Experts tracking the spread of Covid-19 in England say infections may have gone up at the beginning of the current lockdown.
Professor Paul Elliot, who is leading the React study at Imperial College London, suggested the current measures may not be strict enough to see a drop in infections and the reproductive rate – the R.
The study on 143,000 people, who were randomly selected, looked at the prevalence of coronavirus including in people without symptoms.
Infections from January 6 to 15 were 50% higher than in early December, the study found.
Prof Elliott told BBC Radio 4's Today programme the current R rate – which represents how many people an infected person will pass the virus on to – was "around 1".
He added: "We're in a position where the levels are high and are not falling now within the period of this current lockdown."
He added: "We're seeing this levelling off, it's not going up, but we're not seeing the decline that we really need to see given the pressure on the NHS from the current very high levels of the virus in the population."
Government data shows that the number of new cases of Covid-19 per head of population has been falling in all regions of England in recent days.
The Government said the React study does not take full account of the current lockdown measures.