Around 1.7 million more people will be added to the shielding list in England, after being found to be at serious risk from COVID-19.
They have been identified due to multiple factors, including underlying health conditions, and will be contacted by a letter from the NHS in the coming days.
Some will have already been offered a coronavirus jab but those who have not will be able to get their vaccines earlier.
The shielding list was expanded after scientists at Oxford University developed a new tool that assesses whether someone is at risk of severe disease or death.
It looks at multiple factors including age, ethnicity, body mass index, other health conditions.
The additional 1.7 million will bring the number on the list to almost four million - all of whom are being advised to shield until 31 March.
Dr Jenny Harries, one of England's deputy chief medical officers, said: "For the first time, we are able to go even further in protecting the most vulnerable in our communities.
"This new model is a tribute to our health and technology researchers.
"The model's data-driven approach to medical risk assessment will help the NHS identify further individuals who may be at high risk from COVID-19 due to a combination of personal and health factors.
"This action ensures those most vulnerable to COVID-19 can benefit from both the protection that vaccines provide, and from enhanced advice, including shielding and support, if they choose it."