The US Centers for Disease Control and Prevention (CDC) announced this Tuesday (28) that the omicron variant already accounted for 58.6% of Covid-19 cases in the country in the week ending Saturday (25).
The agency’s announcement comes after a review of data released in relation to the new strain, from 73% to 22% of infections linked to omicron in the week that ended on the 18th. According to the CDC, additional data, added to the rapid spread of the variant, are among the reasons for the discrepancy.
“We received more data from that period, and there was a reduction in the proportion of omicron,” explained the organ through a spokesperson. “It’s important to note that we continue to see a continued increase in the proportion of omicrons.”
Experts consulted by the American newspaper The New York Times said they were not surprised by the revisions, as the CDC’s estimates are approximate guesses, with a wide possibility of values, known as “confidence intervals”.
Even so, they highlighted that the work of communicating about the uncertainty of the estimates was not well done. “The 73% received much more attention than confidence intervals, and I think this is one example among many in which scientists try to project an air of confidence about what’s going to happen,” virologist David O’Connor told the newspaper at the University of Wisconsin-Madison.
Nathan Grubaugh, an epidemiologist at the Yale School of Public Health, also pointed out that one must be aware that the numbers are estimates, not confirmed sequencing. “With omicron specifically, it has been very difficult to do any kind of projection because everything is changing so quickly.”
The founder of the pollster site FiveThirtyEight, Nate Silver, was uncritical when talking about the review. On his social network, he wrote that “we must assume that the CDC method is crap and should be ignored from now on”.
Former drug regulatory agency commissioner Scott Gottlieb reasoned that if the new estimate is correct, it suggests that a good portion of hospitalizations could still be caused by the delta.
Before the arrival of the omicron, the delta was responsible for almost all the sequencing (99% in the week that ended on November 27th). Today, infections are divided between omicron, which already has the largest share with its 58.6%, and delta, which is at 41.1%.
The difference in estimates between the two variants could still impact the chosen treatment, O’Connor told the New York Times. One of the great challenges of Omicron is the ability of the variant to prevent two of the three treatments with monoclonal antibodies, which can prevent serious illness in patients with Covid-19.
Hospital managers in New York, for example, said they would no longer use the two treatments that proved ineffective against omicrons. “If there are still cases of delta, discontinue [esses tratamentos] it means that all those people who could benefit from them will no longer be getting paid,” explained O’Connor.
The omicron, which is spreading with unprecedented speed according to the WHO, was first sequenced in South Africa and had its first confirmed case in the US on December 1, in a passenger —completely vaccinated— returning from the African country.
Since then, the strain has spread rapidly not just across the US, but across multiple regions, leading to spikes in infections, flight cancellations and holiday fears.
Americans have been facing an average of more than 237,000 cases a day, heading towards the peak of the pandemic, recorded in January this year, when the rate reached nearly 252,000 cases, according to data from Our World in Data.
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