2.2 Seroconversion

Seroconversion refers to the point in time when an infected person has detectable antibodies against the infectious agent. For SARS-CoV2, seroconversion occurs at 7−14 days after the infection and antibody titres typically increase to a maximum at about one month after infection and then gradually decline, provided that the person does not become reinfected.

Different classes of antibody last for different lengths of time in plasma in vivo, as shown in Table 2. The measure of persistence is the half-life of the antibody – the amount of time in which half of the original amount is lost. Notice also, that in humans there are four different subclasses of IgG (IgG1 – IgG4) and two of IgA (IgA1, IgA2), which have slightly different characteristics and functions.

While antibodies may only last for a few months, antibody production from B cells and plasma cells lasts for many months after a secondary antigen challenge. and the memory of how to produce these antibodies, residing in memory cells, can last for many years. Consequently, when a person has undergone seroconversion, they usually have detectable antibodies against the pathogen for years. This is a generalisation, and it varies with the pathogen and the individual.

One important question during the COVID-19 pandemic was how long the antibodies and immunity would last. The short answer is that antibodies and antibody production continued for many months following natural infection, and a level of immunity to severe disease lasted much longer than that. The main problem with SARS-CoV2 was not a decline in immunity to the original infection but that the virus mutated to evade the antibodies produced against earlier strains. You will return to this subject in Week 8 and delve further then.