I am not an epidemiologist but most of this stuff is out there. I welcome anyone more qualified to correct any misconceptions below.
Any virus that is aerosolized will be seasonal as its "R0" will vary throughout the year based on changing humidity and temperature as well as normal behavioral changes associated with the seasons that will put people inside unventilated areas more often. Also, the humidity/temperature in the unventilated (and ventilated for that matter) areas will also have an effect on R and that tends to change during the year. See the link below. The first sentence (shown below the link) is a statement of seasonality.
Low relative humidity can aid in the transmission of COVID-19 in three primary ways. Learn what you should do to create a safer environment at home.
www.uhhospitals.org
"As Americans head indoors for the winter, they find themselves at increased risk of contracting and spreading COVID-19 due to lower levels of humidity in the air."
As far as the factor to apply, ask
@dad4. He may have an idea. It will vary depending on the virus and it's especially difficult when a virus is first introduced - hence the reference to the "novel" corona virus. The best case is that the COVID variants aren't produced as efficiently as the flu and the combination of getting it and getting vaccinated eventually pushes it to R < 1 regardless of the "season" and variant. I believe the next best scenario is what
@Desert Hound describes - a "flu-like" seasonality where the R0 > 1 for part of the year. The worst case is well all get it and die - although, at close to a 99% survival rate, that may take a while.