COVID-19 activity increased in small counties following political rallies in August and September, according to a preprint study by Harvard University and Israeli statisticians.

The median rate of infections per 100,000 people per day increased from 8.6 to 12.6 in the two weeks following rallies in 20 counties, including Minnesota’s Beltrami, Blue Earth and St. Louis counties after rallies by President Donald Trump.

By comparison, the U.S. rate increased from 12.9 to 13.1 in the two weeks following Sept. 15.

The study comes loaded with limitations, including that it was posted without the peer review vetting of scientific journals and reveals only a correlation rather than cause and effect. Lead author Oren Miron acknowledged that infections already were increasing in St. Louis County at the time of a Sept. 30 Trump rally in Duluth. However, infections rose in Blue Earth County following the Aug. 17 Trump rally — when case growth wasn’t as rapid.

Infections tied to rally attendees have been small. The Minnesota Department of Health has reported 16 infections among attendees of the Sept. 18 Trump rally in Bemidji and four cases among counterprotesters. Four infections were reported among attendees at the Sept. 30 Trump rally in Duluth, but two involved people who lived together.