Double vaccinated people can easily spread the Delta variant of Covid-19 in their households, a British study has shown.
Unvaccinated people were most at risk of transmission, the Imperial College London study found. Contacts were less likely to get infected if they had been vaccinated.
The study illustrates how the highly transmissible Delta variant can spread even in a vaccinated population.
Peak viral load among vaccinated people is similar to that seen in unvaccinated people, which may explain why they can still readily pass on the virus, according to the study published in The Lancet Infectious Diseases.
Researchers say unvaccinated people should get their shot as soon as possible, with people now more likely to spend time indoors due to the winter months.
Vaccines remain highly effective at preventing severe disease and deaths from Covid-19 but studies suggest they may be less effective against the Delta variant.
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.
“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,” Dr 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 ongoing transmission we are seeing between vaccinated people makes it essential for unvaccinated people to get vaccinated to protect themselves from acquiring infection and severe Covid-19, especially as more people will be spending time inside in close proximity during the winter months,” Dr Singanayagam said.
The study, which enrolled 621 participants, found that of 205 household contacts of people with Delta per cent Covid-19 infection, 38 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 College London epidemiologist Neil Ferguson said that the transmissibility of Delta meant that it was unlikely Britain 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 programme is so important.”
