[this_user]

> Now picture what would have happened if we had been willing to do challenge trials early on for COVID.

Nothing much most likely. The mRNA vaccines were designed very rapidly after the virus itself had been sequenced. The thing that took time was moving through the different trial phases. Given that there was more than enough spread of the virus in the wild, deliberately exposing people might have shaved off a few weeks at most.

But the real bottleneck afterwards was production and roll-out of the vaccines anyway. So realistically, challenge trials would not have had any meaningful impact.

[s1artibartfast]

>The thing that took time was moving through the different trial phases. Given that there was more than enough spread of the virus in the wild, deliberately exposing people might have shaved off a few weeks at most.

I dont think that is accurate. You can run a challenge trial from exposure to outcome in weeks, whereas it takes many months and tens of thousands of people to get enough cases in the wild.

It may still be that production was the critical path, but challenge trials are much faster.

[dredmorbius]

In the case of COVID-19, for persons circulating in public, exposure was pretty much guaranteed during peak periods of the pandemic, most particularly (Northern Hemisphere) Summer 2020 and Winter 2020 (June -- August and November -- February, mostly). Challenge exposure would have been largely redundant, though it might well have been a utilised protocol.

The time constraints were the sequencing of multiple innoculations (most vaccinations require at least two doses spaced at 2+ weeks for preferred effect, with additional subsequent boosters increasing effectiveness), time from final innoculation to infection, observing course of illness, if any, and monitoring for any fall-off in immunity over time. Even on an accelerated basis that's a 90-day period, roughly, and longer-term assessments of 1--2 years are of clinical interest. Though once it became clear that vaccination showed greatly-increased immunity within weeks of treatment, authorisation of the vaccine was virtually assured.

At which point ramping up production and distribution were the challenges.

The initial mRNA vaccines were prototyped within days of the SARS-COV-2 virus being sequenced. It took nearly 18 months to go through the testing, production, and distribution of the vaccine to the point that anyone who wanted a vaccine could obtain one. Little of that lag had much to do with trial phases, though identifying better treatments (several of the vaccine lines proved less effective, notably the Chinese-created vaccine AFAIU) also had value. Parallel development and testing was effectively pursued in this case.