[set fair (OP)]

We read about vaccines resulting in x% of trial particpants producing antibodies. What about specific T cells?

[evan_au]

It's not easy to count the number of T-Cells which react to SARS-CoV2 (or any other specific target).

Your body has many different strains of T-cells, each targeting a different pathogen to which you have been exposed in th epast (or against which you have been vaccinated). Sometimes different strains of T-cells could be targeting different aspects of the same pathogen. There could be hundreds or even thousands of T-Cell strains.

The challenge here is to identify and count the individual T-cells which respond to the coronavirus (out of many T-Cells in a blood sample). This is not easy, as they T-Cells must interact with other cell types (eg dendritic cells) to activate the T-Cells.
- This T-Cell selection process is not easy - just look at the prices being charged for monoclonal antibody treatments

While some of these T-Cells circulate in the bloodstream (and are thus easy to collect), other T-Cells hide in lymph nodes or in particular tissues, and so can only be collected by a biopsy (much more intrusive).

See: https://en.wikipedia.org/wiki/T_cell#Cytotoxic_CD8+_T_cells