Almost half all counties in the U.S. are currently at the highest level of COVID-19 community transmission, according to Centers for Disease Control and Prevention (CDC) data.

The data, taken from a time period between Monday July 19 to Sunday July 25, shows 1,495 counties are at "high" levels of community transmission compared to 1,724 that are in the "substantial", "moderate" and "low" categories.

Meanwhile the CDC updated its mask guidance on Tuesday to recommend that even fully vaccinated people wear masks indoors in areas of high or substantial transmission—the majority of all counties.

Previously, the health agency had said fully vaccinated people "can resume activities that you did before the pandemic without wearing a mask or physically distancing, except where required by laws, rules, regulations, or local guidance."

A CDC seven-day community transmission map is available here, and allows people to view which counties in which state are at a certain level of transmission.

Some states, such as Florida, Arkansas and Louisiana appear to be entirely in the "high" transmission category.

Others, such as North and South Dakota and Montana, are a mixture of "low" to "high" transmission counties.

In a media briefing on Tuesday, CDC director Dr. Rochelle Walensky said she has seen "worrisome" new data showing that the COVID Delta variant "behaves uniquely differently from past strains of the virus" and that the mask guidance had been updated accordingly.

Walensky said: "Information on the Delta variant from several states and other countries indicates that in rare occasions some vaccinated people infected with the Delta variant after vaccination may be contagious and spread the virus to others.

"We continue to strongly encourage everyone to get vaccinated. Getting vaccinated continues to prevent severe illness, hospitalization and death even with Delta. It also helps to reduce the spread of the virus in our communities.

"Vaccinated individuals continue to represent a very small amount of transmission occurring around the country."

The Delta variant is thought to account for 82.2 percent of all new U.S. cases in the two weeks ending July 17th—the most recent CDC dataset on Wednesday morning.

CDC data also shows there were 53,772 new cases across the country on Monday, down from this month's peak of 67,023 on July 23. However, the seven-day moving average has continued to increase.

The COVID Delta variant was first detected in India in December 2020 and the first U.S. case was reported in March this year.

Evidence suggests the variant is highly contagious and more transmissible than other variants of COVID, meaning it spreads more easily.

The variant is also affecting unvaccinated people more, according to the UC Davis university health department.

A person wears a face mask as they walk through New York in April 2020. The CDC has updated its face mask guidance amid the spread of the Delta variant.Angela Weiss/AFP/Getty