Andrew Badley, M.D., COVID-19 Research Task Force Chair, Mayo Clinic: Why you need two doses is that the amount of immune protection that you get increases with each dose. We know that after one dose there's a degree of protection, but after the second dose there's more protection.
The time from receiving the vaccine until you are immune varies. We know that immune response normally takes about two to three weeks. And we know that if you've received one dose of vaccine, then you have partial immunity to it.
After the second dose of vaccine, there's more immunity. So in general, after two to three to four weeks, you should have an immune response to the vaccine and have immunity.