Right, but the point is: Australia etc. are in winter. Right now. We have that data; there's more to the world than just the top half.
(Incidentally, COVID precautions appear to be working well on flu, too. https://www.wsj.com/articles/covid-19-measures-have-all-but-...)
Sydney's temperature tomorrow is a high of 17, low of 10.
Glasgow's temperature tomorrow is a high of 19, low of 10.
It's plenty chilly in southern Chile.
NZ - Christchurch 12C, Wellington 15C, Aukland 16C.
South Africa, Cape Town high of 19, Joburg high of 18, similar to Glasgow in Summer.
South America? Yes it's chilly in Southern Chilie. There's barely anybody living there. Same in Argentina - Ushuaia has a population of 130k and a high of 6C today. Arkangel is 3 times the size and has an average high of -9C in winter. Fairbanks population about 100k and high of -17 in the winter.
The population south of Viedma, Neuquén and Valdivia (highs today of around 13) is well under 3 million, with towns mostly less than 100k.
Santiago's high today is 33. BA has a high of 15. That's not the same as a northern winter.
Compare with Beijing, population 20 million, average highs of 2C in winter. Chicago, 1C, 9 million. Moscow -4C, 12 million.
Comparing a Southern winter with a Northern winter is disingenuous - it's milder, there's far fewer people, and far smaller cities with far lower density.
Instead, the replication of rhinovirus depends on if the air temperature in the nasal cavity is above or below 33-35C[1]. The winter in most temperate regions gets low enough to trigger that.
[1] https://www.pnas.org/content/early/2015/01/02/1411030112
The key point is this: Australia's winters get cool enough to have a distinct cold/flu season every year.
The reason the UK locked down was to "protect the NHS". The lockdown worked - dramatically, spread was cut to almost zero overnight, the extra hospitals that were built weren't needed.