Far fewer Britons have coronavirus antibodies now than at the peak of the first wave, according to a Government-led study.
It raises concerns that protection against the disease is short-lived and that people may be able to get reinfected just months after recovering the first time.
The REACT-2 project — which sends out tens of thousands of DIY blood tests to work out how much of the population has been infected — found 4.4 per cent of people in England in September had Covid-19 antibodies, proteins in the blood trained to fight off the disease.
By comparison, the first round of the study in June found 6 per cent tested positive for antibodies, marking a fall of 26 per cent in three months.
Worryingly, the biggest drop was spotted in the over-65s, who are most vulnerable to falling ill or dying from the disease.
Imperial College London scientists, who led the research, said they suspect natural protection against Covid-19 lasts between six to 12 months. They believe that most people will be vulnerable to reinfection after that time.
However, antibodies are just one of several key components involved in immunity. For example, the study did not look into T cells, types of white blood cells found in Covid-19 survivors that also play a major role in preventing reinfection.
Scientists insisted the truth on immunity is still murky and said it was possible that the body could still rapidly produce antibodies in the future, even if someone no longer tests positive for them. They said this may not protect them entirely but lead to a milder illness.
Experts believe the coronavirus kills around 0.5 per cent of everyone it infects, the equivalent of one death for every 200 patients. But the disease poses a much graver threat for the elderly than it does under-40s.
Using this infection-fatality rate (IFR) to estimate the true scale of England’s Covid-19 crisis, it would suggest the country has really had 8million cases — the equivalent of 14.3 per cent of the population.
The REACT-2 project found 4.4 per cent of people in England had Covid-19 antibodies in September. By comparison, the first round of the study in June found 6 per cent tested positive for antibodies and the second study found 4.8 per cent had the proteins
The biggest drop proportionate was spotted in the over-65s, who are most vulnerable to falling ill or dying from the disease. Antibodies were most common among younger people who were actively encouraged to help stimulate the economy in the months after lockdown was lifted
Prevalence remained highest in London, at 9.5 per cent compared with 1.6 per cent in the South West of England
When someone becomes infected with coronavirus, their immune system produces antibodies which store memories of how to fight off a specific disease.
They come in different forms and may attack viruses and destroy them themselves, or may force the body to produce other kinds of immune cells and white blood cells to do the dirty work for them.
Once antibodies have been created once, the body usually retains a memory of how to make them and which ones are needed to fight off different viruses.
This memory can fade quicker in people with weakened immune systems, including the elderly and people with pre-existing health conditions.
Unveiling the REACT-2 results on Monday, Professor Wendy Barclay, a virologist at Imperial, said: ‘What this study shows is that when you take a snapshot of the population, we see a fall in detectable in antibodies overall.
‘Seasonal coronaviruses can reinfect after six to 12 months. We suspect the way the new coronavirus infects people is similar to that.’
But independent scientists said the findings were not so clear-cut. Professor Jonathan Ball, a virologist at the University of Nottingham, said: ‘This study confirms suspicions that antibody responses – especially in vulnerable elderly populations – decrease over time.
‘What is less clear from this kind of study is the relationship between waning immunity and susceptibility to reinfection and the resulting severity of any subsequent infection, and this is important to know.
‘Antibodies are likely to be important in protecting us from future infection and disease, but other arms of the immune system, for example cellular immunity, might also be key.
‘Therefore, it is essential that we gain a better understanding of what protective immunity looks like and this can only be gleaned by measuring all aspects of immunity following infection and seeing how this relates to reinfection risk.’
Dr Alexander Edwards, associate professor in biomedical technology at the University of Reading, added: ‘When people are ill, antibody levels rise, and when you heal, antibody levels do drop naturally – this is not exactly the same as losing immunity.
‘What is not clear is how quickly antibody levels would rise again if a person encounters the SARS-CoV-2 virus a second time. It is possible they will still rapidly respond, and either have a milder illness, or remain protected through immune memory.
‘So even if the rapid antibody test is no longer positive, the person may still be protected from re-infection. But we don’t know this yet- it takes time to work this out, by following large groups over many months, and this type of study is ongoing yet hard and slow.’
As part of the REACT-2 study, researchers sent finger-prick antibody blood tests to the homes of 350,000 adults in England between July and September.
People were of all ages and were randomly selected from the NHS patient list, which includes anyone registered with a GP in England.
Participants had to do the test themselves and send the research team a picture of their results to be verified. In total, 17,676 people tested positive for antibodies throughout the three-month period.
About 100,000 people were recruited during the first round of the study in July, three months after the initial peak of the pandemic in early April.
The second phase saw 105,000 people tested in August, while the third round of the study collected 160,000 samples.
The proportion of people testing positive for antibodies declined from 6 per cent in July, to 4.8 per cent in August, and finally 4.4 per cent in September. It marked a fall of 26.3 per cent over the three rounds.
Prevalence was highest among 18-to-24-year-olds, who were actively encouraged to help stimulate the economy when lockdown ended, and lowest in those above 75, who have been advised to stay at home and shield as much as possible.
London had the highest number of people with antibodies, at 9.5 per cent, compared with 1.6 per cent in the South West of England, where prevalence was lowest.
The capital was bombarded by the virus during the first wave when an influx of cases were imported from Europe and Asia by virtue of being the country’s transport hub.
It meant that more Londoners were exposed to the disease before it migrated to other parts of the UK.
People of black (13.8 per cent) and Asian ethnicity (9.7 per cent) were more likely to have had the disease than white people (3.6 per cent).
The discrepancy between races has been echoed in numerous other studies. Experts believe ethnic minorities are more at risk because they work public-facing jobs, use public transport more often and live in deprived areas.
Health and social care workers were the most likely to have Covid-19 antibodies in all three rounds of testing. Prevalence did not change over the course of the three months for people in these jobs.
Writing in the paper, Imperial researchers said: ‘We observe a significant decline in the proportion of the population with detectable antibodies over three rounds of national surveillance, using a self-administered lateral flow test, 12, 18 and 24 weeks after the first peak of infections in England.
‘This is consistent with evidence that immunity to seasonal coronaviruses declines over six to 12 months after infection.
‘The relevance of antibody waning for the potential for reinfection by SARS CoV-2 is currently not resolved… Understanding the ongoing risks of reinfection for the population is key to understanding the future course of the epidemic.
‘In addition it is currently not clear what contribution T cell immunity and memory responses will play in protective immunity during re-exposure.
‘As such, it is not possible to say with certainty the loss of antibody positivity… would correlate with an increased risk of an individual being reinfected.
‘However, at a population level, the waning we have observed may indicate an overall decline in the level of population immunity.’
Scientists still do not know for certain whether people can catch Covid-19 more than once or if they become immune after their first infection.
With some illnesses such as chickenpox, the body can remember exactly how to destroy it and becomes able to fend it off before symptoms start if it gets back into the body. But it is so far still unclear if people who have had coronavirus can get it again.
