Could we effectively double the supply overnight for both Pfizer and Moderna vaccines against COVID-19? A new study from Canada concludes that the short-term effectiveness of both vaccines is nearly identical from a single dose as it is from the two-dose regimen. However, “short term” might also be the problem:

Single doses of the Pfizer and Moderna vaccines are more than 92 percent effective in preventing COVID-19 illness after two weeks, Canadian researchers are now saying.

The FDA’s own data show that a single shot of the BioNTech-Pfizer vaccine is 92.6 percent effective after two weeks, and a single Moderna jab is 92.1 percent effective, the researchers note in the New England Journal of Medicine.

Getting that second shot of Pfizer’s vaccine hikes the efficacy only marginally, to 94 percent, according to a separate study based on real-world data from Israel’s vaccination program.

And so the prescribed second doses should be given instead to those in priority groups who are still waiting for their first shot, “given the current vaccine shortage,” the researchers urge.

Here’s the catch, though:

“There may be uncertainty about the duration of protection with a single dose,” the researchers said.

“But the administration of a second dose within 1 month after the first, as recommended, provides little added benefit in the short term, while high-risk persons who could have received a first dose with that vaccine supply are left completely unprotected.”

The recommendation to go all in on single doses would make sense if the data shows rapid and all-but-full protection against COVID-19, even in the short term, for as long as the supply shortages of vaccines exist. We need to vaccinate as many people as quickly as possible to stop transmissions and the variants that continue to emerge from the virus’ spread. It’s better to get more people inoculated with short-term protection than to drag it out trying to guarantee long-term protection for fewer people.

However, that strategy depends on just how “short term” this protection will be. If it’s only going to last three or four months off of one dose, that might not be enough. We would still need to inoculate large numbers of people when those first recipients would become susceptible to both transmit and develop COVID-19 on that time frame. It would turn into a game of whack-a-mole, and eventually we would have to return to a two-dose regimen to resolve it and stop new variants from emerging all over again. If it lasts as long as a year, that might be enough; we already do annual flu shots, and those have much less efficacy overall. If one dose lasts longer than a year, then we might not need the second dose at all.

The fact is that we just don’t know how long one or two doses will remain effective. If this had not been an emergency, the pharmaceutical trials would have lasted much longer and answered some of these questions. Researchers believe that the mRNA vaccines will have a very long span of effectiveness, based on some experience with SARS vaccines and the mechanics of the immune-response strategies used by Pfizer and Moderna, but that’s a hypothesis, not a proven fact. Right now we’re flying blind on the longevity question of either regime. It might be years before we get that kind of data, too, which means the FDA is not likely to authorize any change in regimen in the near term, even with the supply shortages. By the time we get that data, the supply should be strong enough to where the question becomes academic.