It’s going to be harder to conduct trials and get valid results with a declining number of cases.
Which will then make the excuse that the trials didn’t show anything.
Below are two Covid related links. The first compares the 4 vaccines most in our US news. The second concerns serious manufacturing problems affecting availability of vaccines.
https://www.prevention.com/health/a35118263/astrazeneca-vs-pfizer-vs-moderna-covid-19-vaccine/
https://www.politico.com/news/2021/04/07/covid-variants-drug-manufacturing-479902