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 of the virus.
The study on 143,000 people, who were randomly selected, looked at the prevalence of coronavirus including in people without symptoms.
Infections from 6-15 January were 50% higher than in early December, the study found. Prof Elliott said the current R rate - which represents how many people an infected person will pass the virus on to - was "around 1".