Almost two million more people are to be told to shield to protect themselves from serious side-effects from Covid-19, officials have said.
A new tool has identified those who are at high risk of severe disease or death.
As a result, 1.7 million additional people in England will be sent letters asking them to shield, with officials advising that shielding continues until March 31.
Around 2.2 million people are currently on the list in England, which will expand to almost four million when the additional people are included.
More than 800,000 of these are aged 19 to 69 and will be prioritised for the vaccination programme, the Department of Health and Social Care confirmed.
The current shielding list includes people with single risk factors such as those with various cancers, people on immunosuppression drugs or those with severe respiratory conditions.
But as the pandemic has progressed, medics have found that some people are at higher risk than others because they have multiple risk factors.
Using medical records, the new tool assesses which people are at higher risk based on multiple factors including age, ethnicity, body mass index (BMI), other health conditions and also postcode, which is indicative of levels of deprivation.
Dr Jenny Harries, deputy chief medical officer for England, said the 1.7 million, if they join the list, will receive the additional support available to those deemed to be clinically extremely vulnerable, including medicine deliveries, priority slots for supermarkets and statutory sick pay.
She said: “All of these people who are identified for the first time are those who have not had an individual clinical condition that we’ve previously been able to identify through the clinically extremely vulnerable group, but they are people who have multiple personal risk factors and underlying health conditions which move them into a higher risk group.”
Dr Harries said the approach was “risk averse” to “protect as many people as possible”.