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This week, UK prime minister Boris Johnson warned that there is no doubt that the third wave of covid-19 infections happening now in mainland Europe will 鈥渨ash up鈥� on the UK鈥檚 shores. In fact, scientists and doctors, including England鈥檚 chief medical officer Chris Whitty, have been warning for weeks that a new surge of infections would happen this year, as lockdown restrictions are eased. Now we have some early figures on vaccine take-up and effectiveness, what can we predict about the UK鈥檚 third wave?

Although some people might have expected that the roll-out of coronavirus vaccines would put an end to infection surges, the new wave is inevitable because there will still be three groups of people who can get sick, even when everyone eligible has been offered a vaccine. These are: under-18s, who aren鈥檛 currently eligible for immunisation in most countries; those who refuse the vaccine; and the people it fails to protect.

In the UK, about 21 per cent of the population is under 18. About 6 per cent of adults don鈥檛 plan to have the vaccine, according to the . And a first dose of the Pfizer/BioNTech or Oxford/AstraZeneca vaccine gives about 75 to 80 per cent protection from severe illness that would lead to hospitalisation, according to several recent studies. The figures for vaccine effectiveness and uptake are better than initial predictions, says Mark Woolhouse at the University of Edinburgh, UK. 鈥淭hat鈥檚 very encouraging.鈥�

Nevertheless, those three groups add up to a sizeable chunk of the UK population who could still get severely ill from the virus. The number of people who could catch it would be higher still, because the vaccines don鈥檛 completely prevent people from catching and transmitting the virus, even if they don鈥檛 become ill themselves. We don鈥檛 yet have numbers for that. On the other hand, those people who have already had covid-19 will have some natural immunity.

Putting all the figures together, Anne Cori at Imperial College London and her colleagues, who have been advising the UK government, have calculated that by July about a third of the UK population 鈥� some 22 million people 鈥� would still have neither vaccine-derived nor natural immunity to the virus, and would be susceptible to a third wave.

How would this wave be different from the previous ones? While hospital admissions and deaths will still rise along with infections, they will do so less than in the first and second waves, because older and more vulnerable people will be 80 per cent protected by vaccination. 鈥淲e鈥檙e going to be seeing fewer hospitalisations and deaths per case,鈥� says Cori.

Putting actual numbers to the hospitalisations and deaths is difficult because they depend on many variables. One of the most important is how fast lockdown restrictions are lifted.

At the moment, England and Scotland have planned timetables for easing restrictions in a series of steps between March and June, with the next step due to take place in England from 12 April. Plans for Wales and Northern Ireland are less definite. This means many restrictions being lifted before everyone eligible has been offered their first vaccine, currently slated to be by the end of July, although the people to get the jab last should be the youngest and least vulnerable.

However, the plan is that each step will proceed only if figures show that the previous one hasn鈥檛 set the country on the path to hospitals being overwhelmed. 鈥淭he current strategy is to pause at every step of the relaxation to see if things are going wrong,鈥� says Cori. This should mean the third wave is more gradual than if restrictions were lifted at once.

Other factors that would affect the size of any future wave include lower vaccine uptake in younger adults, or new coronavirus variants spreading that are more transmissible or that can evade vaccines.

Despite the uncertainties, Cori鈥檚 team and another group at the University of Warwick, UK, have offered estimates of the number of deaths likely in a third wave in the UK. Using computer models, both groups estimated that even if all goes well, there could be another This work was presented to government advisors in February and helped inform England鈥檚 timetable.

To put that into context, there were about 40,000 deaths in the UK鈥檚 first surge, most of which were in April and May of 2020, although a third wave could be more spread out so hospitals would not necessarily be overwhelmed. 鈥淲e should hopefully avoid any of these mass hospital surges we鈥檝e seen before,鈥� says Sam Moore at the University of Warwick.

On the other hand, Moore says that if one or more of the unknowns go in the wrong direction, that could easily take deaths and hospital admissions higher than the optimistic estimate 鈥� enough to risk overwhelming hospitals and forcing stricter social distancing rules to be reintroduced. 聽鈥淢y main worry is the emergence of variants that would jeopardise the effectiveness of the vaccination campaign,鈥� says Cori.

鈥淧eople really shouldn鈥檛 think: 鈥極n 13 April I will be able to do this thing鈥�, because I think it鈥檚 unlikely we can make a plan and stick to it,鈥� says Cori. 鈥淭hings evolve.鈥�

However, Moore believes that the current timetable for lockdown easing is unlikely to have to go into reverse 鈥� unless a vaccine-resistant variant starts taking over in the UK. 鈥淭hat is the biggest worry,鈥� he says.

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