[set fair (OP)]

In the last 5 weeks the number of daily cases worldwide has halved. Something must be going on, anybody heard an explanation? Could the virus be inherrantly unstable?

[Bored chemist]

People are probably trying quite hard to avoid catching it, and are learning how to do so.

[alancalverd]

The Christmas and New Year rush to get infected has finished, travel bans are beginning to bite, and Satan has vacated the White House.

[Bored chemist]

To some extent, we must be running out of unexposed people.

[chris]

Countries are in lock-downs, supressing spread.

[alancalverd]

The oddity seems to be India where apparently the infection rate is anomalously low - the the extent that Test cricket is now permitted in half-filled grounds.

[set fair (OP)]

Not as marked as India but the fall in the US is pretty dramatic - in terms of new cases per unit of population, it is very similar to the UK with (much?) less stringent measures.

[evan_au]

the fall in the US is pretty dramatic

Vaccination in the US can only account for at best 50 million people out of a population of 330 million- but in reality, much less than that, since it takes a few weeks to build a good immune response from the first dose of vaccine, and a few weeks to build an improved response to the second dose, 3 weeks later.

But maybe people see the vaccine as being so close, it would be a pity to catch COVID now, so they are being more careful?

I would put part of the fall down to Twitter (finally) implementing their house rules about distributing false and misleading information, and tweets that endanger public safety (ie banning Trump).
- That allowed the softer voices of infectious disease specialists like Dr Fauci, and the mask-wearing president-elect to be heard.

In the last 5 weeks the number of daily cases worldwide has halved

The US was responsible for a fair fraction of the diagnosed cases worldwide (many poorer countries did not have the capacity to test symptomatic or even dying patients).

So a fall in cases in the the US represents a fall worldwide.
- But perhaps the spectacular fall of a president caused people to question some of the free health advice he had been dispensing?

[evan_au]

I heard that a disease modelling expert predicted this fall last November, as he considers the disease to have a seasonal component (like other coronaviruses).
- But he is concerned that the variants now circulating in South Africa could lead to a third wave, at the end of the Northern summer.

[set fair (OP)]

It throws up a lot of surprises, behaving differently in different countries. I heard some US body thought a US variant woud do what the UK variant did here. I'm not even guessing.

[charles1948]

I heard that a disease modelling expert predicted this fall last November, as he considers the disease to have a seasonal component (like other coronaviruses).
- But he is concerned that the variants now circulating in South Africa could lead to a third wave, at the end of the Northern summer.

I think that the virus does its own thing, comes and goes in waves.  The measures that we try to take, such as lockdowns and vaccinations, probably don't make much difference. 

We like to think they do, obviously,  to keep up morale.  It makes us feel that we're doing something.  But given the nature of viruses,  I doubt that our efforts have more than a marginal effect on how the pandemic progresses

[Kryptid]

Vaccination absolutely does make a difference. Ask Israel.

[Bored chemist]

I think that the virus does its own thing, comes and goes in waves.  The measures that we try to take, such as lockdowns and vaccinations, probably don't make much difference. 

You really think that?
You don't know about New Zealand (and a few other places)?

Wow!
I can't imagine how you could pay so little attention to what is currently one of the most important things happening in the world.

Vaccination in the US can only account for at best 50 million people out of a population of 330 million

Yes and no. Give it a while and it's 50 million, plus the people that those 50 million might have infected plus the ones they would have infected and so on.

We like to think they do, obviously,  to keep up morale. 

Smallpox was not eliminated by wishful thinking.

Why do you ignore obvious facts?
It makes you look like a fool or a troll- and neither option makes you look clever.

[alancalverd]

Vaccination absolutely does make a difference. Ask Israel.

Or anyone who doesn't remember smallpox or polio. Or who hasn't had tuberculosis, measles, mumps, rubella, tetanus, cholera, typhoid or yellow fever despite being exposed to them.

[max_thunder]

The photoperiod is known to influence immune functions, such as innate immunity. My own guess is that it explains why cases peaked around a week or two after the winter solstice, when innate defenses would start getting stronger and prevent infections, after doing the opposite in December and accelerating transmission. I also think that because innate defenses vary significantly in the population, it makes the threshold for herd immunity much lower, as the number of highly susceptible people go down with time; current models assume everyone is as likely to catch it and transmit it if they haven't caught it before or been vaccinated. This would explain why some US states had case counts peak even well before early January despite much less than 60%+ of their population having been infected.

In India, it seems cases have peaked in September just after monsoon season. Maybe by keeping people inside and by being rainy all the time, it lowers people's defenses. The flu is known to also peak at this time of the year. Countries with rainy seasons seem to have very different flu seasons from ours in North America. Australia had a relatively low case count peak a couple weeks after its winter solstice, when it was our summer in the northern hemisphere.

The main one I don't understand is South Africa and why their cases have peaked right in the middle of summer. They did peak during their normal winter, like in Australia, before this recent summer peak. I really hope their variant does not have some unique capability to spread faster in summer. The nearby countries have less precise data but seem to experience the same general pattern. The pattern is really strange because the first wave aligns with countries like Australia, just like when the flu would peak, but the second wave of South Africa aligns with the northern hemisphere and peaked in early January before going down fast. I would be curious to know what pattern coronaviruses typically follow in South Africa; in America, coronaviruses cause most fall colds and the coronavirus season mostly aligns with the flu season.

[alancalverd]

cases peaked around a week or two after the winter solstice,

Call it Christmas and blame herd stupidity

some US states had case counts peak even well before early January

or Thanksgiving, ditto

Australia had a relatively low case count peak a couple weeks after its winter solstice

not a public holiday, and sensible national quarantine

South Africa and why their cases have peaked right in the middle of summer.

Christmas + summer holidays = stupidity squared!

[max_thunder]

cases peaked around a week or two after the winter solstice,

Call it Christmas and blame herd stupidity

some US states had case counts peak even well before early January

or Thanksgiving, ditto

Australia had a relatively low case count peak a couple weeks after its winter solstice

not a public holiday, and sensible national quarantine

South Africa and why their cases have peaked right in the middle of summer.

Christmas + summer holidays = stupidity squared!

This is ridiculous unscientific, singular events can't cause the Rt to hit a certain level throughout a whole month and suddenly reverse it the next month. The events would cause a singular spike in Rt and it is not what happened. Are you suggesting that Australia's winter peak, and typical flu season in the southern hemisphere, is caused by people in the southern hemisphere celebrating their winter solstice on June 21? Or that the flu has been peaking for hundreds of years at around the same time just because of Christmas and people shopping in malls for gifts during the whole month of December?

[alancalverd]

Rt to hit a certain level throughout a whole month and suddenly reverse it the next month.

Oh yes you can. That's the whole point of quarantine where the principal vector is other humans. It has been demonstrated time and again for at least the last 5000 years and forms the basis of infection control wherever it is practicable.

[max_thunder]

Rt to hit a certain level throughout a whole month and suddenly reverse it the next month.

Oh yes you can. That's the whole point of quarantine where the principal vector is other humans. It has been demonstrated time and again for at least the last 5000 years and forms the basis of infection control wherever it is practicable.

You realize that many places in the world have reversed a significant part of their restrictions, including reopening schools months ago, and cases are still going down? You're suggesting that leaders all across the world, but almost only in the northern hemisphere, called each other to ensure their measures would be synced, even though they all used different measures and some places didn't add any and still saw cases drop similarly.

Your suggestion that "quarantines" would explain why the flu and coronaviruses tend to peak around December/January in the northern hemisphere is very dubious.

By the way, what we are doing right now has never been done before and is very different from the concept of quarantining new arrivals. Quarantining started in the 14th century with the plague, not 5000 years ago. SARS-cov-2 virus is already spreading in almost every single country, locking new arrivals is not going to stop it.

I think you're using quarantine liberally to mislead on purpose; I've heard people like you refer to social distancing/NPIs in that way, I'm not sure why you would do that. You seem to have no interest in having a scientific discussion; I'm done talking to you.

[evan_au]

The photoperiod is known to influence immune functions

And the hours of daylight (related to photoperiod) also correlates with temperature. Increasing hours of daylight leads to rising  temperatures, and people are more inclined to go outside.
- Rise in temperature has a lag after the December 22 solstice, but usually more than 2 weeks.

Quarantining started in the 14th century with the plague, not 5000 years ago.

It is true that the Italian word "Quarantine" was coined in Venice during the plague. It represented the 40 days a ship had to anchor in the harbour before they could have contact with other people.

But there are older references, for example in the Jewish Torah/Christian Old Testament:

“Anyone with such a defiling disease must wear torn clothes, let their hair be unkempt, cover the lower part of their face and cry out, ‘Unclean! Unclean!’ As long as they have the disease they remain unclean. They must live alone; they must live outside the camp.

And quarantine is partly innate: if you have a viral infection, the natural interferon response makes you lethargic (less likely to go out and mix with other people) and produces a runny nose and other symptoms that people associate with a cold (which makes other people keep their distance).
- A recent paper showed that these effects happen in bats too.

See: https://en.wikipedia.org/wiki/Quarantine#History