Covid-19 infections could double over Christmas, scientists advising the government have said. The warning comes in the latest tranche of files from the Scientific Advisory Group for Emergencies (Sage). The documents were presented to government before a decision to relax Covid restrictions over the festive period. “Notes on Festive Period” was written by the Scientific Pandemic Influenza Group on Modelling (SPI-M), which caveats the document as being based on preliminary modelling. The document contains stark warnings about a substantial mixing of people over a short period of time. “Any relaxation over the festive period will result in increased transmission and increased prevalence, potentially by a large amount,” the document says. Later, they add: “SARS-Cov-19 has demonstrated high secondary attack rates in households (with estimates of up to 50% in one household becoming infected from one infected member). The prevalence could easily double during a few days of the festive season, with further multiplicative increases as new infections go back to their ‘routine’ networks.” The warning comes as the government and Sage revealed the R number for England specifically, and the UK as a whole, was now 0.9-1.0 for the first time since August. Because of time lags in the data, Sage says this figure captures some of the impact of the latest lockdown in England, but not its full effect. The overall R number masks considerable variation around the UK. While R was revealed to be between 0.7 and 0.9 in the north-west of England, it was 1.0-1.2 in the south-east, suggesting that the epidemic was shrinking in the former, and remaining steady or growing in the latter. But while signs are growing that lockdown is beginning to have an impact on infection rates and hospital admissions, it could be some weeks before such changes are reflected in the daily death toll. On Friday 521 people in the UK were reported as having died from Covid. Boris Johnson announced on Tuesday that three households would be allowed to mix for up to five days over Christmas, forming a “bubble”. However, Sage experts reveal in the files that Christmas bubbles will still be risky. “If all households were to form a bubble with one other household, the number of out-of-bubble contacts would have to reduce to the levels seen in April 2020 (<1 external contact per person) in order to prevent exponential growth,” one document notes. The experts also caution that while forming bubbles may be better than limiting contacts to a specific number, it is crucial that people stick to just one bubble. “Allowing people to form different bubbles over the course of the festive period would be worse than limiting them to a single bubble,” the modelling team say. Sage experts believe that a key factor in reducing the impact of Christmas socialising is getting prevalence as low as possible before the festivities – not only to reduce the chance of meeting someone infected, but because a rise in infections if prevalence is already high is likely to have a greater impact on hospital admissions and deaths. “Households can significantly reduce their risk of introducing infection into an event by quarantining for two weeks,” they say. “However this raises equity issues as many families would struggle to do this, including frontline workers.” The documents also shed light on the timeframe allowed by the government for mixing over Christmas. “Bubbling would be most effective if time-limited to less than one generation time of infection (about one week),” the modelling experts write.Prof Liam Smeeth, professor of clinical epidemiology at the London School of Hygiene & Tropical Medicine, said: “A period of tiered measures in December is warranted now in order to help allow some social mixing at Christmas that is so important to many. I suspect a further circuit breaker in January or possibly February may well be needed because Christmas will place such upward pressure on transmission rates.”