Some level of social restrictions will need to be in place for a month or two, a UK Health Security Agency expert has said.

Dr Susan Hopkins chief medical adviser for the UKHSA, said that if Omicron modelling so far is correct then the country will “need some level of restrictions in place for the next four to eight weeks”.

She warned that the country faces “a very difficult four weeks ahead” with rising case numbers both in the community and in hospitals.

And she urged people to take a lateral flow test before they socialise as she appeared before the Science and Technology Committee of MPs.

Greg Clark, chairman of the committee, asked: “Based on the modelling that you’ve seen so far, when do you expect the restrictions to be able to be lifted?”

Dr Hopkins said: “I think that if the modelling that the London School (of Hygiene & Tropical Medicine) has done, I would expect that there’s going to be needing some level of restrictions in place for the next four to eight weeks.”

She said that Omicron would “displace Delta” but in some regions the two variants would be circulating at the same time.

In London, 44% of cases are Omicron.

“We are seeing increases in the total number of cases in London, so London has doubled its cases over the last four weeks,” she said.

“We expect to see Omicron displace Delta, but they are going to live together in parts of the country for longer and we are going to continue to see hospitalisations from Delta for the next two weeks baked in from the numbers that we have.

“And then we will start seeing the Omicron case numbers come into hospital.”

She warned that with previous variants the risk of reinfection – where someone who has had Covid tests positive again within 90 days after their initial positive test – was “rare”, but Omicron appears to carry a higher risk of reinfection.

“In less than three weeks from when we’ve identified to the first case, we are seeing reinfections and higher rates of reinfections in Omicron compared to Delta with a rate of three to eight times the reinfection risk for Omicron compared to what we’ve seen with Delta,” she added.

The growth rate of the Omicron variant seems to be “shortening rather than lengthening”, she added.

“We have been studying this virus for three weeks today and in that time period we have shown that in the UK this is growing very fast, with a growth rate initially two to three days and that growth rate seems to be shortening, rather than lengthening, at the moment,” she said.

She added: “We are concerned with the large volume of individuals who are being infected every day in the population that we are going to have a very difficult four weeks ahead with cases in the community which will, of course, cause individuals to stay off work and school, and then those cases to transfer into admissions to hospital.”

Dr Hopkins said that the UKHSA was also “concerned” about the effectiveness of one of the drugs used to treat Covid.

“We’ve got monoclonal antibodies, one of those monoclonal antibodies ronapreve, we are worried about the effectiveness of that, so that is actually taking away a drug that we have been using in the NHS in the past weeks and months to treat people who are antibody negative so that is a challenge,” she said.

She urged people to take a lateral flow test before they socialise.

“Our strong public health recommendation is if people are going out to venues to socialise, they should do lateral flow before they go to reduce the risk of going into that venue with asymptomatic infection and therefore transmitting to others.”

Professor Wei Shen Lim, chairwoman of the Covid-19 panel of the Joint Committee on Vaccination and Immunisation, told the MPs that the expert group was actively discussing whether or not children as young as five should be offered a Covid-19 vaccine.

The Medicines and Healthcare products Regulatory Agency is currently assessing whether or not it will approve vaccines for use in children aged five to 11.

MPs also heard from Dr Paul Burton, chief medical officer of Moderna, who said that he believes the Omicron variant could be just as severe as Delta.

He said: “I actually do not think that Omicron is a milder, less severe version of the current virus.

“I also think that the idea that essentially it will push Delta out of the way and takeover may occur in the future, but I think certainly in the coming months, these two viruses are going to coexist.

“It will also lead to a situation where individuals will become co-infected, which gives the opportunity for these viruses to further evolve and mutate, which is a concerning and worrying situation.”