What You Need To Know About Immunity To Covid-19

Immunity in the UK is “waning quite rapidly”, a new study suggests. Here's what that means.

“Have I had coronavirus and, if so, am I immune?” It’s the question many of us are asking – but sadly it isn’t an easy one to answer.

Immunity is defined as being able to resist a particular disease. Specifically, it’s the “protection against a particular disease or illness by particular substances in the blood”, according to the Cambridge Dictionary.

So, if you were immune to Covid-19, it would mean you could be exposed to the virus again and it wouldn’t affect you or cause any nasty symptoms.

But can that actually happen? We asked an expert to explain the science – here’s what he wants you to know.

What are antibodies and do they give me immunity?

Some people who have tested positive for coronavirus have developed antibodies in their blood – these are molecules produced by the immune system to fight infection. In theory, it’s thought people with antibodies would be able to fight the virus if they caught it again.

But as it stands, the World Health Organisation says there’s “no evidence” that people who have recovered from Covid-19 and have antibodies are definitely protected from a second infection. Professor Danny Altmann, an immunology expert from Imperial College London, isn’t aware of any evidence either.

That said, there’s a growing belief that antibodies may protect us from the virus. Harvard scientists did a small study with macaques, where they deliberately infected monkeys with the virus. A month later, the monkeys were reinfected and they didn’t become ill – the scientists found they had developed antibodies.

Test tube experiments have also shown a certain type of antibody – the ‘neutralising antibody’ – can prevent the virus from entering cells. But, not all antibodies are created equal and therefore not all of them can prevent the virus from wreaking havoc again.

It also depends on how many antibodies are present – the higher, the better. A study of people who had recovered from Covid-19 found just 1% of them had high levels of antibodies that could neutralise the virus.

Why do we have antibody tests then?

Antibody tests – also called serology tests – are designed to check if a person has detectable antibodies in their blood. It can, therefore, show if someone has had Covid-19.

Is a test an effective solution to finding out if people have some form of immunity, then? Given the evidence so far, Prof Altmann believes so – although only up to a point. ”[There is] more and more evidence that antibody levels variably wane within a few months,” he says.

Multiple studies have found antibody levels decline quite quickly over a short space of time. Some 365,104 adults took part in three rounds of testing for a study by Imperial College London between late June and September to measure the prevalence of coronavirus antibodies in England.

The study found that antibody levels fell by 26.5% overall during the three-month period. Just 4.4% of adults had some form of immunity against Covid-19 in September, when UK cases began to increase again. This is compared with 6% found to have antibodies between June 20 and July 13, and 4.8% between July 31 and August 31.

Experts said the data from the Real-Time Assessment of Community Transmission (React-2) study showed immunity was “waning quite rapidly”, which could lead to an increased risk of reinfection.

Graham Cooke, professor of infectious diseases at Imperial College London, said: “The big picture here is that after the first wave (of coronavirus), the great majority of the country still did not have evidence of protective immunity.

“Although we are seeing a decline in the proportion of people who are testing positive, we still have a great majority of people who are unlikely to have been exposed. So the need for a vaccine is still very large if you want to try and get a large level of protection in the population.”

Another study from King’s College London (KCL) found a steep decline in patients’ antibody levels three months after infection, The Guardian reported. Blood tests of 90 patients and healthcare workers revealed that while 60% of people had a “potent” antibody response when they first developed the virus, only 17% retained the same potency three months later. In some cases, antibody levels became undetectable.

This, in theory, could mean after a few months of immunity, you’re back to square one and could become reinfected with the virus. There have now been a few cases of people becoming reinfected with Covid-19 around the world – including a 25-year-old man who experienced more severe illness the second time around.

Does everyone develop antibodies?

Some people who have a positive swab test – showing they have the virus – have later found they don’t have any antibodies when having an antibody test.

This is “not uncommon,” says Prof Altmann. His theory is that if the viral load initially was quite low when a person became infected, the innate immune system (the body’s first line of defence) might’ve been able to fight it off without having to fire up an antibody response.

Initially, your immune system tries to fight off the virus with white blood cells (called B cells and T cells). If you become more ill, your immune system produces antibodies to kill the virus. So the antibody response is basically your body’s second line of defence, which tends to kick in about a week after becoming infected.

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Are some people more likely to develop antibodies than others?

There are trials under way in the UK to see if blood from people who’ve had the virus can be used as a treatment for those worst affected by symptoms. Researchers want to see if the antibodies in the blood could help people who are unable to develop their own immune response.

Figures from NHS Blood and Transplant show that 43% of male blood plasma donors had high antibodies, compared to 29% of women. It’s generally agreed that the higher the antibodies, the better. As a result, men are being urged to donate their blood if they’ve had a positive Covid-19 swab test and recovered.

Explaining why men might have a higher antibody count, Professor David Roberts, from NHS Blood and Transplant, said: “Our studies and many others around the world show men with Covid-19 are more likely to become seriously ill than women. This makes them better plasma donors once they’ve recovered.”

Antibodies aside, are there other ways to find out if I have immunity?

Antibodies are one way to find out if people may have immunity. Another is T-cells. The body’s second line of defence to fight off a virus involves the release of B-cells, which produce antibodies, as well as T-cells, which kill infected cells.

T-cells are important, according to a growing body of research. A pre-print study (which means it’s not been peer-reviewed by other scientists) from the Karolinska Institute and Karolinska University Hospital revealed many people with mild or asymptomatic Covid-19 demonstrate “T-cell-mediated immunity” (a.k.a Covid-19 immunity) – even if they did not test positively for antibodies.

According to researchers, this means public immunity is probably higher than antibody tests suggest. “T-cells are a type of white blood cell specialised in recognising virus-infected cells, and are an essential part of the immune system,” said Marcus Buggert, assistant professor at the institute’s Centre for Infectious Medicine.

Roughly twice as many people have developed T-cell immunity compared with those with antibodies, the results showed, which led researchers to suggest more people might have some ability to react to and fight off the virus again.

This could be important, suggests Prof Altmann, as T-cell immunity for coronaviruses seem to last for some years (rather than the mere months that antibodies have been found to last).

It’s also informative, he says, “because T-cell immunity has been detectable in people who never show a positive antibody test”.

In short, immunity isn’t simple.

If there’s one thing Prof Altmann wants us to know, it’s that immunity to viruses is very complex, “encompassing different types of white blood cells making very diverse molecules, from different antibodies to different cytokines”.

“We don’t yet fully understand what protective immunity to the virus looks like, so can’t possibly define who or how many have it,” he says.

“In line with this, the 100 or more vaccines under development will have very different levels and modes of immune stimulation (and safety), so the first may not be the best and we have to be canny in comparing and analysing them.”