https://www.lrb.co.uk/the-paper/v42/n10/rupert-beale/short-cuts

Measuring immunity is important, but it isn’t easy. The most obvious way is to look for the presence of antibodies. But antibodies to what? The virus has many components. Its main entry weapon is known as Spike. This is a large, sugar-coated protein complex that can rip a hole in the membrane of a cell to allow the virus to enter. Block Spike, and you keep the virus out. It’s easy enough to measure antibodies to Spike, but not all of them actually prevent the virus from entering cells. To find out whether the antibodies are doing their job effectively, you have to culture the virus in a high-containment facility, titrate tiny amounts of serum extracted from the test subject’s blood into the virus culture, and demonstrate that the serum blocks the virus. It’s painfully slow.

...

There are four ‘seasonal’ coronaviruses – 229E, OC43, NL63 and HKU1 – that cause mild disease in nearly everyone, only occasionally causing pneumonia. They can be given to healthy volunteers to study the immune response. They cause the ‘common cold’, and in experimentally infected humans they give rise to an antibody response. That response wanes after a few months, and the same people can be experimentally reinfected, though they tend to get milder symptoms the second time round. It is thought that adults get reinfected on average about once every five years. Sars-CoV-2 causes mild disease in most cases, and gives rise to antibody responses in nearly all cases. We don’t know how long these responses will last, but it is likely that people who suffer only mild disease will be susceptible to reinfection after a few months or years. Humanity has never developed ‘herd immunity’ to any coronavirus, and it’s unlikely that Sars-CoV-2 infection will be any different. If we did nothing, a likely possibility is that Covid-19 would become a recurring plague. We don’t know yet. It may have seemed like an aeon, but we have been aware of this virus for only a few months.