Fully vaccinated adults can harbour virus levels as high as unvaccinated people if infected with the Delta variant, according to a sweeping analysis of UK data, which supports the idea that hitting the threshold for herd immunity is unlikely.

There is abundant evidence that Covid vaccines in the UK continue to offer significant protection against hospitalisations and death. But this new analysis shows that although being fully vaccinated means the risk of getting infected is lower, once infected by Delta a person can carry similar virus levels as unvaccinated people.

The implications of this on transmission remain unclear, the researchers have cautioned. “We don’t yet know how much transmission can happen from people who get Covid-19 after being vaccinated – for example, they may have high levels of virus for shorter periods of time,” said Sarah Walker, a professor of medical statistics and epidemiology at the University of Oxford.

“But the fact that they can have high levels of virus suggests that people who aren’t yet vaccinated may not be as protected from the Delta variant as we hoped.”

Positive tests, hospitalisations and deaths linked to Covid have been rising slowly in the UK recently. In the week to 18 August, 211,238 people had a confirmed positive test result, an increase of 7.6% compared with the previous seven days. Over the same period, there have been 655 deaths within 28 days of a positive test, a rise of 7.9% versus the previous seven days. Hospitalisations have also risen slightly, with 5,623 going into hospital with coronavirus between 8 August 2021 and 14 August 2021, a rise of 4.3% compared with the previous seven days.

The study, which is yet to be peer-reviewed, found vaccine performance has waned against Delta versus the previously dominant Alpha variant.

The analysis did not directly investigate whether the lower level of vaccine protection against Delta affected jabs’ ability to prevent severe disease. However, Dr Penny Ward, a visiting professor in pharmaceutical medicine at King’s College London, noted: “The low incidence of hospitalisation seen to date suggests that in this respect at least the vaccines are protecting individuals from developing severe Covid.”

The study – conducted by Oxford researchers in partnership with the Office for National Statistics and the Department of Health and Social Care – compared the results of about 2.6m nose and throat swabs taken from more than 384,500 adults between December 2020 and mid-May 2021, and more than 811,600 test results from 358,983 adults between mid-May and 1 August 2021 (the period of Delta’s domination).

The UK findings on peak virus levels after Delta infections in vaccinated people echoed data from a small study cited by the US Centers for Disease Control and Prevention (CDC) last month. The agency indicated those findings had underpinned its decision to recommend that people wear masks in some indoor settings, regardless of their vaccination status, especially in areas of “substantial or high” virus transmission.

These datasets highlight the potential for vaccinated individuals to still pass Covid on to others, and the importance of testing and self-isolation to reduce transmission risk, said Dr Koen Pouwels, a senior researcher in Oxford University’s Nuffield department of population health. This potential for transmission makes achieving herd immunity even more challenging, he suggested.

The concept of herd or population immunity relies on a large majority of a population gaining immunity – either through vaccination or previous infection – which, in turn, provides indirect protection from an infectious disease for the unvaccinated and those who have never been previously infected.

The hope had been that we could vaccinate enough people to protect the unvaccinated, added Walker, who also serves as the chief investigator for the UK analysis. “I suspect that, partly, the higher levels of virus that we’re seeing in these [Delta] infections in vaccinated people are consistent with the fact that unvaccinated people are just going to be at higher risk.”

The researchers also found that compared with the Oxford/AstraZeneca jab, two doses of the Pfizer/BioNTech vaccine does have about 15% greater initial effectiveness against new infections, but the protection declines faster compared with two doses of Oxford/AstraZeneca. About four to five months after being fully vaccinated by either vaccine, the protection offered is relatively the same, said Walker.

“Even with these slight declines in protection against all infections and infections with high viral burden, it’s important to note that overall effectiveness is still very high because we were starting at such a high level of protection,” added Pouwels.