The samples of the staff in the study indicate that the T-cell response was unchanged after nine months. The T cells are a type of white blood cells that are a significant part of the body’s immune system. They learn to recognize different types of invaders and attack them, for example by killing infected cells.
The hypothesis is that the vaccine provides a sustainable defense against covid via the T cells, then in particular protection against severe illness and death. The study mentions that other studies need to be done on larger groups to strengthen the view