Covid seems to go in four-month waves

We know that each country’s Covid reporting is flawed – some over-report, some under-report, for whatever reason. It seems highly unlikely that similar and neighbouring countries (Austria v Switzerland; Czah Rep vs Slovakia etc) would show such large differences.

But if you add up world totals, then all the under- and over-reporting should largely cancel out and the real trends are probably fairly reflected.

Worldometers.info prepare exactly such charts, it is pretty clear that:
1. The case numbers for the first 2020 wave were almost certainly under-reported, but that’s understandable
2. It has been going in four-month waves, it’s not just winter peaks (although would be interesting to see Northern/Southern Hemisphere split). That’s the interesting bit.
3. The number of deaths relative to cases is declining, but only slowly. That might simply be the effect of the majority of people in wealthier countries having had their jabs.