COVID-19 isn’t going away any time soon, and the number of cases continues to go up in many parts of the world, including the US and UK. If you’re pretty sure you've already had the virus, you might be thinking that you're now safe. But can you get COVID-19 twice?

As worrying reports come in of people catching coronavirus for a second time, we talk to Professor Tim Spector about the latest science on COVID-19 immunity and the chances of reinfection.

Can you catch COVID-19 again?

You may think that once you've had COVID-19, which is caused by infection with the SARS-CoV-2 coronavirus, you’re unlikely to get it again.

While this is the case for some viruses, like chickenpox and measles, there are plenty of viruses we can catch more than once. This includes common colds  - which can be caused by other types of coronavirus that can infect people multiple times - and the flu.

The good news is that there have been very few reports of COVID-19 reinfection coming in from around the world so far.

"We've had six months experience of this virus now, and the number of confirmed people who have had it twice is extremely small," says Tim, explaining that he has only heard of around four or five cases of proven reinfection across the whole of Europe.

Let’s look at why this might be happening.‍

COVID-19 antibodies might protect you from reinfection

Antibodies are molecules produced by special cells in the immune system (known as B cells) in response to infection. They bind to viruses and prevent them from entering our cells.

Antibodies tend to stay in the blood for some time, so if a person encounters the same virus again, they can mount an immune response quickly. But they don't stick around forever.

Research suggests that antibodies for the SARS-CoV-1 coronavirus, which caused the SARS pandemic in 2003, start to disappear about three years after infection.

A small study from researchers in the UK showed that antibodies against the COVID-19 SARS-CoV-2 virus can last for a few months, which could explain why we haven't seen many cases of reinfection so far. In some people however antibody levels had started to go back to normal after 60 days.

The big question is what happens when these antibodies start to disappear. Will people become vulnerable to reinfection?

"We thought that because some people don't appear to develop antibodies, or they disappear quickly after the infection, that there would be lots of people getting reinfected, but that doesn't seem to be the case," says Tim. "So there's more to immunity than just antibodies alone."

Our immune system is more than just antibodies

Besides antibodies and the B cells that make them, we have another important weapon in our immune arsenal: T cells.

T cells have a range of jobs including protecting us against invading pathogens like bacteria and viruses, destroying infected cells, and controlling the production of antibodies by B cells.

After infection, our immune system creates so-called memory T cells. As the name suggests, these cells ‘remember’ previous encounters with pathogens and respond quickly to repeat infections.

While antibody immunity often decreases relatively quickly over time, immunity resulting from memory T cells can last much longer.

Memory T cells can also respond to viruses that look similar to ones we have previously encountered, even if we have never been infected with the new threat. This explains why a recent study has shown that 20-50% of people in the US have T cell responses to SARS-CoV-2 even if they have never had it, which could offer some degree of immunity.

"The latest research into COVID-19 immunity suggests T cell responses might be much more important than we thought, and even people exposed to recent cold viruses might be given some protection" says Tim.

What about herd immunity?

There has been a lot of talk about the concept of ‘herd immunity’. This is the idea that COVID-19 will no longer be a threat when enough of the population has developed immunity to the virus, either by being directly exposed or through vaccination.

Given that a vaccine is still some way off, is there any chance that a significant proportion of the population could be protected by having caught COVID-19 already?

"We tested around 400 of the participants in the TwinsUK study living in the South East of England, and 12% of them tested positive for COVID antibodies,” Tim says. “However, around half of people who have been infected may have had only a brief antibody or immune response so the proportion of people who have some degree of immunity may well be higher."

Although the TwinsUK participants may not be fully representative of the whole population, it’s reasonable to assume that at least one in ten people in the South East may have had COVID-19 already. This may explain why the second wave now seems to be hitting the North of the UK much harder than the South.

It could also be because the rates of immunity are higher in younger, fitter people who are now going out and about compared with older people and those with underlying health conditions who are tending to stay home.

This could be creating a temporary ‘immunity buffer’, slowing down the rate of transmission and buying some time to flatten the growth in infections. But given how rapidly rates are now rising, it’s not clear how long this will last.

Even so, researchers think that around 50-70% of the population need to be immune to coronavirus to benefit from herd immunity long term. This is unlikely to be achieved any time soon based on the number of people who have contracted COVID-19 so far.

More importantly, allowing COVID-19 to spread through the population unchecked would likely lead to the deaths of many tens of thousands of vulnerable elderly people and cause significant long term health problems such as ‘Long Covid’ in up to one in ten people.

It's worth noting that if some of us have pre-existing protective memory T cells, herd immunity thresholds will be lower and fewer of us have to contract the disease to protect the whole population. But right now, it’s too soon to tell.

Help us understand COVID-19 immunity and reinfection with just one minute a day of your time

Millions of users of the COVID Symptom Study app are providing vital information about COVID-19 immunity and reinfection. Their data is helping us to discover how long immunity lasts, whether people can experience symptoms a second time, and if there are areas of the country that have already reached some level of partial herd immunity.

Even if you have had suspected or confirmed COVID-19 already, Tim urges you to download the COVID Symptom Study app and take just one minute a day to log your health.

“Did you get infected earlier in the year and stop logging because you think you can't get it again? Or have you had a positive test but have never felt ill? We need you to come back and be part of this vital study so we can find out more about reinfection and immunity to help protect everyone."

Whether you’ve tested positive for coronavirus, think you might have had COVID-19 or are sure that you haven’t, we need as many people as possible to download the COVID Symptom Study app and encourage friends and family to join too.

It takes just a minute every day to contribute to the world’s biggest COVID-19 science project, providing life-saving insights that will help us through the months ahead.

Find out more:

• Longitudinal evaluation and decline of antibody responses in SARS-CoV-2 infection – MedRxiv
• Coronavirus immunity: Can you catch it twice? – BBC
• SARS-CoV-2 T cell immunity: Specificity, function, durability, and role in protection – Science Immunology
• Covid-19: Do many people have pre-existing immunity? – The BMJ
• Estimates of the rate of infection and asymptomatic COVID-19 disease in a population sample from SE England – MedRxiv
• What the immune response to the coronavirus says about the prospects for a vaccine - Nature