Except... that isn't what actually happened.
Keep in mind that reports of new cases are delayed by 2-3 weeks, in part because it takes about a week for symptoms to appear, and then tack on another 7-10 days because of delays in testing.
There was a surge in reported cases in last spring/early summer last year (which seemed big at the time, but in retrospect looks small). It started to recede in early August, because the hardest-hit states started social distancing. There was then a small bump as the virus spread to new states, mostly the Midwest, in no small part because those states were taking fewer precautions.
Then, in the winter, the number of cases exploded. This was partly because people were congregating indoors, as well as ignoring social distancing to do things such as visit family and friends during the three major holidays (Thanksgiving, Xmas, New Year's). Cases then started to fall, in part because of vaccination, and because people went back to social distancing.
Note how cases started to fall in early January -- long before most Americans could socialize outdoors.
And again... Those reports are all at least 3 weeks late. Meaning the initial surge was in early spring (not summer), and started to recede in late July (when temps really start to rise). The big winter surge also really started receding after Xmas.
Also note how the US, which is around 50% fully vaccinated,
isn't seeing a big surge yet this summer -- and that's despite an early heat wave hitting numerous states.
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We see the same pattern in states that have intense heat during summer (driving people indoors) and mild winters (where weather is less likely to drive people indoors).
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Whatever "seasonal effect" there is of heat driving people indoors, it is clearly overwhelmed by the ways summer weather reduces the virus -- e.g. more people able to socialize outdoors, and the virus getting killed fairly quickly by the UV in sunlight.
Now, we
can see from the above graphs that the lower vaccination rates have not resulted in a major surge in the US. So far, the bumps are small -- which isn't too surprising, as even the slowest states have fully vaccinated at least 30% of their population.
However, those bumps
aren't happening in the hottest states. They're happening in Alaska, Missouri, and Oklahoma.