Things have greatly changed in a week.

Data from the Centers for Disease Control and Prevention reported a week ago had just nine counties in Pennsylvania with substantial or high transmissions of COVID-19, the marker that the CDC is using to recommend that people start masking again indoors.

As of Monday afternoon, 42 counties were showing at least a substantial rate of COVID-19 transmission, with five of those counties at a high rate. This includes Perry County, who just a week ago had a low rate of transmission.

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Over the weekend, high rates of transmission were reported throughout most of central Pennsylvania, but those levels dropped back to substantial on Monday for all but Perry County.

The CDC determines the transmission rate by studying new cases per 100,000 people and the positivity rate of new tests, both over a rolling seven-day period. This means transmission rates can vary from day to day, with counties showing substantial rates of transmission one day, high rates the next day, then dropping back to substantial on the day following that.

What has remained consistent over the past week is the growing number of counties that are seeing substantial transmissions of COVID and are not dropping back down to moderate. Most of Central Pa. showed this uptick last Tuesday, with Cumberland and York counties joining them on Wednesday.

This matches the trend in case data being reported by the Pennsylvania Department of Health, as hospitalizations and case numbers increased once again over the weekend. On July 4, the rolling 7-day average of new cases in the state was at 163. Now, it’s close to 1,500.

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On the CDC tracker, counties with substantial transmission have between 50-99 new cases per week, those with high transmission have more than 100.

As for positivity rates, counties with substantial transmission have between an 8% to 9.99% positivity rate, while those with high transmission have a positivity rate higher than 10%.

If case rate or positivity rate fall in two different categories of transmission, the higher one is taken.

A growing number of businesses are once again mandating masks, as well as pushing for employees to get vaccinated. Amazon workers began masking at its warehouses again on Monday. All military troops are required to get vaccinated by September 15. There is currently no plan for a statewide mask mandate in Pennsylvania schools.

RELATED: These major retail stores and businesses have revised their policies on masks as COVID-19 cases rise

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