WASHINGTON, Aug 5 (CNN) - Fully vaccinated people who get a Covid-19 breakthrough infection can transmit the virus, US Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said Thursday.

"Our vaccines are working exceptionally well," Walensky told CNN's Wolf Blitzer. "They continue to work well for Delta, with regard to severe illness and death -- they prevent it. But what they can't do anymore is prevent transmission."
That's why the CDC changed its guidance last week and is now recommending even vaccinated people wear masks indoors again, Walensky said.

Last week, the agency released a study that showed the Delta variant produced similar amounts of virus in vaccinated and unvaccinated people if they got infected -- data that suggests vaccinated people who get a breakthrough infection could have a similar tendency to spread the virus as the unvaccinated.

"If you're going home to somebody who has not been vaccinated, to somebody who can't get vaccinated, somebody who might be immunosuppressed or a little bit frail, somebody who has comorbidities that put them at high risk, I would suggest you wear a mask in public indoor settings," Walensky said.

The dangerous Delta variant has fueled the country's latest surge of Covid-19 cases and if more Americans don't get vaccinated and mask up, the country could soon be seeing "several hundred thousand cases a day," similar to the winter surge, Walensky said.

And while states across the South -- including Florida and Louisiana -- have seen exponential rises in cases, Walensky said, they have not reached their peak just yet.