Covid infections remain high with one in 35 people infected in England, say researchers who did random sampling in February.
The results from 94,950 swabs also showed that infections may be rising in the over-55s while falling in children and younger adults.
Among the population as a whole, it is a drop from rates in January when the REACT monitoring study found a rate of 1 in 23 people were infected but is the still the second highest level since the research started in May 2020.
A genomic analysis of positive samples also showed that the BA.2 Omicron variant is becoming more common accounting for almost half of cases by 21st February compared to just 0.8% the previous month.
It suggests this variant spreads more easily, the researchers said and noted the highest rates of BA.2 were found in London.
Professor Paul Elliott, director of the REACT programme at Imperial College London said: ‘It’s encouraging that infections have been falling across England, but they are still very high and the possibility that they are rising in older adults may be cause for concern.
‘The good news is that this is a highly vaccinated group, however a high number of infections will lead to more people becoming ill, so it’s important that people continue to follow public health guidance to avoid fuelling further spread of the virus,’ he added.
It comes as the latest data from the ZOE Covid study suggested that cases are once more on the rise after weeks of declining rates.
Their figures show there are currently 175,189 new daily symptomatic cases of Covid in the UK on average – an increase of 20% from the previous week.
Professor Tim Spector, co-founder of the ZOE Covid symptom app said the increase was a worry.
‘This increase was predicted when all restrictions were lifted. We are likely to continue to see high infection and prevalence rates of 1in 30 people for the foreseeable future.’
He added that the official dashboard of cases was providing a misleading picture with the ZOE app recording five times the number of cases.
Commenting on the REACT study data, Dr Jenny Harries, chief executive of the UK Health Security Agency, said: ‘These data confirm that cases have declined substantially following the peak of the Omicron wave.
‘However, the increasing presence of the BA.2 sub-lineage of Omicron and the recent slight increase in infections in those over 55 show that the pandemic is not over and that we can expect to see Covid circulating at high levels.’
Dr Simon Clarke, associate professor in cellular microbiology at the University of Reading said reported increases in infections could be because people are mixing more or because of a drop in population-wide immunity.
‘Most likely, it’s a bit of both, but at the moment nobody can say for sure. The ONS data will reveal this in time.’