Why did the CDC update its mask guidance? Some breakthrough cases may be infectious, data shows

SAN FRANCISCO (KGO) -- Some vaccinated individuals could be spreading COVID-19 if infected, that's the finding that led to the mask update by the CDC on Tuesday.

The CDC reversed its indoor mask policy citing their latest data "8 in 10 sequenced samples contain the highly transmissible Delta variant."

"In areas where substantial and high transmission CDC recommends fully vaccinated people wear a mask in public indoor settings," said Rochelle Walensky, CDC director.

RELATED: California health officials recommend wearing face mask indoors even if you're vaccinated

The CDC's recommendation applies to places with substantial or high transmission -- defined as more than 50 new cases per 100,000 people, diagnosed within seven days.

In this CDC map, you can see the southeast covered in red, that's high transmission. California is barely below that, in the orange category.

Our ABC data team noticed multiple Bay Area counties are surpassing the CDC's transmission standard for the indoor mask guideline and are in the red category.

What led the CDC to take a step back and reinstate this mask guidance is a concerning scientific finding. "Information on the Delta variant from several states and other countries indicate that in rare occasions some vaccinated people infected with the Delta variant after vaccination may be contagious and spread the virus to others," said Walensky.

RELATED: Doctors say the Bay Area 'could avoid' another lockdown if mask mandate is reinstated

At the CZ BioHub, epidemiologist Dr. Patrick Aycue and his team have been supporting the state's health department with their COVID sequencing and understands why the CDC took a step back.
"What that indicates is that what we are currently doing is not sufficient to keep case numbers down," said Patrick Ayscue, D.V.M, epidemiologist & CZ BioHub senior biosecurity fellow.

Some of the latest data indicates that as the Delta variant continues to mutate, transmissions in some cases are happening faster.

"In similar areas where are seeing very low case numbers like Australia or New Zealand where they have been able to track how people have gotten infected just people walking past each other in the hallway or more glancing contact has been sufficient for transmission in indoors," said Ayscue.

Yesterday during a press conference we asked Gov. Gavin Newsom

Luz Pena: "Is California considering to reinstate a mask mandate as the Delta Variant spreads?"

VIDEO: Pres. Biden expected to announce COVID-19 vaccine mandate for federal workers

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Late Tuesday, ABC News confirmed that President Joe Biden is likely to announce a federal employee vaccine mandate.



Gov. Newsom: "Our focus is on vaccinations, so there will be no need if we could extinguish this virus and put behind."

On Wednesday, California health officials announced a recommendation that everyone wear masks indoors in public settings, regardless of vaccination status.
According to the CDC, COVID-19 vaccines continue to be highly effective. The Mrn vaccines are 90 to 95% effective against severe disease and hospitalization, but there is still potential for a breakthrough case.

In a place of high transmission where you are in contact with lots of people the CDC says one in 10 to 20 contacts could potentially lead to a breakthrough infection.

Doctors are recommending for the vaccinated population to wear one to two masks indoors depending on the mask.

Now the concern is what's next?

"Every time another person gets infected it gives the virus an opportunity to mutate again and we don't know what it could do in the future," said Ayscue.

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