Originally Published: 11 AUG 21 10:37 ET

By Priya Krishnakumar, CNN

(CNN) -- More than 98% of US residents now live in an area where there is a "high" or "substantial" risk of Covid-19 community transmission, up from 19% of residents only a month ago. This sharp turn has been driven in large part by the highly infectious Delta variant and low vaccination rates in many regions.

The Centers for Disease Control and Prevention (CDC) measures community transmission using two metrics -- the number of cases per 100,000 people and the test positivity rate, both measured over the previous seven days.

The latest figures from the CDC show an alarming rise in the number of counties that are considered at high risk of community transmission. On Monday, 2,361 counties in the United States were listed in the "high" tier, a stark increase from 457 counties in the same tier at the beginning of July. A look at the past five weeks shows just how rapidly community transmission has increased across the country.

An area with a low transmission rate has 10 or fewer cases per 100,000 people and a positivity rate below 5%, while an area with a high transmission rate has 100 or more cases per 100,000 people and a positivity rate of 10% or higher.

The increase in community transmission has led to renewed mask mandates and an urgent push for vaccinations across the country, especially in areas where vaccination rates are low. The CDC now recommends that people wear masks indoors if they are in areas with "substantial" or "high" levels of spread, regardless of whether or not they are vaccinated. You can look up your county's transmission level on the CDC's website, which updates multiple times per week.

Vaccines are the most effective tool in combating the spread of the new Delta variant, according to available data, with studies indicating that all three commonly available vaccines are able to prevent severe illness or death from Covid-19.

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