I think this is happening all over the world. Many, many more cases than have been confirmed. Remembering that 80% of cases are mild and unlikely to be tested for or known about.
EDIT- Here is a link to the info Annie has posted-
"Sick Time: Dozens of Diseases Wax and Wane with the Seasons, Will COVID-19?" a March 20, 2020 Science Magazine article features an interesting chart of prior Epidemics by Season."
As you say no one knows if COVID-19 is seasonal at this point. @Hip