https://www.scientificamerican.com/article/the-risks-of-rushing-a-covid-19-vaccine/

The excitement and enthusiasm for a COVID-19 vaccine by the end of 2020 is both palpable and understandable. We all hope for a rapid end to the pandemic and an effective vaccine would be a surefire solution. But there are risks that come with a fast-tracked vaccine delivered end of this year, not the least of which are the risks related to the safety of the vaccine itself.

Telescoping testing timelines and approvals may expose all of us to unnecessary dangers related to the vaccine. While preclinical trials to evaluate the potential safety and efficacy of vaccine candidates are likely to include tens of thousands of patients, it is still unclear whether that number will be large enough and a trial will last long enough to evaluate safety for a drug that would be administered to so many. The US alone plans to vaccinate hundreds of millions of people with the first successful candidate. One serious adverse event per thousand of a vaccine given to 100 million people means harm to 100,000 otherwise healthy people.