Covid19: How to explain the decline of the virus?

“The Covid 19 virus no longer exists clinically in Italy.” This bold statement was made on Italian television on Sunday 31 May by Dr Alberto Zangrillo, director of the San Raffaele Hospital in Milan. According to him, “the samples taken over the last ten days show an absolutely infinitesimal viral load compared to those taken a month or two ago”. In other words, recently infected patients would have much less virus in their airways than patients at the beginning of the epidemic.

How can we explain the decline of the virus, not only in Italy but in many other countries around the world?
As for me, the best explanation has something to do with John Sanford’s genetic entropy. And you, what do you think?

To my knowledge this is commonly observed in viral epidemics. The most harmful strains get more attention and care not to pass the infection (or possibly kill their hosts before infection can be passed) leaving the milder strains to carry on. This is natural selection acting on the virus.

The virus is definitely still exists, but if it isn’t making people sick they are unlikely to go to a clinic to get tested.


How about social distancing inadvertently selecting for reduced symptoms, which indirectly selects for reduced viral load?

It’s been some time since I read Sanford, but if he is correct then it’s not clear why we should have viruses at all. They should have all decayed to non-functional status long ago (along with every other living thing). IIRC.

Not if some viruses can exist in their natural reservoirs in a kind of dormant state.

I just don’t see how that can be relevant when we know there are active and mutating natural reservoirs in the wild (bats and pangolins in this case) producing new strains. How could you tell the difference between a long-dormant strain and a new strain?


Yes. It seems to me that coronavirus is likely to just become another seasonal flu-like virus that will persist in the human population for a long time if we don’t manage to vaccinate ouselves to herd immunity.

I’am not sure to follow you here.

Agreed. And if it’s flu-like them we’ll be new vaccines for every new strain that comes along.

I am hopeful that the burst of research activity on COVID might lead to improved vaccination methods. We shall see!

No, Sanford is still wrong. So is Dr. Zangrillo.

First, those statements from Italian physicians are based on unpublished (not even in preprint) observations of viral load. We don’t know what the data look like or the methodology, if the testing procedure has been validated for this kind of quantitative analysis, if the differences are statistically significant, what the clinical status of the patients was, or any number of other things. It is entirely possible for example that they are seeing lower viral loads because they now have the capacity to test more patients including those who are not as sick. So it is not even clear that there is a change that needs to explained.

Second, cases counts are increasing in some countries, and countries where transmission is largely controlled are still seeing significant localized outbreaks (for example, the cluster of cases in South Korea traced to a single club patron).

Third, some respiratory illnesses have a seasonal component, which could be due to decreases in transmissibility, changes in human behavior, weather-related changes in immune system function, or some combination of all of the above. While we do not know if COVID-19 will be seasonal in this way, such seasonality could be a possible explanation if it is determined that there actually is substance to what this physicians are reporting.

All of those issues, plus the relatively low (compared to some other RNA viruses like influenza) rate of fixation, need to be considered before we get to discussing possible selection effects or genetic entropy.


I think mutation is going on and they are getting fixed in isolated populations. Mutation will eventually weaken binding and replication. One of the papers posted here showed the sequence variation of the coronavirus across the world.

One thing is clear. The covid19 epidemic is declining rapidly in most countries. In fact, we see that this epidemic, like many other epidemic due to respiratory viruses, has displayed a characteristic bell curve. It is this bell curve shape that needs to be explained. As far as I can tell, no one has provided a good explanation for this mysterious phenomenon. In this context, I think that Sanford’s genetic entropy is worth considering.

Around here (Seattle) there is nothing to explain. New diagnosed cases have dropped by about half, or a bit more than that, as a result of the social distancing. But the number of new cases is declining very slowly after that.

As far as I can tell, overall, we are quite ignorant of the life of viruses in their natural reservoirs.

Yes it displays well known characterististics*, this behavior makes it a not so mysterious phenomenon. I do not intent insult, but “As far as I can tell” is hardly an objective criteria, and neither of us are experts. How could we know a good explanation if one was offered?

* ETA @AndyWalsh (below) is correct that it is not a bell curve, but I took your meaning to be “the common characteristics displayed”. I don’t think we need to quibble over it being a bell curve or not.

I agree there is more to be learned, and much that I personally do not know. It’s not fair to say “we are quite ignorant” because there is a great deal known. I happen to work with some epidemiologists, and I know that ral expertise in the area exists. You and I might be ignorant, so let’s not fall into the Dunning-Kruger trap of think no other knowledge exists.

Yes, due to mitigation efforts such as social distancing, mask-wearing, isolation of known cases, contact tracing and self-quarantine of case contacts, stay-at-home/shelter-in-place orders, etc.

No, in general a bell curve is a poor fit for the epicurves of COVID-19. Look at data for the US or the UK in particular. The epicurve can be strongly skewed. This is not consistent with a Gaussian distribution, but is consistent with epidemic models, particularly SEIR models.

Epidemic models illustrating precisely this behavior–exponential growth, a peak in incidence, and then decline–have been studied for nearly 100 years.


My city, Tuscaloosa, is being overrun by the virus now. Increasing and spreading like crazy

There is no genetic evidence whatever of mutations making SARS-CoV-2 less virulent. There was one early mutation that came to dominate the epidemic in Europe and in much of the United States; this mutation might make the virus more transmissible, but it shows no evidence of being associated with changes to viral load or disease outcome. Besides that, there are no mutations that have become really common in multiple regions, and no sign of the same mutation occurring multiple times and becoming more common in multiple places.


Weird, it’s almost like it’s an entire field of study. Almost.