Two competing forces will determine Omicron’s impact on the nation over the next few weeks. The power of booster jabs to give last-minute protection against Covid-19 will be pitted against the new variant’s ability to elude existing immunity. The outcome will decide whether our festive season is going to be muted or miserable.

If enough arms are jabbed with booster vaccines, while Omicron turns out to have poor powers to evade immunity, then there is hope hospital cases will be contained and the NHS will be protected. Severe restrictions in the new year – including the prospect of lockdowns – could be avoided.

But if Omicron is found to evade existing immunity quite easily, while booster campaigns provide poor overall protection, then the country faces a very grim winter with strict restrictions needed for some time.

According to a study by scientists at the London School of Hygiene & Tropical Medicine, the first scenario – poor Omicron escape from immunity matched by effective booster jab protection – would trigger a wave of infection that could lead to 175,000 hospital admissions and 24,700 deaths by the end of April. Closure of some entertainment venues and restrictions on indoor hospitality would be enough to control case numbers.

By contrast, the most pessimistic scenario – high immune escape from vaccines and low effectiveness of booster jabs – would see 74,800 deaths while there would be 492,000 hospital admissions, a figure twice as high as the peak seen in January 2021. Far stricter restrictions, including lockdowns, would then have to be considered.

“These results suggest that Omicron has the potential to cause substantial surges in cases, hospital admissions and deaths in populations with high levels of immunity, including England,” the team state in their paper, which has not yet been peer-reviewed. Nicholas Davies, co-leader of the study team, described Omicron’s threat to the UK as “worrying”.

Presented with evidence like this, many scientists have warned that urgent action should now be taken to hold back infections while booster campaigns are accelerated and given time to take effect. “Cases are doubling every two to three days which means there is a real risk the curve is going to get very steep around Christmas and New Year,” said the vaccine expert Peter English.

“That means panic measures could be brought in at the last minute and disrupt people’s festive plans. I am also desperately sad for my colleagues in clinical practice who face a January that is going to be worse than anything we’ve seen so far and at a time when they are now exhausted.”

However, the epidemiologist Prof Mark Woolhouse, of Edinburgh University, counselled caution. “Lockdown interventions buy time, that is true, but they also cause wider harm. Other, more sustainable measures – such as wider use of self-testing – may prove to be more viable.”

The crucial point about making strict interventions was not to prevent the population getting Covid but to avoid too many people getting it at the same time, he added. “I don’t expect to live out my years without getting Covid once or twice some time in the future – and that applies to the rest of the population. Individual risks have not changed. The problem is that it looks as though an awful lot of us are going to encounter it in the next few weeks.”

Other scientists said they were slightly more optimistic. The virologist Prof Ian Jones, of Reading University, said the grimmest scenarios did not take into account the availability of new antiviral drugs that have been shown to lessen the impact of the virus if given soon after infection. “If this better clinical picture is factored in, the link between infection and severe disease may not be as high as assumed here, and the outcome not as alarming.”

This point was backed by Prof Paul Hunter, an infectious diseases expert at the University of East Anglia, who said there was evidence that Omicron was associated with less severe disease and that models overestimated hospital admissions, possibly substantially. “I suspect these models overstate risk of hospitalisation and deaths and the worst case scenarios are unlikely to be seen.”

If the dangers of Omicron turn out to be exaggerated, the prospects of further variants disrupting society cannot be ignored, said the global health researcher Michael Head, of Southampton University, who accused rich countries including the UK of hoarding vaccines instead of sending them to nations with less well-developed health services. “We don’t know how often this coronavirus can change its clothes and emerge with a new look but the risks of new variants emerging are higher in under-vaccinated populations. The coronavirus has not finished with us.”