reporting, according to this article:
Oscillations in U.S. COVID-19 Incidence and Mortality Data Reflect Diagnostic and Reporting Factors
https://msystems.asm.org/content/5/4/e00544-20
"By analyzing New York City (NYC) and Los Angeles (LA) testing data, we demonstrate that this oscillation in the number of cases can be strongly explained by the daily variation in testing."
but disputed by some who cite the global trend of these oscillations:
Globally Coherent Weekly Periodicity in the Covid-19 Pandemic
https://www.medrxiv.org/content/10.1101/2020.06.02.20120238v2.full.pdf