The emergence of a new COVID variant with a similar death rate to MERS, which kills one in three infected people, is a "realistic possibility", the government's scientific advisers have warned.
However the experts also say the virus could result in "much less severe disease" in older people and those who are clinically vulnerable in the long term.
In a paper published on Friday, the scientists outline the chances that a new variant will evade current vaccines, saying one of the causes is "almost certain" to happen.
It said the eradication of the virus "will be unlikely" and the scientists have "high confidence in stating that there will always be variants".
They considered a scenario where a variant causes severe disease in a greater proportion of the population than has previously occurred, with similar death rates to other coronaviruses SARS (10%) or MERS (35%).
The experts said this could be caused by a "recombination" between two variants of concern or variants under investigation, such as between beta and alpha or delta variants.
SAGE warned that the likelihood of a more severe variant under these circumstances was a "realistic possibility".
It set out measures the government should consider to combat this, including minimising the "introduction of new variants from other territories (to reduce risk of recombination between variants)".
Ministers confirmed this week that England would allow fully vaccinated visitors from both the EU and the United States to arrive without needing to quarantine from 2 August.
SAGE also looked at a scenario where the coronavirus becomes like one "that causes common colds, but with much less severe disease predominantly in the old or clinically vulnerable."
It said that while it was "unlikely in the short term", there is a "realistic possibility in the long term".
Meanwhile, SAGE said a scenario where a variant evades current vaccines because of a process known as "antigenic drift" is "almost certain".
It suggests that the UK needs to continue vaccinating vulnerable age groups "at regular periods with updated vaccines" to the dominant variants to increase their protection.
Clinical epidemiologist Dr Deepti Gurdasani said the SAGE paper was a "stark warning".
Writing on Twitter, she said: "Given the impact Delta has already had, and in light of recent evidence from the CDC (Centers for Disease Control and Prevention), we cannot afford any more new variants emerging - we need to take preventive action now."
She added that the SAGE paper "makes clear that the virus becoming less virulent is unlikely in the short term".
"So for all those who suggest that we should live with it, and it'll become like seasonal coronaviruses and benign, doesn't look like that's likely to happen anytime soon," Dr Gurdasani wrote.
COVID infection rates have continued to rise across England, Wales and Northern Ireland - but dropped in Scotland, according to the latest estimates.
Data from the Office for National Statistics shows that in the week ending 24 July, approximately one in 65 people were estimated to have had the disease in England.
More than 5.8 million coronavirus cases have been recorded in the UK during the pandemic, with nearly 130,000 people dying within 28 days of a positive test, according to government figures.
Watch: Woman freed from hotel quarantine to see dying father