[acqq]

> we test more people, register more mild cases.

That, and the age structure of the infected changed: the older people are doing what they can to avoid the infection, if they are able to do that.

Also, across the world, different measures are still in place, changing the dynamics of the spread, compared to the start of the pandemic when the spread was practically unconstrained. The dynamics of the spreading is also different in different settings.

There's no any scientific reason to believe that "the virus" changed in any way biologically. It's the world that does its best to adapt. Also, the schools and the universities are not opened for students at the moment in many countries, also slowing down the spreading.

Edit: answer to: "Viruses don’t mutate?"

The coronaviruses have additional mechanisms to correct the copying, slowing down the mutations, compared to e.g. flu viruses.

Edit2: answering: "The main reason for why Coronaviruses don't mutate much is because they don't have to": I wouldn't call that "the main reason", but it's a part of their success. Coronaviruses have longer genome than many other viruses, and having uncontrolled mutations in that longer genome would make them degrade too fast, so there is that molecular mechanism they have, correcting the copying errors. Additionally, they have other mechanisms to recombine their genetic material, something like "sex between (corona)viruses" where even more than two parents could be possible. But that's different from mutations and happens under different conditions. Knowing all that, and all that what est31 mentions, it is indeed true, the coronaviruses really simply don't need to "mutate fast."

Edit3: "There different strains" is not true. They are different isolates, where the completely minor differences exist, but for all it is known, until somebody proves otherwise, and nobody has, there is still just a single "strain" of SARS-CoV-2.

Edit4: Thanks to Gibbon1, yes, comparing with influenza is tricky, but maybe it's good to give readers the idea once again that this is surely not flu and that the viruses aren't the same and don't behave the same.

[Izkata]

> Edit3: "There different strains" is not true. They are different isolates, where the completely minor differences exist, but for all it is known, until somebody proves otherwise, and nobody has, there is still just a single "strain" of SARS-CoV-2.

A month or so ago, there was a research paper (admittedly an unreviewed preprint) that suggested there are different strains circulating in the US - the part I remember is that the New York City and Chicago strains did indeed act slightly but measurably differently. I think there was a third as well.

[acqq]

> a research paper (admittedly an unreviewed preprint)

As Dr. Fauci would say, it doesn't matter, even if it were a peer reviewed it can still be bad. (1)

There are a lot of bad studies, especially on the "preprint servers" that in more quiet times would not appear at all, or which nobody would take a bit serious. Now there is a lot of wishful thinking or bias involved even in their perception.

The scientific process doesn't protect anybody from some studies simply being bad, the process is there that eventually the bad ones are going to not be reconfirmed, and the really good ones are those that have many confirmations and have even the power to make new predictions, that remain true.

There are known issues with these studies claiming different "strains" too easily.

1) https://www.rev.com/blog/transcripts/dr-fauci-hydroxychloroq...

[RalfWausE]

Would it not be possible that through quarantine of infected people we "outbreed" the dangerous strains of covid19? People with symptoms going to be isolated, so their strain of the virus can not infect as much people as a hypothetical other strain which don´t produce symptoms... just an idea.

[raducu]

The problem is in as much as 50% of the cases, the same deadly virus causes no symptoms in some, so you really don't know if you are asymptomatic because of the strain or your immune system.

[andialo]

That would be my guess too.

More testing does not explain falling CFR in South Korea for one thing.

[zaroth]

Not ventilating people at the first sign of O2 dropping helps a lot.

[nmfisher]

> There's no any scientific reason to believe that "the virus" changed in any way biologically.

Viruses don’t mutate?

[est31]

Viruses have mutations all the time in random places in their genome, in fact in the course of an infection you have a "quasispecies" of viruses in your body with a diversity of different genomes, but most of those mutations don't do anything relevant. Think of someone modifying a // comment in your code or adding/removing an empty if clause somewhere. In some viruses, mutation is very relevant to fighting the disease, e.g. for HIV it's immensely large. But for the Coronavirus the mutation rate is rather low, and even though it does mutate, no variant has been discovered yet that's functionally different to already existing variants.

The main reason for why Coronaviruses don't mutate much is because they don't have to. While influenza constantly comes up with mutations so that it can come back seasonally, it seems that Coronaviruses take a different approach by evading adaptive immunity instead. E.g. feline Coronaviruses can infect cats over and over again, without large increases in ability for cats to fight it. That's also the case a bit for SARS-Cov-2, there are reports of humans getting infected a second time, but they are rather rare reports, we'd have far more of them if humans had no good adaptive immunity.

[hinoki]

They do, it’s just GP is saying that’s not the most likely thing to have caused the reduction in infection rates.

There different strains, but apparently the virulence is pretty similar [0].

[0] https://coronavirusexplained.ukri.org/en/article/cad0013/

[simiones]

They do, but there has been no sign of this one mutating in any significant way as far as I know.

[Gibbon1]

One thing to be careful about is using a influenza as reference. Influenza is a segmented virus. The viral genome is composed of 8 separate segments. In the wild new strains occur due to reassortment where the RNA sequences from two separate strains are shuffled to produce a third. That makes influenza annoying because it's regularly swapping in RNA segments from bird and swine influenza viruses. Makes developing an universal vaccine impossible and occasionally you get nasty novel strains like in 1918.

Single stranded viruses are a lot more stable.