3.1 Boosters and the duration of immunity

As you may have noted from your laboratory investigation, antibody titres decline gradually after 6 months. This is partly due to the progressive loss of antibodies from the serum and partly because the antibody producing plasma cells stop production and die after 1 or 2 months. To maintain protection, it is necessary to give booster doses of vaccine. How often boosters are needed varies depending on the infectious agent.

A major problem with SARS-CoV2 and influenza-A is that the viruses mutate regularly. Mutation particularly affects the external proteins involved in attachment to target cells, ie the spike protein of SARS-CoV2 and the haemagglutinin of influenza-A. Mutation of these proteins means that protective antibodies against them may no longer bind; the degree to which this occurs varies between individuals.

At this point, it is worth repeating that protection against reinfection is primarily due to antibodies, but protection against disease involves T cells as well as antibodies. The antibodies mostly recognise epitopes exposed on proteins on the surface of the virus, but T cells recognise peptides (presented on MHC molecules) which may come from any part of the viral structural proteins, or even from non-structural proteins expressed in an infected cell. Consequently a variant that has evaded recognition by antibodies, may still be recognised by memory T cells.

Next week will look at viral variants and how they evade immune responses.