Almost all COVID-19 survivors have the immune cells necessary to fight re-infection for at least eight months after the initial infection, according to a US study.
The research at La Jolla Institute for Immunology, San Diego, California, is believed to be the biggest study, for any acute infection, that has measured all four components of immune memory – antibodies, memory B cells, helper T cells and killer T cells at the same time.
The team, co-led by Professor Alessandro Sette with Professor Shane Crotty, research assistant Professor Daniela Weiskopf, analysed 188 COVID-19 survivors and found that although antibodies decline over time, they had T cells and memory B cells to fight re-infection.
“Our data suggest that the immune response is there and it stays,” said Prof Sette, whose findings are published in Science.
It means COVID-19 survivors could have protective immunity against serious disease from the SARS-CoV-2 virus for months, perhaps years after infection. However, the authors caution that protective immunity varies significantly between individuals.
They found that virus-specific antibodies remain in the bloodstream months after infection and that memory B cells could reactivate and produce SARS-CoV-2 antibodies to fight re-infection.
When they looked for memory B cells specific for the SARS-CoV-2 spike, they found that they increased in the blood six months after infection.
COVID-19 survivors also had T cells ready to fight re-infection, while memory CD4+ "helper" T cells and many memory CB8+ "killer" T cells also remained, ready to destroy infected cells.
The researchers said the different parts of the adaptive immune system work together, which meant that COVID-fighting antibodies, memory B cells, memory CD4+ T cells and memory CD8+ T cells in the blood more than eight months following infection was a good sign.
“This implies that there’s a good chance people would have protective immunity, at least against serious disease, for that period of time, and probably well beyond that,” said Prof Crotty.
However, the researchers saw a 100-fold range in the magnitude of immune memory, which could mean that people with a weak immune memory may be vulnerable to a case of recurrent COVID-19 or could be more likely to infect others.
Finding that immune memory against SARS-CoV-2 is possible is also a good sign for vaccine developers, say the authors.
“It is possible that immune memory will be similarly long lasting similar following vaccination, but we will have to wait until the data come in to be able to tell for sure,” said Prof Weiskopf.
“Several months ago, our studies showed that natural infection induced a strong response, and this study now shows that the responses last. The vaccine studies are at the initial stages, and so far, have been associated with strong protection. We are hopeful that a similar pattern of responses lasting over time will also emerge for the vaccine-induced responses.”
The researchers are to continue analysing samples from COVID-19 patients and hope to track their responses 12 to 18 months after the onset of symptoms. They are also examining how immune memory differs across people of different ages and how that may influence COVID-19 case severity.
Dan JM, Mateus J, Kato Y et al. Immunological memory to SARS-CoV-2 assessed for up to eight months after infection. Science 7 January 2021; doi: 10.1126/science.abf4063
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