UK's coronavirus vaccination strategy could raise a 'small' risk of yet ANOTHER mutant strain because delaying second doses gives virus more time to evolve

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Britain's coronavirus vaccination strategy could increase the risk of yet another mutant strain of the virus evolving by giving it more time to mutate.

Professor Chris Whitty, England's chief medical officer, admitted at a Downing Street briefing today that extending the time between doses could let the virus evolve.

The Government last week unveiled its controversial policy which will see people given a single dose of a Covid jab without a second one lined up.

Both the vaccines approved so far – one made by Pfizer and the other by Oxford University – rely on two doses to be most effective, with them ideally spaced three weeks apart.

But in a scramble to stop the devastating second wave of Covid-19, Britain has abandoned this rule and decided it will extend the gap to 12 weeks so it can give more people a single dose as soon as possible.

The benefit will be that millions more people end up being vaccinated in the coming weeks. But it's possible the vaccines won't work as well in the long run.

Officials switched to this schedule because they want to vaccinate around 13million people by mid-February so that lockdowns can start to gradually be lifted.  

And Professor Whitty said this afternoon it may also raise the risk that an 'escaped mutant' version of the virus evolves to resist immunity produced by the jabs.

Britain's vaccination strategy, which originally planned to hold back second doses so people who had had their first could be fully immunised, has been changed so the maximum number of people can get their first jab as soon as possible (Pictured: 82-year-old Brian Pinker is vaccinated in Oxford)

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'That is a real worry but quite a small real worry within the system,' Professor Whitty said.

'The general view was the size of the increase of the risk is sufficiently small that, measured against this ability to double the number of people who actually are vaccinated, the public health arguments are really strongly for doing what we have decided to do.'

He added: 'Clearly, if we had infinite vaccine we might have taken different approaches, but we don’t.

'At this point in time, for the next three to four months, the number of vaccines we have available is going to constrain our ability to get through the 25 to 30 million people we must do.

'Whilst this is such a fast-moving virus at this time, our view was very strongly, on the balance of risk, the benefits to the UK for us at this point in the epidemic were in favour of doing this.'  

HOW COULD A VIRUS BECOME VACCINE RESISTANT? 

Vaccine-induced evolution can develop in a similar way to antibiotic resistance, which is more widely known about.

It is possible because vaccines are not 100 per cent effective and don't train every person's body to kill every virus they come into contact with.

This means that some people who have been vaccinated who come into contact with the virus can get infected, whether it makes them ill or not, so they have both the virus and the immune substances made by the vaccine in their body at the same time.

If the virus is exposed to parts of the immune system that should kill it – substances called antibodies and T cells, for example – but survives, it has the opportunity to pass on its genetics to more viruses.

Therefore, the versions of the virus that become more common and keep spreading are the ones that have evaded vaccinated people's immune systems in the past.

This increases the chance that one of them will pass on a mutation meaning it can get around the immunity completely and the vaccine won't work against it. 

This evolution becomes more likely the longer a virus is exposed to a vaccinated person, particularly if the vaccine does not have high effectiveness. This is why spreading out the doses in the UK could contribute to the risk – because people would spend longer with a weaker immune response against the virus.

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Vaccine-induced evolution can develop in a similar way to antibiotic resistance, which is more widely known about and considered one of the top threats to human health.

It is possible because vaccines are not 100 per cent effective and don't train every person's body to kill every type of the same virus they're designed to protect against.

This means that some people who have been vaccinated who come into contact with the virus can get infected, whether it makes them ill or not, so they have both the virus and the immune substances made by the vaccine in their body at the same time.

If the virus is exposed to parts of the immune system that should kill it – substances called antibodies and T cells, for example – but survives, it has the opportunity to pass on its genetics to more generations.

Viruses are constantly mutating at random and some of the mutations become permanent because they help it to survive or reproduce. 

Therefore, the versions of the virus that become more common and keep spreading may be the ones that have got past vaccinated people's immune systems before.

This increases the chance that one of them will pass on a mutation that will make a vaccine much less effective or completely useless.

This evolution becomes more likely the longer a virus is exposed to a vaccinated person, particularly if the vaccine does not have high effectiveness. This is why spreading out the doses in the UK could contribute to the risk – because people would spend longer with a weaker immune response against the virus.

Just because this is scientifically possible, however, doesn't mean it is likely and scientists don't yet know whether they should expect it to happen.

It has never happened to measles, for example, and that virus is still stopped by a vaccine that was invented decades ago.  

But on the flipside the flu virus mutates so much and in such big ways that multiple vaccines are needed and they depend on which versions of the virus are circulating in that year. 

Experts say coronavirus could go the same way, and have reassured the public that now they know vaccines work it should easy to make new ones if they're needed.