Does infection with the novel coronavirus protect you from re-infection? Should we issue “immunity passports”? A thread. 1/14

There is now plenty of evidence to suggest that people infected with the novel coronavirus develop an immune response. Nearly everyone who has recovered from COVID-19 has antibodies in their blood that recognise bits of the virus. 2/14

However a few people don’t make a strong antibody response. There are other ways the immune system can fight viruses though. “T cells” can become activated and kill virus-infected cells. Once its host cell has been killed, the virus can’t get out and spread. 3/14

So can infection with the novel coronavirus activate our T cells? It’s too soon to know for sure but the early data look promising - most people who have been infected with SARS-CoV-2 appear to have mounted a T cell response. 4/14

Antibodies take 1-2 weeks to appear and T cell responses also take time. We also have innate immunity that acts straight away. If innate immunity is strong, it may clear the virus before it can trigger an antibody response. 5/14

So if people are making good immune responses to the novel coronavirus (antibodies, T cells, innate immunity) why can’t we just issue “immunity passports”? It’s a bit more complicated than that and here’s why… 6/14

Immune responses are not just black & white (either you’ve made one or you haven’t) – the quality of the immune response matters. It’s not like pregnancy testing, where you are positive or not. You may have antibodies to the virus – but the question is are they any good? 7/14

The crucial antibodies are the ones that act to stop the virus getting into cells. These are called neutralising antibodies - they literally neutralise the ability of the virus to infect. Standard antibody tests don’t measure this neutralisation capacity. 8/14

More complicated lab tests can be used to measure neutralising antibodies. Limited data so far suggests that most people who have recovered from SARS-CoV-2 infection have made neutralising antibodies, with some people making higher levels than others. 9/14

However we don’t yet know whether people with neutralising antibodies will be protected from re-infection and how long such protection might last – this is mainly because these individuals need to be followed over time, and not enough time has gone by yet. 10/14

Importantly, we still need to work out which of the standard antibody tests gives us the best measure of protective immunity. The tests need to be properly validated to ensure they are sensitive and accurate. 11/14

The immune response to this sort of virus also tends to fade over time so we need to find out how long immunity lasts. Your antibody response may be strong today, but weak if re-tested 2 years from now. We don’t know how strong it needs to be to offer protection. 12/14

It seems likely that most people who have been infected will have “immune memory”, at least for a while, meaning that when exposed to the virus a second time they are protected from infection or have a milder disease course. 13/14

Overall, because we don’t have widely available tests to pinpoint which individuals have protective immunity, and because we don’t know how long immunity lasts, it’s hard to translate these encouraging findings into public health policy just yet. 14/14