Kaushic is a professor of pathology and molecular medicine at McMaster University. Catherine Hankins is a professor of public and population Health at McGill University. Both are members of the COVID-19 Immunity Task Force.

This year is starting on an optimistic note, as tens of thousands of Canadians receive first doses of vaccines that can ultimately confer 95 per cent protection against COVID-19. However, there are new and confusing headlines in the media every day. Should we hold on to half the vaccine stockpile to complete the two-dose regimen or should we administer all available vaccine doses to a maximum number of people? Is one dose sufficient? Are two doses necessary? And what exactly is the optimal spacing of doses?

As always with this epidemic, there are some answers and some uncertainties.

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First things first: we need to act fast. Many provinces are in the midst of an intense second wave of COVID-19. Deaths are tragically concentrated again among vulnerable seniors in long-term and residential care facilities, and health care workers face intensified risks of exposure. Both Pfizer and Moderna mRNA Phase 3 vaccine trial data indicate that, starting 14 days after the first dose, very few new SARS-CoV-2 infections with symptoms were seen. Using up the current vaccine supply will therefore save lives and keep our health care systems from getting overwhelmed.

When will Canadians get COVID-19 vaccines? The federal and provincial rollout plans so far

This doesn’t mean giving up on second doses. The evidence for longer-term protection is based on studies with two doses spaced three or four weeks apart. Unless new research paints a very different picture, finishing the two-dose series is crucial. That’s especially true given reports of more infectious variants of the virus from the U.K. and South Africa. Hundreds of thousands of new infections are still occurring globally each week. We don’t know what will happen if large numbers of individuals fail to complete two full doses of their vaccine series, but if that leads to sub-optimal immunity, it’s a perfect set-up for the virus to evolve in ways that escape control by vaccines.

So, if getting both doses into people is vital, there’s still the issue of how far apart the two doses can be. That’s an issue because rapid initial deployment could create gaps in the supply chain such that second doses are greatly delayed. Unfortunately, the mRNA vaccines are so new that it is not clear how long that first dose will offer meaningful protection. That matters more when, as in the first quarter of 2021, we’re focused on vulnerable and high-risk populations.

A best-case scenario, based on the trial data, is to ensure we have a steady and secure supply of vaccines so that all available doses can be administered as quickly as possible, knowing that a second dose of the matched vaccine will arrive within the recommended time. But there’s also room for pragmatic alternatives that maximize coverage through smart logistics and risk management. For example, the first doses could be rolled out as fast as possible, while monitoring supply timelines to limit the risk posed by marked delays in giving second doses. In this regard, public health units all over the country have experience and expertise in exercising flexibility in vaccination schedules.

Looking ahead to the second quarter of 2021, supplies of vaccine will be plentiful, supporting a shift to wider coverage aimed at breaking the chains of community spread, while expanding immunization of individuals in specific populations at risk. Different vaccines will also be available. Some may be single-dose by design. Others, more conventional than the mRNA vaccines, may be understood well enough that longer deferral of a second dose raises few concerns. In both cases, we can move at top speed to get at least one dose into as many Canadians as possible.

What, then, is the bottom line? We need to act quickly right now to protect vulnerable groups of Canadians. Second doses remain essential for existing mRNA vaccines, given mutation risks and limited data on duration of first-dose effectiveness. Thus, the timing of vaccine supply must be as clear and firm as possible, so that different jurisdictions can risk-manage logistics to ensure second doses are completed without unreasonable delays.

As all this unfolds, we must undertake meticulous vaccine surveillance to determine which vaccines work safest and best in different groups of Canadians. That means tracking who got which vaccine, which dose, when, and with what outcomes. Above all, Canada needs to stay the course with public health measures that protect citizens until vaccines can work their magic on the population at large. The end is not at hand, but it is definitely in sight.

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