The Imperial College London study illustrates how the highly transmissible Delta variant can spread even in a vaccinated population.

The researchers underlined that did not weaken the argument for vaccination as the best way of reducing serious illness from COVID-19 and said booster shots were required.

They concluded infections in the vaccinated cleared more quickly but the peak viral load remained similar to the unvaccinated.

"By carrying out repeated and frequent sampling from contacts of COVID-19 cases, we found that vaccinated people can contract and pass on infection within households, including to vaccinated household members," Anika Singanayagam, co-lead author of the study, said.

"Our findings provide important insights into... why the Delta variant is continuing to cause high COVID-19 case numbers around the world, even in countries with high vaccination rates."

The study, which enrolled 621 participants, concluded that of 205 household contacts of people with Delta COVID-19 infection, 38 per cent of household contacts who were unvaccinated went on to test positive, compared to 25 per cent of vaccinated contacts.

Vaccinated contacts who tested positive for COVID-19 on average had received their shots longer ago than those who tested negative, which the authors said was evidence of waning immunity and supported the need for booster shots.

Imperial epidemiologist Neil Ferguson said that the transmissibility of Delta meant that it was unlikely the United Kingdom would reach "herd immunity" for long.

"That may happen in the next few weeks: if the epidemic's current transmission peaks and then starts declining, we have by definition in some sense reached herd immunity but it is not going to be a permanent thing," he told reporters.

"Immunity wanes over time, it is imperfect, so you still get transmission happening, and that is why the booster program is so important."

Australian Associated Press