T-cells are cells in the blood that provide a further way of attacking the virus. There are three ways viruses can be attacked two of which are through antibodies but the other is through T-cells. T-cells attack cells in the body that contain the virus or may even attack the virus directly. Memory T-cells are T-cells that have a memory of a previous infection and spring into action if the body encounters a virus from a previous infection. It may be that T-cells provide a long term way of fighting COVID-19 infections since it appears that the antibody response to the infection may be short-lived (weeks to months). T-cells on the other hand may be active for much longer. If COVID-19 vaccines are to be successful they need to produce not only an antibody response but also T-cell immunity.