Covid immunity: Can you catch it twice?

Coronavirus is a completely new infection in people.

Nobody had immunity at the start of the pandemic - and knowing more about immunity is crucial for understanding what happens next.

Our immune system is the body's defence against infection and it comes in two parts.

The first is always ready to go and leaps into action as soon as any foreign invader is detected in the body. It is known as the innate immune response and includes the release of chemicals that cause inflammation and white blood cells that can destroy infected cells.

But this system is not specific to coronavirus. It will not learn and it will not give you immunity to the coronavirus.

Instead you need the adaptive immune response. This includes cells that produce targeted antibodies that can stick to the virus in order to stop it - and T cells that can attack just the cells infected with the virus, called the cellular response.

This takes time - studies suggest it takes about 10 days to start making antibodies that can target the coronavirus and the sickest patients develop the strongest immune response.

If the adaptive immune response is powerful enough, it could leave a lasting memory of the infection that will give protection in the future.

It's not known if people who have only mild symptoms, or none at all, will develop a sufficient adaptive immune response.

Understanding of the role of T-cells is still developing, but a recent study found people testing negative for coronavirus antibodies may still have some immunity.

For every person testing positive for antibodies, it was found two had T-cells which identify and destroy infected cells.

"They look rather durable and are being made in almost all exposed people," said Prof Danny Altmann from Imperial College London.

The immune system's memory is rather like our own - it remembers some infections clearly, but has a habit of forgetting others.

Measles is highly memorable - one bout should give lifelong immunity (as the weakened version in the MMR vaccine does). However, there are many others that are pretty forgettable. Children can get RSV (respiratory syncytial virus) multiple times in the same winter.

The new coronavirus, Sars-CoV-2, has not been around long enough to know how long immunity lasts.

But a recent study led by Public Health England (PHE) shows most people who have had the virus are protected from catching it again for at least five months (the duration of the analysis so far).

Some are reinfected, however, and, even if asymptomatic, can then harbour high levels of the virus in their noses and mouths, which can be passed on to others.

PHE will continue to monitor the people in this study, who are all healthcare workers, to see how long immunity lasts.

Other clues may come from studies involving other coronaviruses.

Four produce the symptoms of the common cold and immunity is short-lived. Studies showed some patients could be re-infected within a year.

Research at King's College London also suggested levels of antibodies that kill coronavirus waned over the three month study.

But even if antibodies disappear, then the cells that manufacture them, called B cells, may still be around. B cells for Spanish Flu have been found in people 90 years after that pandemic.

If the same is true with Covid, then a second infection would be milder than the first.

It is also not understood what happens to T cells in the long term. But T cells against the original Sars (Severe Acute Respiratory Syndrome) have been found 17 years later.

Maybe.

The jury is still out on the field of "cross-reactivity" but there may be some infections that look similar enough to the virus that causes Covid that people may gain some protection.

Laboratory tests show the T cells some people made to fight Sars or common cold coronaviruses can also react against the new coronavirus.

How common this is and how much protection it gives is still unknown.

There were early reports of people appearing to have multiple coronavirus infections in a short space of time.

But the scientific consensus is that testing was the issue, with patients being incorrectly told they were free of the virus.

PHE's ongoing study on immunity in healthcare workers found 44 potential re-infections in a group of 6,614 people who had previously had the virus.

Researchers conclude reinfection is uncommon but still possible and say people must continue to follow current guidance, whether they have had antibodies or not.

Scientists from Hong Kong recently reported on the case of a young, healthy man who recovered from a bout of Covid-19 only to be re-infected more than four months later. Using genome sequencing of the virus, they could prove he caught it twice because the virus strains were different.

Experts say re-infection isn't surprising, but it's likely to be rare, and larger studies are needed to understand why this might happen.

This is not guaranteed and that is why the World Health Organization is nervous about countries using immunity passports as a way out of lockdown.

The idea is if you pass the antibody test then you are safe to go back to work. This would be particularly valuable for staff in care homes or hospitals who come into contact with those at risk of developing severe symptoms.

But while you will find some antibodies in nearly every patient, not all are equal. Neutralising antibodies are the ones that stick to the coronavirus and are able to stop it infecting other cells. A study of 175 recovered patients in China showed 30% had very low levels of these neutralising antibodies.

That is why the World Health Organization says "that cellular immunity [the other part of the adaptive response] may also be critical for recovery".

Another issue is that just because you might be protected by your antibodies, it doesn't mean you cannot still harbour the virus and pass it onto others.

Everyone's immune system is unique - some may mount more successful defensive responses than others.

And people must continue to follow the current guidance as some may become reinfected and pass the virus on to others, PHE says.

People who have already had Covid-19 should still have a vaccine when offered one, officials say.

Dr Julian Tang, at the University of Leicester, says:"Having the vaccine after recovering from Covid-19 is not an issue... and will likely boost the natural immunity.

"We also see this with the seasonal flu vaccine."

PHE scientists will also monitor the immunity of the participants in its Siren study who go on to have vaccines.

It matters for obvious personal health reasons and whether you will get Covid-19 multiple times and how often.

Immunity will also affect how deadly the virus is. If people retain some, even imperfect, protection then it will make the disease less dangerous.

Understanding immunity better could help ease lockdown if it is clear who is not at risk of catching or spreading the virus.

If it is very difficult to produce long-term immunity, then it could make a vaccine harder to develop. Or it may change how the vaccine needs to be used - will it be a once a lifetime or once a year like the flu shot.

And the duration of immunity, whether by infection or immunisation, will tell us how likely we are to be able to stop the virus spreading.

These are all big questions we still lack answers to.

Follow James on Twitter