I think the 'deaths' graphs are the most important, here are the latest Feb/March 2022 graphs which are interesting because they show how the figures go up and down like a roller coaster.

Why are there such widely varying peaks and troughs, is it because of seasons of the year or what?

https://www.worldometers.info/coronavirus/country/us/

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Edited March 18, 20223 yr by Dropship

Deaths are related to number of infections, who got infected, available treatment options, availability of health care, vaccination rates, public health orders, compliance to health orders, viral variants etc.

Generally speaking case numbers declined with firm health orders in place and with delays also the deaths. During summer time health orders were lifted and by fall cases increased sufficiently to create a new wave (also driven by new variants). In the US health orders were more spotty in the UK so the decline during summer was much less uniform (especially 2020). 

Looking at more worldwide data, the seasonality of COVID-19 is a bit overblown. While folks are more outdoors which could reduce infections a bit, in many areas there is only a short time frame (1-3 months in the summer) where SARS-CoV-2 infections were really low. And in most cases there were health orders in place before that. Without, the seasonality effect is even weaker.