We'll soon find out from Canada what the effects of a 4 month gap does to response for all age groups.
In other words, the supply was enough to make sure that all of those people could get one dose in twelve weeks, so that's the dose interval that was set.
That goal (quickly vaccinate all of the at risk population with one dose) was pretty much achieved. I guess it remains to be seen whether it was the right goal, but early indications (deaths/day down from 1,000 to 10 or so) are positive. (Obviously correlation is not causation and so on, and there undoubtedly a ton of confounders.)
It might take a while, as immunity wanes, to see what these different intervals really mean.
Right, reduces, but the full dose is needed to render the virus mostly harmless. My partner's coworker has Covid and one does of the vaccine, they are still severely ill.
It takes ~14 days after the first dose to get the ~80-90% protection (Pfizer/Moderna).
Please correct me in case you have different information
Second of all, the choice was never between giving two doses to 16 million people (as of today) or giving one dose to 16 million people. The choice was giving two doses to 8 million people or one dose to 16 million people. The total death toll from the pandemic would be lower the sooner we get to the point of herd immunity. Giving everyone one dose gets us there sooner and ultimately saves lives.
https://www.canada.ca/en/public-health/services/immunization...
In an ideal world, you'd do a few clinical trials and figure out the optimal dose regime and timing. (I'm not really sure why they didn't run 5 parallel trials with different timings to begin with).
In the world we inhabit, alas we didn't have those studies. But that doesn't mean we have no reason to believe that the first shot is protective! You can still look at how vaccines in general work, and our basic understanding of immunology. In general, for vaccines with multiple shots, the model seems to be that the first shot gets you >50% of the protection, and the second shot mainly extends the protection over time. This might not be how the COVID vaccines work, but your priors should be fairly strong that this is what the clinical trials will ultimately show, because that's what they have shown for a bunch of different vaccines with different mechanisms.
If you're in an ideal world where inaction doesn't cost you anything, sure, do the studies.
In the world we inhabit, thousands were dying every day, and so inaction is morally horrifying. Giving a pair of doses as first shots to two >65yo results in far fewer people dead than giving that pair of doses as a full course to one 65yo. The expected value is pretty clear on this one, even though the clinical trials were not.
You should probably have started to feel tentatively confident that the first-dose-first strategy was sound circa mid-Feb, when it the UK's death rate had been falling rapidly for about two weeks: https://lh4.googleusercontent.com/GGlgMxEKIoaxyuZbuHjjEBH2er... -- while it's hard to disentangle lockdown effects, EU vs. UK was mainly a difference between "locked down with vaccines" and "locked down without vaccines", at risk of oversimplifying.
If I had to summarize in a sentence: use Bayesian reasoning rather than applying unjustifiably strict error bars on your decision process.
In the US, though, I don't think we have ever been forced between these two scenarios: we have always had enough vaccine doses for people in our high risk classes. The question thereby is more whether it is better to have all the essential workers and everyone over 65 years old along with maybe third of everyone else with a single dose or to have the first two classes of people fully dosed... and "I dunno", right? ;P
I'm not sure; January->April the US was constrained by vaccine supply. The initial rollout to at-risk groups was not a case of "we got a shipment of 100m doses, now we need to figure out how to use them", it was more "we're manufacturing 1-2m doses per day and trying to ramp that up". It took months to go through the gradually broadening risk tiers. (e.g. see https://investors.modernatx.com/news-releases/news-release-d..., which says "deliver 100m vaccines by end of March, 100m more by end of May").
I believe the US was keeping shots back in favor of using them later as second doses, rather than giving them immediately as first doses, though I don't have a citation for that to hand. If that's right, the US could have ~doubled its rate of vaccination, i.e. got to the April 15 "all age group open vaccination" milestone in 2 months instead of 4 months, with first doses only administered to that population. At the very least, we were giving second doses in Feb (21-28 days after first doses in Jan) that could have been given as first doses to the higher-risk groups we were still prioritizing through April.
There's a follow-up question on which I have not run the numbers -- are there any X,Y pairs for which you'd rather hold back a shot to give as a second dose later for a >X year old, instead of giving it immediately as a first dose to a <Y year old. Interesting question, I don't think that's what you were getting at though.
We don't have time to RCT test all scenarios. That's how life works. The long term immunity as a function of delta between the first and second shot has not been tested at all. J&J vaccine is one shot mostly because it can only be one shot yet it still works. There are other aspects that have not been tested in the trials. We need to make reasonable decisions under uncertainty in an ever changing landscape.