Boris Johnson must be ready to "move fast" with 'Plan B' Covid-19 measures, the government's chief scientific adviser has said as he warned that the UK is still in a "very uncertain phase'" of the pandemic.

Sir Patrick Vallance told BBC Radio 4's Today programme that there is currently "considerable uncertainty" as to which direction Covid-19 rates are heading.

Sir Patrick said the country needed to "absolutely be prepared" for the government's 'Plan B' contingency measures, which could include a return to mandatory mask-wearing in some settings and home working where possible, as well as the introduction of Covid-19 vaccine passports for venues such as nightclubs.

READ MORE: 'Sensible steps now' will stop us from 'considering lockdowns' this winter, says Andy Burnham as Covid-19 cases rise

"As soon as you start thinking ‘am I, or am I not going to do this? It looks close’ that is the time you need to push beyond your natural reluctance to do it and do it," he said.

“This is obviously something the government will have to consider carefully but we need to be ready to move fast if that occurs.”

Speaking on BBC Breakfast, the scientist said models around what will happen in the coming weeks are “quite uncertain at the moment”.

“Nobody is really clear which direction this is going in," he added.

Sir Patrick explained that there are two factors that will determine which way the numbers go.

“One is waning immunity," he told the programme.

"So if immunity wanes faster than expected, you’ll see a bigger increase, and that’s why it’s so important to get booster shots going in the vulnerable and the elderly in particular.

“The second is the behavioural change, how quickly we return to pre-pandemic behaviours… if you aggregate the models, most are saying ‘Actually, it looks fairly flat, don’t expect the very big peaks we’ve had in the past, it looks fairly flat, but at a very high level at the moment.’

“So the high level remains a concern and from a high level you can go up quite quickly.”

He said a surge in infections in children is likely to level off as immunity builds from vaccination and infection in younger age groups.

Asked if more than 40,000 Covid cases a day is a level that can be dealt with and is acceptable, he said: “Well, that’s a societal question.

“There are high levels, and those high levels, of course, translate into levels of hospitalisation, but the levels of hospitalisation are very much reduced by vaccination.

“The lower the levels, the better in terms of overall outcome, but there are costs and consequences of decisions in both directions there.

“So that’s a societal question about what levels are acceptable."

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Sir Patrick added that Covid-19 infections will reoccur year on year, particularly during the winter months.

"Gradually, as immunity builds, the protection will be there, the consequences will be reduced, but we’re not there yet," he said.

Earlier this week, Downing Street said it was “too early” to draw conclusions from recent figures which suggested a potential levelling off of coronavirus cases.

The Prime Minister’s official spokesman said: “It’s always encouraging when you see reductions like that and including, I believe, a levelling off of admissions.

“But it’s too early to draw full conclusions from the case rates and we would continue to urge the public to abide by the guidance as set out and those eligible to get booster doses.

“Prevalence remains relatively high even if it has dropped off to a certain extent.

“There isn’t anything in the statistics currently to suggest a move to Plan B but it is too early to draw conclusions from the recent few days’ statistics which has shown drops in cases.”

The estimates range from 90.0 per cent in Wales to 92.2 per cent in England, with 90.8 per cent for Northern Ireland and 91.3 per cent for Scotland.

The ONS said there is a “clear pattern” between vaccination and testing positive for Covid-19 antibodies, but “the detection of antibodies alone is not a precise measure of the immunity protection given by vaccination”.

Once infected or vaccinated, the length of time antibodies remain at detectable levels in the blood is not fully known.

It is also not yet known how having detectable antibodies, now or at some time in the past, affects the chance of getting Covid-19 again.

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