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.
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.