What’s the science behind CDC’s decision to say fully vaccinated people don’t need masks?

A fresh batch of data from a big study of health care workers across the country helped prompt the US Centers for Disease Control and Prevention to say fully vaccinated people can go without masks in most circumstances, the agency said Friday.

The study found that real-life use of the Moderna and Pfizer vaccines provided 94% protection for the front-line workers immunized at the beginning of the vaccine rollout. A single dose provided 82% protection, the CDC-led team reported in the agency’s weekly report, the MMWR.
It was the findings from the new study, on top of earlier studies, that pushed CDC to decide to loosen its advice on who needs to wear a mask, and when CDC Director Dr. Rochelle Walensky said.
“This report provided the most compelling information to date that COVID-19 vaccines were performing as expected in the real world,” Walensky said in a statement Friday.
“COVID-19 vaccines are effective at preventing COVID-19 disease, especially severe illness and death,” CDC says on its new web page describing guidance for the fully vaccinated.
After weeks of telling people that even fully vaccinated people might carry a virus in their noses, mouths, or throats and breathe or spit it out onto others, the CDC says the evidence shows this is unlikely.
The reason — viral load. At least three major studies have shown that fully vaccinated people are not likely to test positive for coronavirus, which indicates they are not carrying it in their bodies, whether they have symptoms or not.
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