https://www.scientificamerican.com/article/fauci-optimistic-about-covid-19-vaccines-though-immunity-unknowns-complicate-development/

With millions of lives on the line, researchers have been working at an unprecedented pace to develop a COVID-19 vaccine.

But that speed—and some widely touted breakthroughs—belie the enormous complexity and potential risks involved. Researchers have an incomplete understanding of the coronavirus and are using technology that’s largely unproven.