Public Health England (PHE) said early findings suggested that “levels of the virus in those infected with Delta who have been previously vaccinated may be similar to levels found in unvaccinated people.”
“This may have implications for the infectivity of people, whether they have been vaccinated or not,” he added.
He stressed that this was an “early exploratory analysis” and that more focused studies were needed to confirm.
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Sky health correspondent Ashish Joshi said PHE’s language was cautious with a number of caveats such as “early exploratory findings”.
But he said if the results were confirmed it could have “huge implications for transmissibility” because “The data has consistently shown that the vaccine slows down and should effectively stop the spread of the virus.”
Dr Simon Clarke, professor of microbiology at the University of Reading, told Sky News the early findings were a “big deal”.
“We will need more studies to find a definitive answer,” he said.
“But if the vaccine only blocks transmission of, say, 50%, you will never get herd immunity, even with 100% uptake of the vaccine. “
The Delta variant, which originated in India, remains dominant in the UK and accounts for around 99% of cases, PHE said in its last variant briefing.
New hospitalization data confirms again that vaccines, while offering high levels of protection, are not 100% effective.
Of recent admissions for the virus, 808 (55.1%) were unvaccinated, while 512 (34.9%) had received both doses of a vaccine, PHE said.
“We must also remember that vaccines do not eliminate all risks: it is always possible to get sick with COVID and infect others,” said Dr Jenny Harries, head of the UK Health Security Agency. .
An update on a recent “variant under investigation” was also given by PHE.
VUI-21JUL-01, or B.1.621, was reported by the health agency on July 21 after apparently spreading to several countries and also producing mutations.
There were 37 confirmed cases in England as of August 4, PHE said.
Cases have been detected in six English regions, most of them in London.
“There is preliminary laboratory evidence suggesting that vaccination and previous infection may be less effective in preventing infection with VUI-21JUL-01,” PHE said.
“However, this data is very limited and more research is needed. There is no evidence to suggest that VUI-21JUL-01 is more transmissible than the dominant Delta variant. “
PHE has currently identified four variants of concern (COV) and 10 variants under study (VUI).
Variant update comes as numbers suggest coronavirus infection rates fell in all countries of origin except Northern Ireland.
During the week ending July 31, around one in 75 people in private households in England (722,300 positive cases) would have had COVID-19[feminine[feminine.
This compares to one in 65 (865,200) people the week before.