If - as we're often told - viruses mutate, how sure can we be that the Covid Delta variant is the last?

The view among scientists is almost unanimous. The government had no choice but to postpone plans to open up on June 21.

There’s good evidence that due to the Delta Covid variant, cases are going to rocket in the coming weeks. Evidence that hospitalisations and deaths will stay low due to the vaccine isn’t good enough.

Polling suggests most of us are facing the facts around the mutated virus and grudgingly accept a delay.  But if - as we’re often reminded - viruses constantly mutate, how reassured can we be that this is the last time?


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When SARS-CoV-2 (the virus that causes Covid) first emerged, many scientists confidently predicted that it wouldn’t mutate that fast. The family of SARS viruses to which it belongs have very large genomes containing lots of genetic instructions.

The longer the genome, the greater the chance of mutations. To prevent that from happening SARS viruses have special “proof reading” abilities to correct mutations.

Surge testing has been rolled out across many areas to try to stem the spread of the Delta variant Credit: PA

When SARS-1 emerged in 2003, and then its close cousin MERS in 2012, disease scientists were worried. They both caused severe disease, but didn’t transmit very easily.

Yet despite thousands of cases, the viruses showed little evidence of mutating, indeed if they had, we could have seen a pandemic much sooner than the one caused by Covid.

The same, scientists hoped, might be true of SARS-CoV-2.  

Indeed, it was many months into the current pandemic, that concrete evidence of the SARS-CoV-2 was mutating at all.



We now know a mutation called D614G helped give the virus some kind of advantage and fuelled its spread across Europe last year.

Then came the Alpha variant (previously referred to as B.1.17 or the “Kent” variant) which was 50% more transmissible than the original strain. And now, barely six months later, Delta (the variant formerly known as “Indian”) emerged.

It’s around 60% more transmissible than Alpha. It is now the dominant strain in the UK.

The idea that Covid wouldn’t rapidly evolve to be both more infectious and (slightly) more deadly, were proved completely wrong.

“Of course, what a virus will try and do is optimise itself," Sir Patrick Vallance the government’s Chief Scientist told me recently.

"I think it is doing it faster than people thought a coronavirus would do it in this way. So it's definitely happening a bit more than people thought.”

If that’s the case, when will it stop? Well, viruses can get a lot more transmissible than the Delta variant currently is.

The measles virus for example, is maybe three or four times more infectious than Covid.

However, that doesn’t necessarily mean that will happen.

Mutations can give a virus an advantage in one regard (infectiousness, for example) but have a disadvantage in another (ability to survive colder weather perhaps). Such trade-offs will ultimately limit a given strain’s overall “fitness”.

But even if the virus reaches some kind of optimal fitness in one situation, it will keep evolving if its environment changes. Vaccination is the best — and currently most relevant — example of that.  

Chief scientist Sir Patrick Vallance Credit: PA

The pressure is now on the virus to survive in the vaccinated environment. Mutations that help it dodge vaccine antibodies will have a major advantage.

It’s those that our surveillance efforts are now focused on detecting. It’s also the reason there are plans to give booster doses of vaccines, possibly new versions optimised against new variants.

The only thing that can slow that process down — barring a “universal vaccine” effective against all variants — is preventing infections.

Because two doses of vaccine are still 80%-plus effective against the Delta variant, so the quicker we are all vaccinated, the lower levels of transmission are and the fewer opportunities Covid has to mutate.

The longer the vaccination programme lingers, the slower people are to get vaccinated, the more opportunities the virus has to find a way around the jab.

Aside from a last-ditch effort to keep people out of hospital in a third wave on infections, this is the other reason our public health officials are pushing for the most rapid and widespread rollout of the vaccine. It’s the best chance we’ve got at slowing down Covid’s evolution.For a year or so, the virus was evolving to become more infectious in the absence of a vaccine.

Now, especially in places like the UK, the US, and parts of Europe it will be encountering more and more people with antibodies due to vaccination.