Nearly two-thirds of people infected with the Delta variant, and more than half of those who have died with it, have not had a Covid vaccine at all, the latest official data suggests.
There have been nearly 30,000 new UK cases of the variant in the past week.
The Public Health England report says it is likely to spread more easily and could make people more seriously ill.
Officials say two doses of the vaccine provide “significantly more protection” than one.
The Delta variant, which was first identified in India, is now the dominant variant in the UK – accounting for 90% of cases.
It is being closely monitored by PHE, who are using new genotyping tests to detect the variant more quickly as cases continue to rise.
These suggest that the Delta variant is roughly 60% more spreadable than the Alpha, or Kent, variant.
Analysis of hospitalisation rates suggests people infected by the Delta variant are twice as likely to end up in hospital than those with Alpha.
The numbers are still small, however.
Out of 33,000 cases analysed by PHE and confirmed to be the Delta variant since February, 223 have been admitted to hospital – most were unvaccinated or had only had only dose, and 20 people were fully vaccinated.
And of 42 deaths in people with Delta variant infections, 23 were unvaccinated and seven had received only one dose. The other 12 had received two doses more than two weeks before.
More than half of all UK adults have now been fully vaccinated with a Covid vaccine and more than three-quarters have had one dose. But that means more than 10 million adults, mostly in their 20s, have yet to have a single dose.
Dr Jenny Harries, chief executive of the UK Health Security Agency, said “vaccination is our best defence”.
“If you are eligible, we urge you to come forward and be vaccinated. Remember that two doses provide significantly more protection than a single dose.”
However, Dr Harries warned that while vaccination reduces the risk of severe disease, it does not eliminate it because Delta is “significantly more transmissible than Alpha”.