3.3 Persistence of immunity

Recall from week 2, that there are two major components of immune defence against viruses – T cells recognise and destroy virus-infected cells, whereas antibodies prevent virus spread within the body. Antibodies are also important for preventing reinfection, while T cells limit the damage produced by an infection and consequently reduce disease severity.

It has been noticeable during the pandemic, that many people have become reinfected with new strains of the virus, but generally the new infections have produced less-serious disease symptoms. This suggests that T cell immunity persists well, even if the new strains can evade antibody-mediated immunity.

The half-life of antibodies in plasma is a few weeks, depending on the class of the antibody. So there is a natural decline in immunity produced by antibodies. However the long-term ability to make antibodies and the ability of T cells to recognise virus-infected cells lies mostly in the memory populations of T and B cells. So some level of immunity lasts much longer than the antibodies.

At this time (2023) we cannot know how long immunity to SARS-CoV2 would last if there were no reinfections. However it appears that new variants will continue to develop as the disease becomes endemic and less serious. Consequently previously infected or vaccinated individuals will be subjected to regular restimulation of their B cells and T cells and will always have some immunity. This can be enhanced by boosters with vaccines against new variants.

In the final section of the course, we look at the prospects for adapting current vaccines to deal with new variants.