They don't HAVE to take 10 - 20 years to develop. They usually take that long because there are delays while officialdom gets round to signing off each stage in the procedure, and while the manufacturers raise the money for the next stage. In an emergency, with all the stops pulled out. things move much fastr than normal.
The developers of the Oxford Zeneca vaccine had a blog post in April 2020 at covid19vaccinetrial.co.uk/blog-how-long-will-it-take-get-oxford-vaccine-deployment where their estimate is that 12 months could be enough. They said:-
"Experts have estimated that it will take 12-18 months to develop a new vaccine at high speed. Under normal circumstances, most vaccine development programmes take more than five years, so this is still a considerably accelerated timescale.
This 12-18-month prediction includes the time taken to develop manufacturing processes to produce the vaccine on a larger scale, as well as preclinical testing in animals and evaluation of the vaccine in human participants in a clinical trial. Scientists need to assess the safety and efficacy of the vaccine over a number of weeks and months through phase I, II and III clinical trials. If the vaccine is safe and efficacious, regulatory approval is needed before the vaccine can be deployed.
. Many of these stages can be undertaken more quickly if there are no unexpected roadblocks. Firstly, the use of a platform technology approach, i.e. a vaccine delivery system that has been used before and can be adapted for a new pathogen, can shorten the initial vaccine development time. Also, in an emergency situation, large scale manufacturing could be carried out concurrently while the clinical trial is ongoing, which can shorten the overall timescale for vaccine development. This would mean that if the clinical trial is successful, the vaccine would be ready in larger quantities, to be deployed immediately. Finally, regulatory review of promising candidates is also undertaken faster in an epidemic, because more staff and resources are dedicated to the review process. .
Oxford University is using all these strategies in order to try to make a vaccine available as rapidly as possible once it is proven safe and effective. "