A new study suggests that the antibodies a person develops for protection after a coronavirus infection may not last long - raising the prospect that a population's collective immunity to the disease would only be fleeting.
Scientists at Imperial College London tracked antibody levels in the British population following the first wave of coronavirus infections, in March and April - and discovered they declined rapidly over the summer.
Graham Cooke is a professor of infectious diseases at Imperial:
"“So the main change we saw between these three rounds of the survey was that the proportion of people testing positive (for antibodies) fell from 6% in the end of June and July through to August, where it was 4.8% and then 4.4% in September. So although those changes sound quite small, because of the numbers of people involved we can be pretty confident this is a real change. But overall, it doesn't change the fact that actually 90%-plus of the general population is still susceptible to infection."
The news comes as much of the world is experiencing a second wave of infections forcing local lockdowns and restrictions, and the northern hemisphere readies for winter.
The colder weather means more people will be socializing and congregating indoors - where the risk of infection grows.
The study has not yet been peer reviewed. Natural immunity to the novel coronavirus is a complex and murky area, as well.
But researchers do say the experience of other coronaviruses also suggests that immunity might not be enduring.
Those whose infection was confirmed with a gold standard PCR test had a less pronounced decline in antibodies, compared to people who had been asymptomatic and unaware of their original infection.
There was no change in the levels of antibodies seen in healthcare workers - which could be due to repeated exposure to the virus.
Similar findings have been discovered in Germany - which showed the vast majority of people did not have antibodies - even in coronavirus hotspots - and that antibodies might fade in those who do.