When Savid Javid revealed on Monday that an estimated 200,000 people a day are getting infected with Omicron, it brought understandable concern – especially as just 4,713 cases of the variant had been confirmed in the UK so far – . So where does this figure come from – and what does it tell us about the trajectory of the surge?
Confirming a Covid case is caused by the Omicron variant requires a full genetic analysis of that person’s swab. According to Prof Paul Hunter at the University of East Anglia, it can take up to two weeks to return a viral sequence, meaning the figure of 4,713 Omicron cases reported by the UK Health Security Agency (UKHSA) was already out of date.
Another way of estimating the number of Omicron cases is by looking at “S gene target failure”. Some of Omicron’s genetic mutations affect the readout of certain routine PCR tests, providing a quick – although not conclusive – indication of whether Omicron is present in a sample. Not every UK testing lab has the technology to do this, but between a third and half do. This provides another way to estimate what proportion of UK cases are Omicron, and how quickly it is overtaking the Delta variant.
According to the latest figures, there were 54,661 new positive Covid-19 cases reported across the UK on Monday – though really this reflects the number who became infected a week ago, as it takes time to develop symptoms and seek a test. Meanwhile, the latest S gene dropout figures suggest 20% of UK cases are now Omicron, translating into approximately 11,000 Omicron cases daily.
However, this only represents confirmed cases – people who have tested positive via PCR. Not everyone with Covid gets tested, so the true number of daily infections is unknown. This is why the Office for National Statistics (ONS) survey data is so revealing: by regularly swabbing a random sample of the population, it provides an estimate of the true proportion that has Covid at any given time.
The UKHSA says Javid’s 200,000-a-day figure stems from projected ONS infection survey data, which estimated there were about 78,000 Delta infections on 7 December, equating to approximately 23,000 daily Omicron infections on the same day. Assuming Omicron numbers are doubling every two days, as current data suggests, the UKHSA estimated that approximately 207,000 people were infected with Omicron on 13 December. As in the tale of the wheat and chessboard, regular doubling rates quickly add up to very large numbers.
The UKHSA has also estimated that if Omicron continues to grow at the present rate, there could be 1m daily Omicron infections by the end of December – and maybe sooner. A staggering figure but one that makes sense when you consider the doubling rate – though the revelation of such a figure may itself prompt the public to take precautions, flattening the curve.
As well as the health impact, growing numbers of people could have to isolate – including hospital staff, teachers, supermarket workers and lorry drivers.
Whether these predictions come to pass remains to be seen. Huge uncertainties remain regarding the extent to which T-cell immunity from previous infections or vaccination will protect against severe disease and hospitalisations, for example. And of course, at some point so many people may have been exposed to the new variant and/or received a booster jab in the UK that infections will begin to slow – although immunity takes several weeks to develop. A peak in January has been suggested.
Prof Matt Keeling, of the University of Warwick, said if cases keep doubling we will effectively hit herd immunity. “Infection cannot keep doubling forever,” he said. “There are only a finite number of people to infect, and eventually the outbreak of Omicron would ‘burn itself out’ – but this isn’t going to happen until a lot of people have been infected, which we clearly want to avoid.
“Getting to herd immunity by infecting lots of people is not a good public health strategy. This wasn’t a viable option back in March 2020 – the last time we saw such a rapid invasion – and it’s not a viable option now.”
The UKHSA also cautions that the doubling rate is changing all the time and will continue to do so. The data in the days to come should clarify the situation, because if Omicron cases really are doubling every two days, we should expect a rapid increase in cases confirmed by positive PCR tests very soon.
What does this mean for acting on the surge? Given these worst-case predictions, experts advocate a cautious approach. If additional measures are brought in, these would take a week or so to prompt any fall in daily cases because of the delay between being exposed to the virus and getting tested for it. The impact on hospitalisations and deaths would take even longer to be realised.
