Pandemic model predicts big national COVID surge

Delta variant will target unvaccinated through December

BOSTON — By October, the U.S. could be mired in yet another Covid surge, this one especially concentrated in areas of the country where vaccination rates are low. And a new model projects upwards of 400,000 cases a day are possible.

“There are some big surges coming back,” said Katriona Shea, PhD, a biology professor at Penn State University and a researcher with the Covid19scenariomodelinghub -- which has been regularly assembling forecasts through the pandemic.

The difference between the coming surges and the others, Shea said, is that they won’t happen everywhere -- because of varying vaccination rates.

The scenariomodelinghub is an ensemble forecast that incorporates the work of several individual forecasters to come up with a single projection. In this case, nine forecasts were used to determine the course of the pandemic through Jan. 1, 2022.

Shea said while the forecasters didn’t agree on everything, they were in agreement on two points: the next six months will be especially perilous for the unvaccinated, and it will be the Delta variant responsible for the coming surge.

“When the peak might be is sometime during the fall,” said Shea. “There are some models that think it might get even higher and not come until the spring, so 2022. The problem is we don’t know so it could just keep going up. But in some places even the ensemble thinks it’s going to be as bad as last winter.”

The hub assesses the next six months using four scenarios -- A, B, C and D. Two assume a high vaccination rate of 80% and two assume a low vaccination rate of 70%.

Each of these is paired with a theoretical Delta variant transmission rate of either 40% higher than the Covid seen last year (known now as Alpha) or 60% higher. Shea said it is now known that Delta transmits at the higher number.

“Recent cases have been increasing faster than we projected in that worst case scenario because that was based on data back on July 4th,” said Justin Lessler, PhD, a professor in the epidemiology department at the University of North Carolina, who also works as a researcher with the modeling hub. “The way the indicators are going now we are firmly in that scenario D, which is the worst case scenario.”

Lessler said it is now believed the vaccination saturation point in the U.S. will probably top off at 70%. That, coupled with the high transmissibility of the Delta variant, will trigger the coming surge.

Something else will likely help move it along, as well -- pandemic fatigue.

“It seems like people have really relaxed,” Lessler said. “They’re very much thinking I’m vaccinated or a lot of people are vaccinated and the cases are low and I’m going to go back to a lot of my pre-pandemic behavior.”

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