While just 6 per cent of people were found to have antibodies in late June, this fell to 4.8 per cent in August and even further to 4.4 per cent in mid-September.
This suggests less than one in 20 people in England had any detectable level of antibodies going into the current second spike of infections.
Antibodies were found to start diminishing three to four weeks after they first become detectable, dropping more sharply in elderly people and those with asymptomatic infections.
In addition, the study found that levels of antibodies decreased more slowly in health and care home workers.
This could indicate “repeated exposure” or “ongoing transmission” in those settings, said Helen Ward, professor of public health at Imperial.
Scientists on the React team said they had found no evidence to suggest this virus acts differently to other coronaviruses, such as the common cold, which typically leave individuals vulnerable to reinfection after six to 12 months.
“We suspect that the way the body reacts to infection with this new coronavirus is rather similar to that,” Professor Wendy Barclay, head of Imperial’s Department of Infectious Disease, told a press briefing on Monday.
“Every virus has a different book of strategies that it’s evolved over years of evolution with a host that interfere with the way our immune systems work, and this group of viruses seem to be pretty good at stopping us making long-lived effective antibodies,” Prof Barclay added.
“That may be their evolutionary strategy.”
The scientists emphasised that it was still unknown what level of antibodies is needed to provide immunity from reinfection, but warned it is possible that reinfected individuals could “sustain the epidemic”.
“[With] some of the reinfections that are documented – and there are only a handful of them being researched at the moment – the amount of virus being shed is quite high, suggesting the possibility of onwards transmission and therefore that people being reinfected could sustain the epidemic,” said Prof Barclay.
The Imperial experts tentatively suggested that the level of antibody required for detection by the home tests used in the study is similar to the threshold for protection.
The findings underscore the need for a vaccine, experts said, adding waning immunity in the community will not necessarily translate to the length of time for which a vaccine is effective.
Prof Barclay said: “All of the vaccines which are currently moving forwards towards trials are based on completely different mechanisms of stimulating immune response than infection with a virus itself.
“So it’s not a given that just because natural immunity does this fairly fast waning … that a good vaccine will also do that.
“A good vaccine may well be better than natural immunity, and there’s an awful lot of new vaccine technologies being tested which we’re hopeful may induce long-lasting antibodies – which may not need as frequent boosting as one might need if you were using natural infection to create immunity.”
It is crucial that we gain a better understanding of what level of antibodies is needed to provide immunity from reinfection, the scientists said – but added that researchers should be able to answer that question in a matter of months.