There’s now a preprint of a study of the efficacy of both the Pfizer/BioNTech and AstraZeneca/Oxford vaccines among over one million people in Scotland. It looks like the British “first doses first” plan was a clever way to save the reputation of the British AZ vaccine after it underperformed in a clinical trial mandating two doses (fortunately, a foul-up led to some participants in AZ’s clinical trial only getting 1.5 doses, which worked better than 2 doses). Unlike AZ, the otherwise similar Russian Sputnik vaccine uses two different vectors for fear that the second dose would trample on the good effects of the first dose.
In the new Scottish study, a single dose of AZ reaches an impressive 94% vaccine efficacy (VE) (i.e., the vaccinated are only 6% as likely as the unvaccinated to wind up in the hospital due to covid) by 28-34 days after a single dose.
Unfortunately, AZ dosing only didn’t begin until January, so the sample size after 21-27 days (when 84% VE was achieved) is small. And it’s too small to from 35 days on, so we can’t tell yet whether AZ needs a booster second shot to maintain that 94% VE in the long run that was achieved among a small sample size from 28-34 days.
The data on the Pfizer vaccine, however, suggests that a booster dose is helpful. The vaccine efficacy of a single dose of this mRNA vaccine peaks at 85% from 28-34 days out, but then falls to 64% efficacy after 42 days.
So the Israeli and American strategy of giving the tested two doses of Pfizer three weeks apart, as was tested in the clinical trial, seems like a pretty good idea.
Keep in mind that this wasn’t a perfect apples to apples comparison: AZ was given primarily to the elderly while Pfizer was also given to middle-aged people with major health problems. And Pfizer vaccinations started four weeks before AZ.
No word on side effects was included in this preprint.