Coronavirus is now largely being spread by younger people who may not even know they have the virus, the World Health Organisation (WHO) has warned.
The proportion of people in their 20s, 30s and 40s who have the virus has been rising and continues to rise - which could put older and more vulnerable people at risk, a senior WHO official told reporters.
Takeshi Kasai, the organisation's Western Pacific director, told a virtual press conference: "The epidemic is changing. People in their 20s, 30s, and 40s are increasingly driving the spread.
"Many are unaware they are infected. This increases the risk of spillovers to the more vulnerable."
It came as the UK Government said that as of 9am on Tuesday, there had been a further 1,089 lab-confirmed cases of the coronavirus.
Overall, a total of 320,286 cases have been confirmed.
However the UK has also seen its death toll from coronavirus drop significantly, with officials this month recording the lowest number of weekly deaths linked to the virus since before lockdown.
Some 152 people died after testing positive for Covid-19 in the week leading up to August 7, according to the Office for National Statistics (ONS), which has been tracking the virus' spread in the UK.
That is fewer than any week since before the lockdown. The last week that saw fewer coronavirus-linked deaths was the seven days to March 20, when 103 people died from the virus.
Case numbers have plateaued in the past two weeks, after a period in July after some restrictions were lifted, according to modelling by the ONS - although real case numbers have increased in recent weeks alongside more testing.
It came as other WHO leaders also issued stark warnings about global progress on coronavirus.
Dr Michael Ryan, head of health emergencies at the WHO, said the world is "nowhere near" herd immunity - the level of coronavirus transmission which would supposedly mean the virus died out without enough new people to infect.
Dr Ryan told reporters on Tuesday: "As a global population, we are nowhere close to the levels of immunity required to stop this disease transmitting. This is not a solution and not a solution we should be looking to."
Some experts have speculated that certain countries and parts of major cities may be close to herd immunity.
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Bill Hanage of the Harvard TH Chan School of Public Health, told the New York Times: 'I'm quite prepared to believe that there are pockets in New York City and London which have substantial immunity."
But it is unclear how long a single person remains immune to Covid-19 having caught it the first time.
Professor Paul Hunter of the university of East Anglia told the Mail Online: "If you look at other human coronaviruses, they can infect people in subsequent years, so probably Covid-19 immunity doesn't last even year.
"And so they will achieve some degree of herd immunity but it won't last."
Meanwhile WHO boss Tedros Adhanom Ghebreyesus told reporters last week that countries should aim to wipe the virus out within their borders completely.
He said: "My message is crystal clear: Suppress, suppress, suppress the virus. If we suppress the virus effectively, we can safely open up societies."