Scientists and experts have challenged new modelling that suggests Britain will pass the threshold for herd immunity by next week.

Professor Karl Friston, of University College London, estimates that the number of people who have protection against Covid-19 – acquired through vaccination or previous infection – will reach 73.4 per cent on Monday.

“Based upon contact rates at the beginning of the pandemic and estimated transmission risk, this is nearly at the herd immunity threshold,” he told The Daily Telegraph.

However, other academics have called into question the findings of the model, saying it is not “internally consistent”, while health secretary Matt Hancock said the government will continue to “watch” the real-world data.

Mr Hancock added that the UK had adopted the “right course in plotting our way to freedom”.

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Britain’s roadmap out of lockdown has been shaped by the government’s Scientific Advisory Group for Emergencies, which has warned that a swift reopening will lead to a resurgence in cases and deaths – as highlighted by the case of Chile, which is experiencing a third wave despite its successful vaccination programme.

The UCL modelling has led to calls for an acceleration of the roadmap – but critics have highlighted previous inconsistencies in Prof Friston’s work.

Dr Adam Kucharski, an epidemiologist at the London School of Hygiene & Tropical Medicine, said he was concerned by the findings.

“Unfortunately, the modelling approach used to produce this analysis has a history of making over-confident and over-optimistic predictions,” he said.

Dr Kucharski highlighted how the UCL model had previously predicted that the UK’s second wave would reach a high of roughly 31 deaths a day. At its peak, nearly 500 fatalities were being recorded during the November surge. A high of 1,361 deaths was later reached in the winter wave.

“There is currently a lot of uncertainty about vaccine effectiveness on reducing transmission, the duration of vaccine protection, characteristics of new variants, and the future control measures countries may keep in place or relax, Dr Kucharski added.

“However, it is not clear from the online model description how it incorporates these uncertainties, or what has been updated since it made its predictions above.

“Models can be a useful tool for exploring potential dynamics under different assumptions about transmission and control measures, but it’s crucial that these assumptions are made clear, and known biases in past model performance are addressed before new results are publicised.”

Rowland Kao, a professor of veterinary epidemiology and data science at the University of Edinburgh, said there were a number of factors that need to be considered when attempting to model the herd immunity threshold.

“The underlying number of people who have already been infected will vary considerably from region to region – as an extreme example, the Scottish Highlands and Islands have had very low infection levels so very little naturally induced immunity in those populations,” he said.

Waning immunity in people over 65 must also be accounted for, Prof Kao said, adding that “the uptake of vaccination has been highly variable” among some ethnic groups and people in deprived communities.

“These are also the groups where individuals are more likely to be seriously affected by Covid-19 infection,” he said.

“Any assessment of when we are likely to attain herd immunity which does not consider these heterogeneities, would be overly optimistic in its predictions of our levels of protection.”

According to figures published last Thursday by the Office for National Statistics, about 54 per cent of people in England had antibodies against the virus in the week ending 14 March.

Since then, a further 7.1 million people have received their first vaccine dose, while nearly 100,000 people have tested positive for Covid-19.

Christl Donnelly, a professor of statistical epidemiology at Imperial College London, said the recent results of the React study provide “strong evidence that herd immunity is not imminent”.

She said that Britain’s R rate stood at 1 between 11 and 30 March, indicating that “with all of the immunity in the population at that point … the social distancing restrictions in place in March were just enough to keep infections from increasing and not enough to make them decrease”.

If the population of England were to reach the herd immunity threshold, she explained, then it “would mean that without social restrictions the immunity level would be sufficient to keep infection rates from increasing and as immunity increased further, infection rates would go down.”

Prof Donnelly added that there would not have been any “dramatic” changes to Britain’s population immunity between mid to late March and early April.