""Most studies have shown that people who have recovered from infection have antibodies to the virus...However,some of them have very low levels of neutralizing antibodies in their blood“suggesting that cellular immunity may also be critical for recovery”" https://www.reuters.com/article/us-health-coronavirus-who-idUSKCN2270FB

https://en.wikipedia.org/wiki/Cell-mediated_immunity

This thread (from a reputable source), on the other hand, compiles some of the evidence in favour of immunization (with links to the relevant articles, and some useful caveats): https://twitter.com/dylanhmorris/status/1254043046695616513?s=20