Butler County and all but two of its neighboring counties have entered the “high” range of local COVID-19 transmission, the highest designation by the Centers for Disease Control and Prevention.

Mercer County, to Butler's northeast, earned the “high” designation Friday, joining Allegheny, Armstrong, Beaver and Lawrence in the CDC's most serious category of community transmission. Butler County moved to the highest designation Aug. 15.

Among neighboring counties, only Clarion and Venango remain in the second-highest category, called “substantial.”

The CDC's four categories of community transmission are low, moderate, substantial and high, from least to most serious.

A county is considered to have high COVID-19 transmission if it has 100 new cases per 100,000 residents, a 10% PCR test positivity rate, or both, within the past 10 days; substantial transmission is when a county has between 50 and 99 cases per 100,000 residents, an 8% to 9.9% positivity rate, or both, in the prior 10 days.

With 358 new cases reported between Aug. 11 and Aug. 20, Butler County has nearly double — at 190 cases per 100,000 — the incidence rate required to enter the “high” range.

In the past seven days, the county has averaged 42.3 new cases, or 22.5 per 100,000, each day.