Public Health England has said that early evidence suggested the levels of coronavirus found in people infected with the Delta variant are similar whether or not they are vaccinated, with possible implications for their infectiousness.
"Some 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," Public Health England said in a statement.
"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."
Wales to reopen nightclubs as most restrictions end
Nightclubs are set to reopen and all restrictions on meeting others are to be removed in Wales, in the biggest easing of coronavirus measures since the pandemic began.
First Minister Mark Drakeford will confirm Wales is set to move to alert Level Zero tomorrow. All businesses will be allowed to open and all legal limits on meeting people in indoor private spaces will be removed.
But he will stress that it "does not mean the end of restrictions and a free-for-all" as he urges the public to continue to help prevent the spread of the virus when restrictions ease.
Face masks will continue to be required in most indoor public places, including on public transport, in shops and in healthcare settings, but will no longer be a legal requirement in hospitality settings where food and drink is served.
Adults who are fully vaccinated and children and young people under 18 will also no longer need to isolate if they are a close contact of someone who has coronavirus.
However, people must continue to isolate for ten days if they have symptoms of the virus or if they have a positive test result.
Some rules were eased on 17 July as the country moved into alert Level 1, after a four-week delay due to the spread of the Delta variant.
Confirming Wales will move to alert Level Zero at 6am tomorrow, Mr Drakeford said: "Alert level zero does not mean the end of restrictions and a free-for-all.
"But it does mean we can all enjoy more freedoms with the confidence there are still important protections in place to make sure our public health is being safeguarded while we are out and about.
"Unfortunately, the pandemic is not over yet and we all need to work together to do everything we can to keep this virus under control - at alert level zero, everything we do will have an impact on this virus.
"Even if you have been fully vaccinated, meeting outside is safer than inside; let fresh air into indoor spaces, get tested even for mild symptoms, and self-isolate when you are required to do so."
Welsh Conservative leader Andrew RT Davies said: "After a long 18 months, I'm relieved that we move to Level Zero in Wales as it's time for us all to enjoy the full fruits of our vaccination campaign, whilst of course remaining vigilant.
"As we approach the very end of restrictions, we must look not only towards the strongest bounce back possible but giving justice to those who have suffered from the virus and lockdown."
Additional reporting PA