Researchers studying Covid-19 immunity have determined that T cells specific to the novel Coronavirus are detectable for at least six months in patients who experienced asymptomatic and mild Coronavirus cases that did not require hospitalization.
The immunity study delivers great news for vaccine research, as many vaccines should also induce neutralizing antibodies and T cell responses.

https://bgr.com/2020/11/03/coronavirus-immunity-duration-t-cell-study-uk-cic/