People who have had both doses of the COVID-19 vaccine are 'half as likely' to catch the virus, according to a new study.
The 'REACT' study, conducted by Imperial College London, was based on more than 98,000 randomly selected people in England.
It established that if a double-vaccinated person came in contact with somebody who had COVID-19, only one in 25 (3.84%) would catch the virus.
And, even then, their case would generally be milder than it would have been if they had not been vaccinated.
Steven Riley, professor of infectious disease dynamics at Imperial College London, said the study showed that every person who gets vaccinated is "taking a decent chunk of potential transmission out of what may or may not happen in September".
He added: "There has been a drop, a plateau now, and I think it is challenging to make any kind of prediction over the summer months.
"There is uncertainty around what might happen in September when schools return and (there is) increased indoor mixing."
The percentage of those who tested positive for COVID-19 was 0.4% among those who were double-jabbed compared to 1.21% among those unvaccinated - a three-fold difference.
Researchers, after adjusting for various factors, found that vaccine effectiveness was 49% among people who said they had been double-jabbed.
"In other words, people who are double-vaccinated are half as likely to be infected," said Professor Paul Elliott, director of the React programme, and chair in epidemiology and public health medicine at Imperial College London.
Prof Elliott added that double vaccinated people also had a lower viral load, meaning they were less likely to have severe infections than those who only had one vaccine dose or those who were unvaccinated.
"There are some double-vaccinated people who will get infected because even with very high protection, it's not 100%," he added.
The study also found that the Delta variant of COVID-19, which was first reported in India, has "completely taken over" from the previously-dominant Alpha variant first reported in the UK.
Every COVID-19 swab the researchers looked at showed people were infected with the newer variant.
In other news, it has been announced today (4/8) that all 16 and 17-year-olds in England will be offered the COVID-19 vaccine.
It has been reported that children will not need the consent of their parents to get a jab.
Additional reporting by Sky News