With the latest hand-wringing over the March surge of COVID in some European countries, there seems to be a lot of head-scratching and finger-pointing as to why. And the usual suspects (lifting restrictions, people letting their guard down, new variants, etc.) don't really seem to hold water this time, if they ever really did. Turns out, it is most likely due to seasonality, and its interaction with the level of herd immunity in the population, and has been all along. See the following tweets below:
Apparently, a study from a year ago found that the virus has an ideal temperature range of 41-52 F, and an ideal low absolute humidity that translates to roughly 47-79% relative humidity in that temperature range. So in Europe and the northern USA, regardless of what restrictions were in place, the virus logically surged in the fall and early winter when passing through the ideal range on the way down, then dropped like a stone from mid-January to mid-February, then is either plateauing or rising again in late February and March when passing through that range on the way up. And after about three weeks or so, it will most likely plummet monotonically to a low baseline through the spring and summer. The Sunbelt of course has a different seasonality, so the virus is still dropping there even with restrictions generally being lifted faster there. Last year, their surge occurred in June and July, when they had a relatively COVID-naive population. But given the combination of herd immunity and seasonality overall, the virus should continue to plummet to a low baseline there as well. Regardless of restrictions.
Of course, there are other factors as well. In the winter, people are indoors more, where the virus spreads much more easily (especially with closed windows and thus poor ventilation), and it is also "broken immune system season" (which stretches roughly from November through April) due to low Vitamin D levels from relative lack of sunlight. But this latest nuance about the ideal temperature and humidity range for the virus explains even more still.
As for vaccines, it is notable that cases began plummeting even well before a significant chunk of the population was vaccinated, and by the time it began falling at a slower rate or even rising again, a good chunk had already received the shots. So seasonality is an even stronger factor that vaccines, apparently. And bringing together a large number of people at a time, including many vulnerable people, at mass vaccination sites is ironically a very efficient way to spike the curve in the short term before the immunity from the vaccines fully kicks in (which takes at least two weeks if not longer). Gibraltar apparently learned that lesson the hard way.
(Furthermore, the ever-insightful Bill Sardi has a theory that there may be a darker reason why many places with the highest vaccination rates are seeing spikes now, but only time will tell if that theory pans out. Or perhaps it briefly increases susceptibility before it fully kicks in. Either way, we should know one way or the other by April or May at the latest.)
So far, as of March 20th, the USA has largely dodged the European surge in March, with only Michigan and New Jersey seeing significant (but modest) recent increases. But many other northern states above the 37th parallel, where it is still chilly and in season for the virus, progress has temporarily stalled. But make no mistake, this stalling and/or reversal of progress is temporary, fairly brief, basically inevitable, and NOT related to the lifting of restrictions too soon or quickly, lack of masks or mask mandates, new variants, Spring Break, or vaccine hesitancy. None of these "usual suspects" jibe well with the statistics.
UPDATE: When looking on Worldometer at the data for three countries/territories that were first out the gate for mass vaccination and had the largest vaccination rates in January--UK, Israel, and especially Gibraltar--we can clearly see a massive super spike in COVID cases (and then deaths shortly after) for a few weeks, followed by a sharp and massive drop to a low baseline upon finally achieving herd immunity (by both infection and vaccination in a two-for-one special, ironically). And no March surge there either. So it looks like these post-vaccination surges are fortunately brief (albeit tragically very deadly, especially in Gibraltar), and may partially explain the March spikes in later-vaccinating and slower-vaccinating countries, including some out-of-season tropical countries like Brazil, Peru, and even India as well. Meanwhile, some of the least-vaccinated and slowest-vaccinated areas (such as Texas) and populations (such as Palestinians) have been interestingly seeing better trends than their highly-vaccinated counterparts. Makes you wonder.
(Correction: Palestinians were seeing better trends than Israeli Jews from December through February, but by March the reverse clearly became true.)
But seasonality seems to predominate overall right now in the non-tropics. And we see that in countries like Italy and Germany, the March mini-waves are already slowing down and curling over before their new lockdowns would have had any effect--kinda like their first waves last spring did too. Which we have also repeatedly seen elsewhere as well, of course.
(Exactly how much of such spontaneous curling over and decline of these epidemic waves is due to seasonality, herd immunity to one degree or another, attrition of the biggest superspreaders from the population early on, voluntary behavior changes in response to rising case numbers, reversion to the mean, or some combination of the above is not entirely clear, but any correlation with lockdowns and related restrictions is clearly spurious.)
Another possible explanation for the apparent seasonality of this and other respiratory viruses is the hypothesis that it has more to do with changes in temperature than absolute temperature per se, and that some strains are more temperature-sensitive than others. This also explains a lot as well.
Also, it seems that HCoV-NL63, one of the four endemic common cold coronaviruses, is currently surging in the Midwest. Perhaps that little pest is being picked up as false positives for COVID-19 by the notoriously inaccurate PCR tests in some places?
And speaking of the Midwest, once again, we see that lifting vs. keeping restrictions (of any kind) seems to be at best quite irrelevant to the COVID-19 virus:
So, remind me again why our "leaders" decided a year ago in March 2020 to throw the hard-won wisdom of the ages out the window like so much garbage, in favor of emulating a brutal totalitarian dictatorship halfway around the world? And then when their voodoo clearly didn't work, they not only persisted in their defection in the face of hard counter-evidence, but actually doubled and tripled down on it?
So call it whatever you want, just don't call it science.