CNN conducted an investigation and found that out of 17 Trump campaign rallies, 14 of the host counties — 82% of them — had an increased rate of new Covid-19 cases one month after the rally.
The 17 rallies occurred between August 17 and September 26. CNN evaluated the rate of new daily cases per 100,000 residents at four weeks before the rally, on the rally date, and four weeks after the rally at the county level and at the state level.
Of the 14 host counties that had increased infection rates, eight of the counties had declining rates of infection in the month before the rally. The other six counties already had increasing rates of infection in that preceding month.