CDC recommends masks for vaccinated people in high transmission areas. What does that mean?

The Centers for Disease Control and Prevention recommends that even vaccinated Americans resume wearing masks indoors if they are in areas with high or substantial transmissibility of the COVID-19 virus.

The guidance also recommends for people with underlying conditions wear masks, along with anyone living with vulnerable people. Teachers, school staff, students and visitors inside schools from kindergarten to 12th grade also fall under new guidance recommending universal mask-wearing.

For vaccinated people in just under half of the counties in the country – those with high transmissibility – that means masks are recommended indoors once again.

Several counties with major U.S. urban centers, including Los Angeles, St. Louis and Miami-Dade, are among the high transmission counties.

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Here's what transmissibility is and how to find out if you're in an area where you should start masking indoors again:

Map of high COVID-19 transmission areas

The CDC provides a COVID-19 data tracker that includes a by-county view of transmissibility rates each week. Check out the map below. You can also visit the CDC's website to enter your state, county or metro area to find out what the transmissibility is like where you live.

What is high transmissibility?

Community transmission is calculated using two metrics: how many new cases per 100,000 persons have occurred in the past seven days, and the percentage of positive diagnostic and screening nucleic acid amplification tests (NAATs) in the past seven days. Counties are categorized as having low, moderate, substantial or high transmissibility.

Transmissibility is considered high when there are 100 or more new cases per 100,000 in the past seven days and 10% or higher positive NAATs in that period. A high transmission designation can vary, with places like Salt Lake County in Utah at about 120 cases per 100,000 to counties like Pulaski County in Arkansas, which has more than 400 cases per 100,000. Substantial transmission occurs when there are between 50 and 99.9 new cases, and 8% to 9.99% positive tests.

The delta variant, which makes up more than 80% of new infections in the U.S., is more than two times as transmissible than the original strains of the coronavirus, and experts believe it could cause vaccinated people to be contagious.

"This new science is worrisome and unfortunately warrants an update to our recommendations," CDC Director Dr. Rochelle Walensky said in a briefing Tuesday.

Where is COVID-19 spreading at a high rate?

Just under half of all U.S. counties, 49.94%, are identified as having high COVID-19 transmission, according to the CDC, an increase of more than 18% from seven days ago.

Counties with the likelihood of substantial transmission make up 16.68% of the U.S.; 24.75% of counties are considered at a moderate level; and 8.6% are at a low level.

In at least three states, Florida, Louisiana and Arkansas, everyone should wear a mask in indoor public spaces, according to the latest CDC guidance, because each county in the states is a high or substantial transmission community.

In Louisiana, every county is considered high transmission. Louisiana also saw its greatest single-day increase in COVID hospitalizations since the beginning of the pandemic this week. On Tuesday, 1,390 people in the state were hospitalized, an increase of 169 from the day before, the Louisiana Department of Health said.

"COVID is surging in Louisiana and it is not slowing down," Dr. Joseph Kanter, state health officer, said. "Mask while indoors and get tested if you suspect you’ve been exposed to COVID-19. These are public health emergency measures that will limit death and suffering during this fourth surge."

States with a large concentration of high transmission counties are mostly in the South and some Western states like Utah, according to the CDC's COVID-19 tracker.

In several states, including Florida, Arkansas, Mississippi, Alabama and Missouri, nearly every county is at a high level on the map. In Florida and Arkansas, all but one county is at a high level.

Florida's largest counties by population also have some of the highest levels of transmission, with Miami-Dade County at more than 408 cases per 100,000 people and Broward County at 310 cases per 100,000.

Arkansas' largest county by population, Pulaski County, has 411.32 cases of COVID-19 per 100,000 people in seven days.

Los Angeles, by comparison, is also listed in the high transmission category and has 178.41 cases per 100,000 people over seven days. Salt Lake County in Utah has 119.18 cases per 100,000.

Transmissibility is lower in areas of the Northeast and some parts of the Midwest. Nearly all the counties in New York, Pennsylvania and Minnesota, for example, are in the moderate or low categories.

Masks are coming back: What that means for vaccinated Americans.

This article originally appeared on USA TODAY: CDC mask map of high transmission areas: What does it mean?