[rleigh]

Two doses is very common for vaccines because this is based upon how the immune system responds to real infections. The first dose will initiate a primary immune response. The second dose will act as a "challenge" to trigger a secondary immune response.

[ed25519FUUU]

It sounds like one dose is enough to make you much less likely to develop serious symptoms or symptoms at all, and two doses gives you close to immunity.

[rleigh]

Absolutely, after the first shot you do have immunity but it's not as fully developed compared with having two, so it will take longer to respond than with two, and that does present a larger window of opportunity for the virus to cause problems.