Are people who have had the coronavirus now immune to it?

If the answer is “yes”, it raises the tantalising prospect that the current lockdown could be eased without having to wait for a vaccine – which is expected to take over a year. There’s even been talk of so-called “immunity passports”.

But the science on Covid-19 immunity is far from settled. Let’s take a look at what we know – and what we don’t.

Antibody tests

There are two categories of coronavirus test. One looks for the presence of the virus right now, the other tells you if have the antibodies needed to fight it off.

If you have those antibodies, then – whether you had symptoms or not – you have been exposed to the coronavirus and your immune system has found a way to defend itself, at least for now.

The UK government announced some weeks ago that it had bought millions of antibody tests, but was still checking whether or not they worked. This week, it emerged that these particular tests are not reliable.

Nevertheless some are still optimistic that once an effective model becomes available, it could form the basis of “immunity passports”, with those found to have the antibody perhaps allowed greater freedom to move around.

Experts urge caution

But there’s still a lot we don’t know about coronavirus immunity.

Daniel M Davis, who is Professor of Immunology at the University of Manchester, told FactCheck that “we don’t know” whether people who are found to have the coronavirus antibody are definitely immune even at the time the test is taken.

“Some level of immunity is almost certain but we don’t know how protective exactly, nor how long protection would last”, he says.

Dr Bharat Pankhania, Senior Clinical Lecturer at the University of Exeter Medical School, told the Science Media Centre: “With respect to immunity testing and the issuing of ‘immunity’ certificates, I advise caution.

“Covid-19 belongs to the same family of viruses as the common cold virus. Infections with the common cold virus do not produce strong immunity and the immunity is usually short lived, usually about three months. We do not yet know how long immunity to the Covid-19 virus would last for.”

Professor Davis says: “One study has shown that antibodies are detectable for at least 20 days but this is anecdotal and we don’t know how long they would stay detectable. For the 2002-3 SARS epidemic it’s thought that protection lasts about a year.”

Polly Roy, who is Professor of Virology at the London School of Hygiene and Tropical Medicine, told FactCheck that the antibody “should be” effective against the virus for “several months to one year or more generally depending on antibody level”.

Dr Pankharia says we should take note of the uncertainty in this area and “continue to maintain infection control measures, post-infection, while research is being done”. In other words, we may need to maintain some or all of the current measures, even if a reliable antibody test becomes available.

What if the virus mutates?

We asked our experts whether a mutation in the coronavirus could leave even those who already have the antibody at risk of re-infection.

Professor Davis told FactCheck that the likelihood of this is “not known” but that “this new virus does not vary as much as, say, influenza”.

Professor Roy says the answer “depends on where the mutation is in the viral genome”, and suggests that it could be a problem if the mutations happen in a part of the virus called the “host receptor bind site”.

What about reports of people being re-infected?

Early on in the pandemic, there were reports – for example in Japan – of a handful of patients testing positive for Covid-19 after having already recovered from the virus.

When the reports first came through in February, Professor Mark Harris of the University of Leeds told the Science Media Centre that “it is unlikely that they would have been reinfected having cleared the virus”. He suggested that the second positive test result might be explained by the possibility that the patient “remained persistently infected”.

Speaking to FactCheck today, Professor Roy said: “Generally, they should not be re-infected but  they might still have some residual replicating viruses in their body.”

And Professor Davis points out that “it’s very difficult to make generalisations from small case numbers. One issue is that we will all respond differently to the virus and levels of immunity are likely to also vary.”

Like with many areas of coronavirus science, we have to wait for more research to say anything for certain.