The U.S. Census Bureau released estimates of the total population as of July 1, 2019 at the county level. The data show that the fastest growing counties since 2010 are Cumberland (+7.6%), Lebanon (+6.1%), Lehigh (+5.6%), Centre (+5.4%), and Chester (+5.2%). Counties with the greatest percentage decreases were Cameron (-12.5%), Cambria (-9.4%), Venango (-7.9%), Susquehanna (-6.9%), and McKean (-6.5%).
By sheer numbers Philadelphia continued to add the most population this decade (+58,052). Philadelphia was followed by Montgomery (+31,075), Lancaster (+26,281), Chester (+25,856), and Lehigh (+19,643). Westmoreland County lost the most population so far this decade (-16,172). Westmoreland was followed by Cambria (-13,503), Erie (-10,856), Fayette (-7,327), and Allegheny (-7,258).
Want to take a closer look? Find the data at: https://www.census.gov/data/datasets/time-series/demo/popest/2010s-counties-total.html