Scientists cannot yet be certain that the coronavirus vaccine being rolled out will be effective against the UK's new Covid-19 variant, as it emerged it has also been identified in three other countries.
Dr Susan Hopkins, of Public Health England, said further studies were needed to look at the strain, which has been circulating in the South East since September and has a high transmission rate.
She told the Andrew Marr Show: "So we can look at the structure - and we have looked at the structure very carefully, what the mutations do - and therefore the likely impact based on that knowledge of that structure. We won't know for definite until we have further studies.
"We are studying individuals who are admitted to hospital at the moment to determine how many of these may have this variant and is there differences in those who have the variant virus versus other viruses of this type?
"The vaccine induces a strong, multiple response, immune response and therefore it is unlikely that this vaccine response is going to be completely gone."
On Saturday, Prof Chris Whitty said there was "no current evidence to suggest the new strain causes a higher mortality rate or that it affects vaccines and treatments although urgent work is under way to confirm this".
Maria Van Kerkhove, the Covid-19 technical lead at the World Health Organization, also appeared on the show.
She said the variant had also been identified in Denmark, the Netherlands and Australia - with there being one case in the latter that did not spread further.
Watch: What is the new Covid variant?
She added: "And so more sequencing that can be done will be helpful to help us determine if this variant is circulating elsewhere. Sequencing will also help us understand any changes in the viruses that are being identified.
"The longer this virus spreads, the more opportunities it has to change. So we really need to do everything we can right now to prevent spread, and minimising that spread will reduce the chances of it changing."
Dr Hopkins, meanwhile, said that the evidence that the new variant was up to 70 per cent more transmissible was based on modelling the rate of increase in the new strain compared with others in circulation.
She added: "We have seen evidence of the virus showing higher viral loads so there's more virus in the people we are detecting with this virus strain."
But she said a higher viral load did not mean people were going to get more ill, adding: "The illness comes from the immune response and how it reacts in your lungs - that's where we know the illness really starts to be driven from and why people need oxygen.
"The higher amount of the virus means that people are likely to be more infectious than they would otherwise be and this means we need to reiterate the social distancing measures."
The great escape
Dr Hopkins said that the main routes of transmission of the new variant were the same as the previous strains.
She said the Tier 4 measures and shorter Christmas bubble announced on Saturday by Boris Johnson were "really important", adding: "This virus will not transmit unless we are in contact with each other and really close contact, indoor spaces, reduced ventilation all help drive the transmission of this virus, no matter what variant it is."
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When asked if she was "completely horrified" by pictures of people cramming into London train stations, Dr Hopkins said: "I understand people's wish to get home to their families and loved ones that they may live with on a normal day-to-day basis and wanted to get out of London last night.
"I hope that when they go to wherever they are moving to they reduce their social contacts and don't contact anyone outside their household for the next 10 days, as that will help minimise the risk of transmission to other parts of the country.
"We know it's in other parts of the country in small amounts but what we are trying to do is prevent more spread and rapid increases across the rest of the country."
The new variant was identified in October from a sample taken in September.
But Dr Hopkins told the show that its greater transmissibility was only confirmed earlier this week. She added: "So there is constant evolution happening in the virus and the virus mutates all the time.
"This particular variant was identified from whole genome sequencing in the middle of October from a sample taken in September.
"It continued to spread and in early December, when we were trying to understand why Kent and Medway were continuing to increase despite the national restrictions, we found a cluster that was growing very fast and that had spread not just from the South East into London and parts of Essex.
"We still did not understand what the difference in transmissibility was. This week the modellers and academics that we work with in Imperial and other partners demonstrated that it was indeed more transmissible than other variants that are circulating."
Matt Hancock, the Health Secretary, has said that keeping the new coronavirus variant under control until a vaccine has been rolled out will be "very difficult".
"We have really got to get this under control. The cases in the Tier 4 areas have absolutely rocketed in the last few days - the last two weeks or so. We have got a long way to go to solve this," he told Sky News's Sophy Ridge On Sunday programme.
"Essentially we have got to get that vaccine rolled out to keep people safe. Given how much faster this new variant spreads it is going to be very difficult to keep it under control until we have the vaccine rolled out."
Watch: How worried should we be about the threat of the new strain?
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