Too early to say if Covid infections are falling in England, experts warn

It is too early to say whether a recent fall in daily Covid cases reflects a drop in actual infection rates, experts have cautioned, after new government data presented a mixed picture for England.

According to figures from the Office for National Statistics, based on swabs collected from randomly selected households, an estimated one in 65 people in the community in England had Covid in the week ending 24 July, up from one in 75 the week before. The survey suggests infection levels have also risen in Wales and Northern Ireland, although Scotland has experienced a decline.

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Although the ONS team said the rise in infection levels in England was showing possible signs of slowing, the findings contrast with daily figures for Covid cases, which are based on people who have come forward for testing, often once symptoms have developed.

According to data from the government coronavirus dashboard, the number of daily cases by date reported plummeted from a peak of 50,955 in England on Saturday 17 July to 20,290 cases on Tuesday 27 July, before rising to 27,524 on Thursday and falling to 26,241 on Friday.

Prof Paul Hunter, from the Norwich School of Medicine at the University of East Anglia, said: “The important point to point out is that the ONS survey largely covers a period prior to the decline in cases, especially as this is a prevalence survey and people can be positive for some time after acquiring their infection.

“We will have to wait till next week before we can see any indication of the recent decline in cases. Generally changes in ONS data lag about two weeks behind daily cases data.”

Duncan Cook, the deputy director of the Covid-19 Infection Survey, suggested the disparity could also be down to the ONS using a random testing approach that picks up both symptomatic and asymptomatic infections.

“These new official statistics do not reflect the recent dip in the daily testing figures in England, so it’s important to understand the differences between the two sources. Our survey tests a large randomised sample of the population and provides an independent estimate of infections in the wider community.

“It’s notable that around 40% of positive tests in the ONS study are from people who show no symptoms of infection. This group is therefore less likely to show up in the daily figures.”

However, Cook said it would take time before the trends became clear. “Together with our partners across the UK we will continue to investigate whether the current wave of infections is stabilising or not. On that it remains too early to say,” he said.

The ONS team said that in the most recent week of data for England, the percentage of people testing positive for Covid increased in all regions except the east and the south-west, with infection levels only clearly rising among those aged over 50 and in children from aged two up to school year 11 (ages 15 to 16).

In Scotland, daily reported Covid cases have been falling since the start of the month. However, a decline in infection levels has only now appeared in the ONS data, with about one in 110 people in the community estimated to have had Covid in the most recent week, down from one in 80 the week before.

The new figures came as the R value for England, which reflects the situation over the past few weeks, was estimated to be between 1.1 and 1.4. This number indicates the average number of people one infected person goes on to infect, and is a slight fall on last week’s estimate of 1.2 to 1.4.

Experts have warned against reading too much into recent fluctuations in Covid datasets, suggesting there could be plenty of upticks and downturns to come.

“Cases are volatile at the moment, so [it is] not good to overinterpret short periods of time,” said Prof Steven Riley of Imperial College London, an expert in infectious disease dynamics and a member of the Scientific Pandemic Influenza Group on Modelling (Spi-M).

“I’m thinking about the next three months in two halves. There are quite a few reasons [why] prevalence will remain stable or in decline up to the start of September, but then the return of schools, increased workplace mixing, bad weather and a genuine attempt at ‘return to normal’ behaviour will bring upwards pressure.”

A document released on Friday from the Scientific Pandemic Influenza Group on Modelling, Operational sub-group (SPI-M-O) and dating to 14 July echoed Riley’s concerns.

“SPI-M-O is concerned that September and October 2021 will be a particularly risky point in the trajectory of the epidemic. It is likely behaviours will take time to return to more normal levels and, if this coincides with the return of schools and universities in the autumn, significant pressures on healthcare could be seen.”

The experts also stressed the need for global surveillance, particularly of new variants, and vaccination to bring Covid infections worldwide under control.

“Any increase in foreign travel over the summer and the return of international students to universities in the autumn is of particular concern,” the document noted.

Prof Rowland Kao of the University of Edinburgh, who also contributes to Spi-M, cautioned it could take time for the picture to become clear, adding the impact of relaxing Covid restrictions had yet to become fully apparent.

“Even if there is a long-term trend downwards, we would expect bumps in the road – and if you look at the overall pattern there have been, in the past, multiple consecutive days of an apparent trend, but then it disappears again,” he said. “Having said that, we are at the stage where the impact of the release of restrictions on the 19th [July] might start to show up more … so we should be watching those trends closely.”