One of the UK’s top public health experts has warned MPs the reproduction “R” rate of the Delta coronavirus variant could be as high as 7 without any mitigation.
Dr Susan Hopkins, speaking two days after Boris Johnson delayed the end of England’s lockdown due to the rapid spread of the variant first identified in India, said “caution” is still needed.
R is the average number of people each COVID-positive person goes on to infect. An R of 7 would theoretically mean every 10 people with the virus would infect 70 other people.
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The Delta variant is thought to be 60% more transmissible than the previously dominant Alpha variant, first identified in Kent.
The Alpha variant was found to be 70% more infectious than the original strain of the virus.
“So if we were in the real world where we had none of the measures that we were seeing right now,” Dr Hopkins told MPs at the House of Commons science and technology committee, “we would estimate R greater than 5 and maybe up to 7.”
Dr Hopkins, strategic response director for COVID-19 at Public Health England, said that at the beginning of the pandemic R was thought to be 2.5.
She explained: “That’s why we need people to have vaccinations because that’s a clear mitigation measure. That's [also] why we need people to take care, take caution – particularly in healthcare settings.”
Johnson had been hoping to drop all restrictions on social contact on Monday, but the Delta variant has changed the outlook of the pandemic.
On Monday, Johnson delayed the end of lockdown until at least 19 July after being warned by advisers that dropping all restrictions could lead to thousands more COVID deaths.
Experts had called for a delay in order for more people to receive a second vaccine and get maximum protection against the virus. As of Wednesday, 42.2% of UK adults – more than 20 million people – still needed a second jab.
The Scientific Pandemic Influenza Group on Modelling (SPI-M-O), which advises the government, described how difficult it will be to reach "herd immunity" – whereby enough people are immune to a disease so it cannot spread through the population – with a R number as high as 7.
A SPI-M-O paper presented to ministers reads: "Whilst the precise herd immunity threshold cannot be calculated, an R of 7 would require over 80% of all people (not just adults) to be immune for herd immunity to be reached and for the epidemic to begin to shrink without further measures.
"Younger adults play a disproportionately large role in transmission but have not yet been vaccinated."
The latest Office for National Statistics data suggest eight in 10 adults have COVID antibodies, amounting to roughly 60%of the population.
Meanwhile, according to Oxford University's Our World in Data website, more than half of the UK's total population – 45.5% – are yet to be fully vaccinated.
The UK's vaccine programme, which has been one of the most successful in the world, opened to over-21s on Wednesday to receive their first dose.
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