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What’s the nature of immunity and how long does it last?
Immunologists are working feverishly to determine what immunity to SARS-CoV-2 could look like, and how long it might last. Much of the effort has focused on ‘neutralizing antibodies’, which bind to viral proteins and directly prevent infection. Studies have found that levels of neutralizing antibodies against SARS-CoV-2 remain high for a few weeks after infection, but then typically begin to wane.
However, these antibodies might linger at high levels for longer in people who had particularly severe infections. “The more virus, the more antibodies, and the longer they will last,” says immunologist George Kassiotis of the Francis Crick Institute in London. Similar patterns have been seen with other viral infections, including SARS (severe acute respiratory syndrome). Most people who had SARS lost their neutralizing antibodies after the first few years. But those who had it really severely still had antibodies when re-tested 12 years later, says Kassiotis.
Researchers don’t yet know what level of neutralizing antibodies is needed to fight off reinfection by SARS-CoV-2, or at least to reduce COVID-19 symptoms in a second illness. And other antibodies might be important for immunity. Virologist Andrés Finzi of the University of Montreal in Canada, for example, plans to study the role of antibodies that bind to infected cells and mark them for execution by immune cells — a process called antibody-dependent cellular cytotoxicity — in responses to SARS-CoV-2.
Ultimately, a full picture of SARS-CoV-2 immunity is likely to extend beyond antibodies. Other immune cells called T cells are important for long-term immunity, and studies suggest that they are also being called to arms by SARS-CoV-23. “People are equating antibody to immunity, but the immune system is such a wonderful machine,” says Finzi. “It is so much more complex than just antibodies alone.”
Because there is not yet a clear, measurable marker in the body that correlates with long-term immunity, researchers must piece together the patchwork of immune responses and compare it with responses to infections with other viruses to estimate how durable protection might be. Studies of other coronaviruses suggest that ‘sterilizing immunity’, which prevents infection, might last for only a matter of months. But protective immunity, which can prevent or ease symptoms, could last longer than that, says Shane Crotty, a virologist at the La Jolla Institute of Immunology in California.
