How does Omicron spread so fast? A high viral load isn't the answer - Nature.com

The results suggest that Omicron’s hyper-transmissibility does not stem from the release of large amounts of virus from infected people.

To compare the viral loads linked to the ever-changing cast of SARS-CoV-2 variants, Yonatan Grad, an infectious-disease specialist at the Harvard T.

The researchers studied PCR-test results of nose and throat swabs collected from infected individuals and found that those who had Delta had a slightly higher peak viral load than did those with Omicron.

“I was really not expecting to see that,” says Grad.

After all, in only two months, Omicron replaced Delta as the dominant cause of US COVID-19 cases.

“Naturally, you’d think that higher transmissibility must cause a higher viral load,” he says.

Meyer and his colleagues took the study a step farther: rather than measuring only viral RNA, they also measured the number of infectious virus particles on swabs collected from a separate group of almost 150 infected people2.

This more stringent method found no significant difference between the viral loads of vaccinated individuals infected with Omicron and those infected with Delta.

Meyer’s team examined samples from people who had been vaccinated but nonetheless became infected with Delta.

They found that about half of the samples still held infectious virus five days after the individuals tested positive.

Grad and his colleagues found that five days after an initial positive test for Omicron, about half of tested individuals had viral loads high enough that they were probably still infectious.

Such results are concerning, Grad says, because guidelines published by the US Centers for Disease Control and Prevention (CDC) allow people infected with the virus to end their isolation five days after either testing positive or experiencing their first symptomsA

Omicron has made public-health decisions all the more difficult, Grad says.

He and his colleagues also found more variability in viral load in individuals infected with Omicron than in people infected with Delta.