A longitudinal study tracking 173 individuals found that T cell numbers and function did recover in most people after Covid by 6 months, but a subset of people exhibited persistent lower CD8+ T cell counts for months. Patients exhibited lower CD3+ T cell stimulation in response to a variety of viral antigens during acute Covid infection, and while functional response improved for most antigens at 6 months, responses remained significantly lower against adenovirus in people who had recovered from either mild or severe Covid.
https://onlinelibrary.wiley.com/doi/full/10.1111/all.15372