What We Do and Don’t Know About COVID-19 Immunity

Those following the development of the COVID-19 pandemic have probably been introduced to the concept of herd immunity – the idea that once a sufficient proportion of the population acquires immunity, it becomes much harder for the disease to spread and infect unprotected individuals. Developing immunity amongst the population, particularly through vaccination, forms and integral part of our strategy against infectious diseases and the current pandemic. However, we still lack an essential piece of information: how long does immunity against COVID-19 last?

When the immune system responds to an infection, some of the immune cells that recognise the offending pathogen will become T and B memory cells. These cells exist ‘on standby’ to respond should the same pathogen ever be encountered again, eliminating it before it has the chance to cause disease. Unfortunately, this population of cells dwindles over time, as do the antibodies produced by specific B cells. Furthermore, the duration of immunity against different diseases can vary, in some cases being lifelong, and in others short-lived.

So are there any indications of how long immunity to COVID-19 might persist? Towards the start of the pandemic, there were reports of some confirmed COVID patients apparently recovering, testing negative, and then testing positive again soon after. However, this could have been due to the second test being a false negative: in other words, the virus never actually left these patients’ systems. Research suggests that, at least in monkeys, re-infection was not possible four weeks after primary exposure. With over 1.1 million people having recovered from the disease worldwide according to the Johns Hopkins coronavirus resource centre, and no clear cut examples of reinfection, most experts are confident that getting COVID-19 does at least grant immunity in the short term.

As for the long term, it is impossible to say with certainty how long immunity will last at this stage. The best we can do is to look at the duration of immunity against other coronaviruses. In the case of Sars-CoV, the coronavirus that emerged in 2002, research suggests that memory T cells lasted up to 11 years. Another study suggested that SARS-specific antibodies persist for an average of 2 years, while antibodies against MERS persist for just one year.

However, the existence of memory cells or antibodies doesn’t guarantee total immunity, or that a person cannot still spread the virus to others. This is one reason many experts have tried to temper enthusiasm around antibody tests and the idea of ‘immunity passports’. We must also consider that even if protection is robust and long-lasting, Sars-CoV-2 might mutate from year to year such that previously acquired immunity becomes ineffective. COVID-19 would then join the ranks of seasonal influenza, requiring a new vaccine each year.

Overall, there is confidence that those that recover from COVID-19 do acquire immunity. Based on what we know of other coronaviruses, we can expect this immunity to last around 2 to 3 years, but it is still far too early to say with any certainty.