COVID is still evolving and if two concerning variants combine, it could become significantly more deadly, Sage papers say.
The mortality rate of Covid is somewhere around 1-3 per cent, according to estimates, meaning less than five in every 100 people who catch it die in the absence of vaccines.
But there is a “realistic possibility” this will increase - maybe even to a rate similar to SARS and MERS, experts say.
SARS and MERS are two other human coronaviruses that are no longer in circulation, first emerging in 2000 and 2012, respectively.
Some 10 per cent and 30 per cent of people who caught SARS and MERS, respectively, died.
The paper said that for Covid to become as deadly as SARS or MERS, it would need to go through more adaptation in the human population.
It could occur if two new variants, already of concern, combined to make a more lethal one, such as Delta, Beta or Alpha.
This event is called “recombination” and happens naturally. It’s been seen with Covid before as well as in flu.
The likelihood of Covid becoming more deadly was described as “likely” to a “realistic possibility” by experts, due to the virus still spreading at such a high level.
Commenting on the paper, Dr Simon Clarke, an associate professor in cellular microbiology, University of Reading, told the Sun: “I would agree with their conclusion on that.
“It should be noted that something can inflict a greater toll on human health just by being more transmissible too.
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“While it is rare for two viruses to combine, it does happen, it’s called a ‘recombination event’ and is perfectly possible. It’s well documented with flu, for instance.
“There are multiple potential outcomes from this, but there’s no reason why it could not generate a more lethal variant.
"But it’s certainly not a given that it would happen, it’s just something that could happen. Most changes however will be much more subtle shifts in genetic code.”
The paper said such an event would have a “high impact”.
But it was referring to the natural mortality rate of Covid, without the impact of vaccines which have hugely driven down fatalities.
It's hard to believe up to three in ten people that catch Covid could die somewhere like the UK, which is edging closer to a fully vaccinated adult population.
But even though the population will be vaccinated, “vaccines do not provide absolute sterilising immunity”, the paper said.
The jabs have prevented hundreds of thousands of deaths so far in the UK. But they are not 100 per cent effective and some will still be unfortunate to get severe Covid.
Vaccine immunity will also wane over time and possibly become less effective as the virus changes over time and becomes unrecognisable to the immune system.
The paper stressed the importance of keeping on stop of booster campaigns - the first of which could start in September - and being ready to tweak jabs as quickly as possible.
In further discussions about how the virus could evolve over time, scientists said there was a realistic possibility a vaccine-evading variant could emerge or that drugs to treat the most critically sick become useless.
It is “inevitable” the coronavirus will keep mutating, little by little, and therefore people will become infected with the virus many times over their lifetime.
It ties into the belief that Covid will one day be endemic, meaning it’s treated the same as the common cold or the flu.
But the paper said it’s unlikely this will happen any time soon, and repetitive vaccination will be the most important way to protect against Covid.
Among the doom and gloom, there is also a chance the virus becomes “less virulent”.
The document was one of many published by the Government today that give an insight into what Sage - the panel of experts who advise ministers - have been debating.
Another document discussed how long vaccination immunity will last, a crucial question that is unanswered because no one has had the jab for very long.
It said vulnerable people, such as the elderly, "may act as ‘canaries’ in signalling the time point beyond which waning vaccine immunity might become an issue”.
Prof Wendy Barclay and colleagues said the first signal that immunity is waning will come from an uptick in hospitalisations, that will suggest that vaccine protectiveness is starting to “fail”.