Monday Wall Street Journal has a front page article
“Covid-19 Vaccines’ Duration Is Uncertain”.

Surveys of recovered Covid-19 patients mostly show that antibodies begin to fade as early as 90 days.

T-cell duration is unknown,
but recovered SARS coronavirus patients from year 2002 still have T-cells that can not only kill SARS infected cells, but many can further kill today’s Covid-19 virus infected cells as well.