Fully vaccinated individuals should now wear masks in certain indoor areas in places with substantial and high coronavirus spread given the highly contagious delta variant, according to the Centers for Disease Control and Prevention (CDC), but which areas are spiking and who is most at risk?

The CDC defines areas of substantial viral transmission as 50 to 100 cases per 100,000 population over a seven-day period, and high viral spread is defined as over 100 cases per 100,000 over a seven-day period. Some areas are experiencing over 300 cases per 100,000 population at the moment, CDC Director Dr. Rochelle Walensky told reporters over a call Tuesday.

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The agency rolled out new mask guidance on Tuesday amid "worrisome" new science "showing that the delta variant behaves uniquely differently from past strains of the virus that cause COVID-19," Walensky said. "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."

The highly transmissible variant is the predominant strain circulating in the U.S., with the variant detected in eight in 10 sequenced samples, Walensky noted.

WHICH MASKS PROTECT AGAINST DELTA COVID-19 VARIANT?

A county-level breakdown of community COVID-19 spread from July 19 to July 25. Per the CDC’s new guidance, in areas with substantial and high transmission, the agency recommends that fully vaccinated individuals wear a mask in public indoor settings to help prevent spread of delta and protect others.

A county-level breakdown of community COVID-19 spread from July 19 to July 25. Per the CDC’s new guidance, in areas with substantial and high transmission, the agency recommends that fully vaccinated individuals wear a mask in public indoor settings to help prevent spread of delta and protect others. (CDC)

As of July 27, more than 46% of U.S. counties were grappling with high community transmission, with 17% of counties experiencing substantial spread, according to federal data, which suggests more counties are seeing an uptick in viral spread, with a 17% increase in areas classified with high viral spread compared to seven days prior. The CDC offers a map where users can toggle for level of county-level transmission across the country. To access the tool, click here.

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A state-level breakdown of community COVID-19 spread in the seven days leading up to July 27. States marked in red are grappling with high levels of community coronavirus spread; orange shading signifies substantial spread and yellow shading indicates moderate spread.

A state-level breakdown of community COVID-19 spread in the seven days leading up to July 27. States marked in red are grappling with high levels of community coronavirus spread; orange shading signifies substantial spread and yellow shading indicates moderate spread. (CDC)

As of July 27, 21 states, territories and jurisdictions were experiencing high viral spread, including Nevada, Utah, Arizona, Wyoming, Nebraska, Kansas, Oklahoma, Texas, Louisiana, Arkansas, Mississippi, Alabama, Georgia, Florida, South Carolina, North Carolina, Tennessee and Kentucky, Missouri and Alaska.

Federal data indicates 60% of U.S. adults are fully vaccinated against COVID-19, with almost 70% receiving at least one dose.