The number of cases of the UK’s B117 coronavirus variant in the US is doubling about every 10 days, a new study estimates.
It comes after the Centers for Disease Control and Prevention (CDC) warned that the variant could become dominant in the US by March – a message reiterated by the agency’s director Dr Rochelle Walensky yesterday.
But experts are divided over what that might mean.
University of Washington computational biologist Dr Trevor Bedford says the variant – as well as others that have emerged or been imported into the US – will likely drag out the pandemic, but he’s not convinced they will lead to a massive spring wave of infections and deaths.
On the other hand, an economist and modeler at Pantheon Macroeconomics estimates that the slow pace of vaccination coupled with lax restrictions could allow new cases of the variant to spike to 400,000 a day by the end of April.
Their conflicting predictions mirror the advice of public health officials like Dr Walensky and Dr Fauci, which can sound paradoxical: The worst of the pandemic may well be behind the US, but it will take work to keep it that way.
US cases of the UK variant are doubling about every 10 days, new research estimates, with the greatest density of cases in Florida and California (dark blue)
As of Thursday, there are at least 1,367 cases of the UK variant in the US.
It has reached nearly every state – all but eight – and has taken a firm hold in two states with similar epidemic curves, but drastically different attitudes toward the pandemic: California and Florida.
The variant rose to dominance within a few months of its discovery in the UK in December.
It is thought to be about 70 percent more infectious and led to a massive surge in COVID-19 cases in the UK and, in turn, the nation’s strictest set of lockdowns, which began in early January.
The US has seen a sustained fiv-week down turn in coronavirus cases, and even daily deaths – which lag behind other metrics – have begun to decline.
US health officials are relieved that the post-holiday surge in infections, hospitalizations and deaths is finally subsiding.
But they warn that this progress is delicate, and could be reversed by the fast-spreading variants.
Public health experts have been by and large been puzzled over the what exactly is driving the downturn, but Dr Bedford isn’t.
‘After a ~2 month plateau from mid-Nov to mid-Jan, the US #COVID19 epidemic has undergone a steady week after week decline and is now back to daily case counts last seen in late October,’ he wrote in a Thursday Twitter thread.
He used daily COVID-19 case numbers in US states and compared them to the Rt – or figure that estimates the rate of transmission according to how many additional cases of coronavirus each infection leads to.
The rate of transmission has declined – though not without some bumps along the way – in every state in the US since November, driving down the cases, deaths and hospitalizations
The Rt – or rate of transmission – is now below 1 (blue) in nearly every US state, suggesting that the outbreak is ebbing, compared to November (top left) when the Rt was about one in every state and cases were about to surge
When that number is above one, the outbreak is likely growing. When it is at one, the pandemic is considered stable, and when Rt falls below one, an outbreak is likely ebbing.
The UK has used this figure as a critical metric to determine whether it needs to tighten restrictions or can begin to relax them.
Dr Bedford notes that the Rt in the US was greater than 1 in November and December – meaning that each infection led to more than one additional infection, on average.
It led to an explosion of cases, which led to surging hospitalizations and, weeks later, a soaring death toll, with more than 5,000 Americans dying of COVID-19 on the nation’s deadliest day.
‘Generally, Rt > 1 in Nov and Dec corresponding to rising cases and drops below 1 in Jan corresponding to falling cases. We’ve seen a steady decline in Rt from Nov to Feb. Thus, current decline is not a sudden shift in circumstance, but resulted from reaching Rt < 1,’ Dr Bedford wrote.
‘Solely based on continued improvements to seasonality and continued increase in population immunity due to natural infection and vaccination I’d expect this trend to largely continue and the US fall/winter surge to be brought further under control.
‘However, the rapid take-off of B117 will push against these gains.’
In the UK, Denmark and Switzerland, the variant had a growth rate of about 0.7, allowing it go from a fraction of a percent to 20 percent of all cases within two-and-a-half months in the latter two countries, and from 20 percent to dominance in the UK.
In the US, the new study – posted as a pre-print, ahead of peer review on MedRxiv.com – the researchers from Helix and the Scripps Research Institute estimated B117 to be 35 percent to 54 percent more transmissible.
That means that cases are doubling about every 10 days, and the variant likely arrived in the US as early as late November.
‘Our study shows that the U.S. is on a similar trajectory as other countries where B.1.1.7 rapidly became the dominant SARS-CoV-2 variant, requiring immediate and decisive action to minimize COVID-19 morbidity and mortality,’ they wrote.
Studies suggest that the variant is not immune to vaccines, but is more transmissible and perhaps some 30 percent more deadly.
Dr Bedford takes this seriously too, but takes a more optimistic view.
‘It’s not clear to me at this point whether biological increase in transmissibility of B.1.1.7 will “win” against further improvements to seasonality and immunity in ~6 weeks time at the end of March,’ he wrote.
He added that California and Florida might be the best bellwethers for how B117 will progress in the US, because they are ‘further along’ with 186 and 416 cases of the variant, respectively.
So far, cases, hospitalizations and deaths are all declining in both states.
But public health officials will certainly be keeping close watch in the coming weeks to see whether the fast-spreading variants change that.