Researchers analyzed the immune response of people who had been infected with SARS-CoV-2 and recovered. The results of this study are very encouraging

“If we had seen only marginal immune responses, we would have been concerned,” says Sette, a professor in the Center for Infectious Disease and Vaccine Research, and adds, “but what we see is a very robust T cell response against the spike protein, which is the target of most ongoing COVID-19 efforts, as well as other viral proteins. These findings are really good news for vaccine development.”
The authors indicate the level and type of response is exactly what you would expect from someone who is immune to the virus. One part which isn't about the vaccine but I found particularly interesting:
The teams also looked at the T cell response in blood samples that had been collected between 2015 and 2018, before SARS-CoV-2 started circulating. Many of these individuals had significant T cell reactivity against SARS-CoV-2, although they had never been exposed to SARS-CoV-2. But everybody has almost certainly seen at least three of the four common cold coronaviruses, which could explain the observed crossreactivity.

It is still unclear, though, whether the observed crossreactivity provides at least some level of preexisting immunity to SARS-CoV-2 and therefore could explain why some people or geographical locations are hit harder by COVID-19.
This is QUITE interesting to me, as it is a VERY important part of the picture. If we could identify people who can already mount a significant immune response we will have a better picture of risk and potential outcomes.

Source: https://www.lji.org/news-events/news...e-development/