Professor Gordon Dougan, from the Institute for Therapeutic Immunology and Infectious Disease at the University of Cambridge
A vaccine expert from the University of Cambridge says he doesn't believe a full lockdown is needed at the moment, after the new variant of coronavirus was discovered in the UK.
One of the first cases of the Omicron strain, which originated in South Africa, was found in Brentwood, Essex, and two others, in Nottingham and London.
Essex County Council said targeted testing was being focused on customers, staff and delivery drivers at a KFC in Brentwood on November 19 and congregants at a church two days later - raising questions over how long the new variant of coronavirus has been in the UK.
Face masks will become mandatory from Tuesday in shops and on public transport in England.
Anyone who enters the UK from abroad will now be required to take a PCR test and self-isolate until they get a negative result.
And those who come into contact with someone who tests positive for the Omicron variant will need to self-isolate for 10 days - even if they are fully vaccinated.
Find out more about the coronavirus data in the ITV Anglia region
At a Downing Street news conference on Saturday, Boris Johnson described the measures as "temporary and precautionary" - and said they will be reviewed in three weeks' time.
There are fears that the Omicron variant, which could be more transmissible and more resistant to vaccines, may affect the plans of millions over Christmas.
Professor Gordon Dougan, from the Institute for Therapeutic Immunology and Infectious Disease at the University of Cambridge, says the natural immunity we have built up could help cushion its effects.
"We do have two things in our favour. One is that we have got a lot of natural immunity to the virus that we have built up and we also have vaccine induced immunity in the UK.
"So we will be looking for is whether there is any evidence that there is an escape in that protection."
Professor Dougan has welcomed the tougher measures, but says the PCR tests won't be able to indicate which variant someone has been infected with.
"The PCR tests will not tell you if you have that variant (Omicron) or not. So you have to use sequencing to prove whether you have the variant which will take extra time.
"There is two issues here. What do we do about people who come in from abroad - yes we are testing those individuals. But the virus is already here, so if we have got an individual who we know has got that variant than we can track and trace around them."
"I don't think we need to go to a full lockdown at the moment, we just need to be vigilant and try to treat each other with respect."