Amid good news about Covid-19 vaccination rates increasing and infections beginning to fall, Boris Johnson shocked listeners to his press briefing on Friday with an unexpected announcement that the more contagious new variant of coronavirus is also more lethal.

After scientists first recognised in mid-December that variant B.1.1.7 was outpacing previous versions of the virus with its rapid spread out from south-east England and across the UK, they had said it was around 50 per cent more transmissible but seemed not to cause more severe symptoms.

How strong is the new evidence that the variant is killing a higher proportion of the people it infects?

The government’s New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) considered studies from three university teams and Public Health England, comparing death rates between people known to have been infected with B.1.1.7 and those with older forms of coronavirus. They corrected as far as possible for other factors that might affect mortality such as age, location and ethnicity.

All the studies found some increase in lethality, though uncertainties about the data resulted in a wide range of estimates. Bringing their results together, Nervtag scientists produced a model showing that someone infected with B.1.1.7 is 30 to 40 per cent more likely to die than someone with a different variant.

Patrick Vallance, the government chief scientist, quoted this estimate at the Downing Street briefing. “I want to stress that there’s a lot of uncertainty around these numbers,” he said, “and we need more work to get a precise handle on it, but it obviously is a concern that this has an increase in mortality.”

Most scientists who have commented on Nervtag’s assessment believe that the available evidence justifies the overall conclusion of higher mortality, though the size of the effect needs to be pinned down. The outer limits of the estimates in the studies considered by Nervtag ranged from a 7 per cent to a 271 per cent additional risk from B.1.1.7.

If the discovery that the new variant is more lethal, how much difference will it make to individuals?

Consider the central estimate that B.1.1.7 carries a 30 to 40 per cent higher chance of dying. The impact of this relative risk on an individual will depend critically on his or her absolute risk — determined above all by age and then by underlying health and other factors. The evidence considered by Nervtag suggests that the variant increases case fatality rates consistently across all age groups.

Sir Patrick took men in their 60s as an example. The average risk was that about 10 in every 1,000 would be expected to die from the infection with the old virus; with the new variant, 13 or 14 would die.

For healthy children and young adults, who are extremely unlikely to die from Covid, the additional lethality of B.1.1.7 would have a tiny effect on their absolute risk. Conversely it would have a big impact on people over 80 who are already at high risk.

It is worth remembering that fatality rates are already considerably lower than they were in the first wave of the pandemic in the spring, because health workers have learned how best to treat Covid-19 patients, when to give dexamethasone steroid to those who are severely ill. Even if the new variant raises risk of death by 35 per cent, it would still be lower than for someone with the original form of the virus back in March.

What impact would a more lethal variant have on overstretched health services?

The increased transmissibility of B.1.1.7 has already raised sickness to a level that is severely stressing hospitals, particularly in London and south-east England where it is responsible for the majority of Covid-19 cases.

Nervtag’s analysis suggests that the new variant does not increase fatality rates among people who are already in hospital with Covid-19, said Rowland Kao, professor of epidemiology at the University of Edinburgh.

“The increase in deaths is a result of more individuals becoming severely infected and hospitalised, rather than more hospitalisations resulting in death,” he said. “As such, it would appear that the new variant is also responsible for the increased, unexpectedly high burdens in hospitals seen especially around London.”

Will the vaccination programme need to change?

A more lethal variant is unlikely to alter the balance of the lively medical debate over how to roll out Covid-19 vaccines — and in particular the government’s decision to inoculate as many people as possible with a first dose, even if they have to wait for as long as 12 weeks for the second jab.

The British Medical Association has asked for the wait for a second dose to be cut to a maximum of six weeks for the BioNTech/Pfizer vaccine. But defenders of the current policy say a more virulent variant makes it even more important to vaccinate vulnerable people as fast as possible.

Yvonne Doyle, medical director of Public Health England, told BBC Radio on Saturday that the current strategy of “bearing down on transmission” would cut deaths and reduce the chance of more dangerous variants of the virus emerging. “The more people that are protected against this virus, the less opportunity it has to get the upper hand,” she said.