The Centers for Disease Control and Prevention on Tuesday recommended that Americans that are fully vaccinated start wearing masks indoors again in places where there are high COVID-10 transmission rates.

While nationwide, the number of COVID cases is on the rise, the recommendation from the agency is directed to certain parts of the country that are experiencing significant spikes in virus cases.

CDC Director Dr. Rochelle Walensky said at a news briefing that masks should be worn in public by everyone, even those people that have been vaccinated, in areas that have “substantial and high transmission.”

The CDC has provided a map on its website to provide a clear understanding of what counties within the U.S. are experiencing “substantial” and “high” transmission rates of COVID-19.

To determine these parts of the U.S., the CDC has broken the nation into risk levels by measuring the number of new cases per 100,000 residents and the percentage of COVID tests that are positive over the past week.

If a county has a positivity rate of 8% to 10%, it is put into the “substantial” risk level for COVID. If its positivity rate is 10% or higher, it is put into the “high” transmission risk level for the virus.

The “substantial” level is determined by 50 to 100 positive cases of COVID per 100,000 residents over a seven-day period, while the “high” level is indicated by 100 or more positive cases of the virus per 100,000 residents also over a seven-day period.

Other states are characterized as “low” or “moderate” for transmission levels of the virus, depending on their positivity rate for the last seven days.

As of Wednesday, 1,495 counties were at the “high” transmission level, and 548 counties of the U.S. were marked as “substantial” – both recommended for mask-wearing indoors, according to CDC data.

Of these counties, three states have every single one of their countries in the advisory: Florida, Louisiana, and Arkansas.

But the recommendation from the CDC has left it up to each individual state to determine if a mask mandate needs to be reintroduced. Some regions of the U.S. have already implemented mask requirements like St. Louis and Los Angeles County.

To determine the transmissibility level of the county you live in, the CDC has also provided a COVID data tracker that is updated with positivity data. Here, you can input your state and county information to determine the transmission risk level of the area where you live so you know if you need to wear a face mask as recommended by the CDC.

Cover up: Spectators wearing protective face masks at Roland Garros on Sunday where a maximum 5,000 fans were allowed on site under Covid-19 restrictions Cover up: Spectators wearing protective face masks at Roland Garros on Sunday where a maximum 5,000 fans were allowed on site under Covid-19 restrictions Photo: AFP / Anne-Christine POUJOULAT