https://www.mayoclinic.org/diseases-conditions/coronavirus/in-depth/coronavirus-vaccine/art-20484859"Coronavirus vaccine challenges
Past research on vaccines for coronaviruses has also identified some challenges to developing a COVID-19 vaccine, including:
Ensuring vaccine safety. Several vaccines for SARS have been tested in animals. Most of the vaccines improved the animals' survival but didn't prevent infection. Some vaccines also caused complications, such as lung damage. A COVID-19 vaccine will need to be thoroughly tested to make sure it's safe for humans.
Providing long-term protection. After infection with coronaviruses, re-infection with the same virus — though usually mild and only happening in a fraction of people — is possible after a period of months or years. An effective COVID-19 vaccine will need to provide people with long-term infection protection.
Protecting older people. People older than age 50 are at higher risk of severe COVID-19. But older people usually don't respond to vaccines as well as younger people. An ideal COVID-19 vaccine would work well for this age group."
