Coronavirus infection levels have fallen in England, figures have revealed, reflecting a recent decline in cases while questions have emerged over levels of the virus in vaccinated people.

According to figures from the Office for National Statistics, based on swabs collected from randomly selected households, an estimated one in 75 people in England had Covid in the week ending 31 July, down from one in 65 the week before. The survey suggests infection levels have also fallen in Wales and Scotland, although Northern Ireland has seen a rise.

The decline follows a dramatic drop in UK cases, from 54,674 reported on 17 July to 21,691 on 3 August. These figures are for those who voluntarily come forward for testing, often with symptoms.

However, experts have noted that trends in the ONS data often lag behind the figures for cases since the survey includes lingering infections as well as new ones.

The drop has also been reflected in England’s R number, which represents the transmission of Covid two to three weeks ago. On Friday it was revealed that R for England, which reflects the average number of people an infected person goes on to infect, has fallen to between 0.8 and 1.1, compared with last week’s reported range of 1.1 to 1.4.

Sarah Crofts, the head of analytical outputs for the ONS Covid-19 infection survey, said it was encouraging that infections had fallen across England, Wales and Scotland.

“Infection rates remain high across the UK, however, and we’ll need to see more data before concluding whether we are over the peak of this current wave,” she said.

In recent days the daily reported case numbers for the UK have risen, with 30,215 new cases reported on Thursday.

Prof Rowland Kao, an epidemiologist at the University of Edinburgh and a participant in the Scientific Pandemic Influenza Group on Modelling (SPI-M), said that while there were some indications that daily case numbers were either stabilising or possibly set to increase again, whether they did would depend on a range of factors.

“These include school holidays, weather impacts, changes in restrictions, the end of the Euro 2020 tournament, the ‘pingdemic’ and possible shifts in human behaviour,” he said.

The figures come as initial findings released by Public Health England (PHE) suggested both vaccinated and unvaccinated people may have similar levels of coronavirus when infected.

However, earlier work by PHE suggested that even one dose of a Covid jab reduced the chance of passing on the virus.

Prof Paul Hunter, a professor in medicine at the University of East Anglia, said the two findings were not necessarily at odds, noting that the levels of virus were derived from samples taken from people who had come forward for a test, and hence probably had symptoms.

“How sick you are, by and large, depends on viral load, so somebody who gets sick whether vaccinated or not will probably have roughly similar viral loads because the severity of illness will be the same,” he said.

But, he added, “very few people get that sick, relatively speaking, once they have had the vaccine”.

In other words, the jabs not only reduce the chance of people becoming infected but decrease the likelihood of people developing symptoms if they do get Covid, meaning overall the chance of passing the virus on is decreased by vaccination.

Others experts added that the estimates of viral load did not take into account the speed at which the virus was cleared, levels of viral shedding by infected people, or whether the virus was viable, while the age of infected people was not taken into account in the latest findings.

The PHE report also reveals that between 1 February and 2 August this year, a total of 5,159 people were admitted to hospital with the Delta variant, 2,960 of whom were unvaccinated and 1,355 of whom had received two doses of the Covid jabs.

The team said that as more of the population is vaccinated, the relative percentage of vaccinated people in hospital is likely to climb.