There are two major types of "memory" immune responses – changes to the body that mean you are able to recall a previous threat in order to mount a rapid protective response on reinfection. The first is driven by B cells, which produce antibodies. Vaccine research aims to generate potent, long-lasting antibodies that can protect us for life, but this is not always achieved. When antibodies wane, booster vaccinations can help. When viruses evolve to escape detection, like the fast-mutating flu, a newly designed vaccine is needed to stop them in their tracks.
The second cell type able to remember an infection is the T cell. T cells may be sufficient to control infection in the absence of antibodies, and act by organising immune defences (so-called "helper" T cells) or directly killing infected cells to restrict new virus production (cytotoxic T cells). T cell responses have been detected in most Covid-19 patients, and first-in-human vaccine trials have reported potent T cell activation. It is possible that T cells' memory of Sars-CoV-2 may last longer than antibodies, as is the case in other coronaviruses.
Can you develop a T cell response without developing antibodies? That seems to be a possibility: a small study of patients and their families shows that an unexpected six out of eight family members who caught the virus at home had T cell responses but no detectable antibodies.
https://www.theguardian.com/commentisfree/2020/jun/25/viral-immunologist-antibody-tests-covid-19-immuity-coronavirus
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