Hopes that the population will become immune to COVID-19 have been dashed by new research showing that antibodies drop quickly after recovery from the disease.
So called collective immunity has been proposed by some scientists as a better alternative to lockdowns to combat coronavirus pandemic.
It would take about 50-60% of the population to have protection against the virus so that it can no longer transmit effectively.
However, a major UK study found that instead of boosting immunity over time, the number of people with antibodies has declined by 26% since the lockdown was eased over the summer.
Researchers at Imperial College London screened 365,000 people in three rounds of tests between June and September.
Results from the REACT-2 study showed that 6% of people had antibodies to the virus by the time the lockdown was eased in late June and early July.
But at the start of the second wave last month, that number fell to just 4.4%.
Professor Helen Ward, one of the researchers, said the new findings strongly suggest that herd immunity is unachievable.
“When you think that 95 out of 100 people are still likely to be susceptible, we are a long way from anything that looks like population-level protection from further transmission,” she said.
“It’s not something you can use as an infection control strategy [for COVID-19] in the population. “
Discovery is another blow to scientists behind controversy Great Barrington Declaration, who had suggested that vulnerable people could be protected at home as the virus spreads through young and healthy people to boost herd immunity.
The proposal has been strongly criticized by many other scientists.
The researchers found that young people, those from Black, Asian and Ethnic Minority (BAME) communities and health workers had higher levels of antibodies, possibly because they were in regular contact with infected people.
The drop in antibodies suggests that people will be regularly re-infected, just as they are with related coronaviruses that cause the common cold.
Professor Wendy Barclay, an infectious disease specialist and one of the researchers, said antibodies peak three to four weeks after symptoms and then drop, as they do with related viruses.
She said: “The seasonal coronaviruses that circulate every winter and cause common colds can re-infect people after six to 12 months.
“We suspect the way the body responds to infection with this new coronavirus is similar to this.”
So far, there have only been a handful of documented cases of reinfection.
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Dr Alexander Edwards, associate professor of biomedical technology at the University of Reading, said: ‘What is not clear is how quickly antibody levels would rise again if a person encountered the virus an second time.
“They may still respond quickly and have milder disease or remain protected through immune memory.
“So even if the rapid antibody test is no longer positive, the person can still be protected from reinfection.”
The study, which has not yet been peer reviewed, only measured antibodies.
It is possible that another arm of the immune system called T cells remains active, but there is currently no test available for them.
Scientists are cautious about using the results to predict the protection offered by a vaccine.
They say immunizations can lead to a more robust antibody response.
Health Minister Lord Bethell said the study “is an essential part of the research, which helps us understand the nature of COVID-19 antibodies over time and improve our understanding of the virus itself “.
He added, “We are relying on this type of important research to inform our continued response to disease, so that we can continue to take the right actions at the right time.
“It’s also important that everyone knows what this means to them – this study will help us in our fight against the virus, but a positive antibody test doesn’t mean you are immune to COVID-19.”