COVID vaccines are “less effective” against a new variant most prolific in Colombia, initial lab studies show.
Public Health England made the warning alongside revealing vaccinated people who are infected with Delta are likely to be just as infectious as those unvaccinated.
It suggests jabs do not prevent spread of transmission of the Delta strain as much as hoped.
The agency made a plea for people to their jabs, which are the best way to prevent infection leading to serious disease or death.
Dr Jenny Harries, Chief Executive of the UK Health Security Agency, said: “Vaccination is the best tool we have in keeping ourselves and our loved ones safe from the serious disease risk Covid can pose.
“However, we must also remember that the vaccines do not eliminate all risk: it is still possible to become unwell with Covid and infect others.
“It is still vital that we exercise caution, particularly while cases are high.
“Remember that meeting outdoors is safer than indoors, isolate if you are told to by NHS Test and Trace, and if you show symptoms stay home and get a PCR test as soon as possible. It is so important that we all continue to play our part.”
The new strain, initially most found in Colombia, is scientifically called B.1.621.
It was labelled a “variant under investigation” by PHE last week.
PHE said in a statement today: “There is preliminary laboratory evidence to suggest that vaccination and previous infection may be less effective at preventing infection with [B.1.621].
“However, this data is very limited and more research is required. There is no evidence to suggest that [B.1.621] is more transmissible than the dominant Delta variant.”
PHE said the variant has mutations “of concern” that have been seen in other dangerous new strains, including from South Africa (Beta), Kent (Alpha) and Brazil (Gamma).
As of 4 August, there have been 37 confirmed cases in England, covering six regions, mostly in London.
At least seven of the infected have a history of travel which include travel from or transit through Mexico, Spain, Dominican Republic, and Colombia.
Meanwhile, the highly contagious Delta variant, first seen in India, is “overwhelmingly dominant across the UK”.
PHE said initial findings “indicate that levels of virus in those who become infected with Delta having already been vaccinated may be similar to levels found in unvaccinated people”.
It continued: “This may have implications for people’s infectiousness, whether they have been vaccinated or not. However, this is early exploratory analysis and further targeted studies are needed to confirm whether this is the case.”
The Delta strain is more able to dodge immunity, too, with studies showing one dose of a vaccine is only 33 per cent effective against symptoms of Covid.
However, two doses of either Pfizer or AstraZeneca give more than 90 per cent protection against hospitalisation.
An unfortunate 1,467 people have been hospitalised with Delta since July 19, of which 512 (34.9 per cent) of people had been double-jabbed.
But PHE said the more people get vaccinated – currently 73.5 per cent of the UK adult population have two doses – the higher proportion of hospitalised people will be vaccinated.
Ultimately more people are unvaccinated (55.1 per cent), and a jab may have prevented their admission.
Dr Harries said: “The latest hospitalisation figures show once again how important it is that we all come forward to receive both doses of the vaccine as soon as we are able to do so.”
It comes as data suggests Covid infections are falling across the UK, other than in Ireland.
For the first time since mid-May, the Office for National Statistics (ONS) said the number of people in the population infected with the virus had fallen in England, to one in 75 people.
The figure for Wales is one in 230, and for Scotland one in 120. But in Northern Ireland, it has soared to one in 55 people.
The survey captures people who do and do not have symptoms of the virus, which is different to the Government data which is only those diagnosed with a test (likely because they had symptoms) – which also shows a downward trend.