antibodies

Levels of protective antibodies in people wane "quite rapidly" after coronavirus infection, say researchers.

Antibodies are a key part of our immune defences and stop the virus from getting inside the body's cells.

The Imperial College London team found the number of people testing positive for antibodies has fallen by 26% between June and September.

They say immunity appears to be fading and there is a risk of catching the virus multiple times.

More than 350,000 people in England have taken an antibody test as part of the REACT-2 study so far.

In the first round of testing, at the end of June and the beginning of July, about 60 in 1,000 people had detectable antibodies.

But in the latest set of tests, in September, only 44 per 1,000 people were positive.

It suggests the number of people with antibodies fell by more than a quarter between summer and autumn.

"Immunity is waning quite rapidly, we're only three months after our first [round of tests] and we're already showing a 26% decline in antibodies," said Prof Helen Ward, one of the researchers.

The fall was greater in those over 65, compared with younger age groups, and in those without symptoms compared with those with full-blown Covid-19.

The number of healthcare workers with antibodies remained relatively high, which the researchers suggest may be due to regular exposure to the virus.

Antibodies stick to the surface of the coronavirus to stop it invading our body's cells, and attract the rest of the immune system.

Exactly what the antibody drop means for immunity is still uncertain. There are other parts of the immune system, such as T-cells, which may also play a role.

However, the researchers warn antibodies tend to be highly predictive of who is protected.

Prof Wendy Barclay said: "We can see the antibodies and we can see them declining and we know antibodies on their own are quite protective.

Poor regulation of antibodies tests - that could indicate if someone has had coronavirus - could be putting the public at risk, doctors have warned.

The U.K. Royal College of Pathologists has written to the health secretary, calling for rules to be tightened on kits sold direct to consumers.

The letter warns they can "mislead the public and put individuals at risk".

No antibodies test has been officially approved for at-home use in the UK - but many different types are available.

It is still not known whether having antibodies will protect people from a second infection.

The doctors say the tests should not be used without "professional back-up", must "give the right result" and be "properly readable".

A Medicines and Healthcare products Regulatory Agency official said it had "worked collaboratively with cross-government agencies at pace to prevent non-compliant antibody test kits being placed into the UK market".

But Royal College of Pathologists president Prof Jo Martin said: "Currently, if you buy a test on the internet or you buy it in certain boutiques or shops, we can't guarantee that the quality of that is of an appropriate standard.

"We can't guarantee that the result will be easy to interpret or that it will be not misleading."

An analysis of 41 antibodies tests sold to the public in the UK, seen by the BBC's Newsnight programme before publication in the medical journal BMJ Open, found almost a third provided incomplete and inaccurate information.

A number claimed to offer "peace of mind".

But just 10% had made documents available to support their claims, academics from the Universities of Birmingham and Warwick found.

What information has been released about how kits were assessed shows most were tested on small numbers of patients only - just a few dozen, all of whom had been admitted to hospital.