Sir Patrick Vallance called the latest government advice a “big intervention”.
It comes as the World Health Organization declared Europe the epicentre of the pandemic, and said about 5,000 people have died globally.
In the UK, the virus has killed 11 people, including one in Scotland.
As of 14:00 GMT there have been 798 confirmed cases across the UK – a rise of 208 compared with Thursday.
But health officials say they believe the actual number of people infected could be between 5,000 and 10,000.
Sir Patrick said he hoped the government’s approach to tackling the virus would create a “herd immunity in the UK”.