Ten percent prevalence on those flights almost defies imagination. I expected far lower. But it might not be entirely bad news. If we have extraordinary prevalence without a huge hospitalization/mortality spike in South Africa, omicron may be less virulent than previous variants.
With 61 infected, the large majority could be clustered—part of a tour group, for example—and have faced a common source of exposure.
More generally, I think we just don't know enough yet. The pieces don't fit together cleanly, and I'm not confident of anything at this point.
More generally, I think we just don't know enough yet. The pieces don't fit together cleanly, and I'm not confident of anything at this point.
One thing to stress is that viruses do not inevitably evolve to become less virulent over time. If this has happened with omicron — and it's too early to tell whether it has — it would be a matter of good fortune.
Many viruses do not attenuate over time. Influenza remains far worse than a common cold. Measles, even more so. Smallpox, worse yet.
When telling tales about virulence evolution, people often invoke myxoma virus, introduced into Australia in the 1950s to control wild rabbits.
When telling tales about virulence evolution, people often invoke myxoma virus, introduced into Australia in the 1950s to control wild rabbits.
Myxoma did indeed become less lethal, from IFR near 1 to IFR near 0.5. But...then it turned around again, and got more virulent.
The point is, it can be quite unpredictable what happens to virulence over evolutionary time.
pnas.org
The point is, it can be quite unpredictable what happens to virulence over evolutionary time.
pnas.org
Another story people tell is that viruses evolve to become less virulent "to keep their hosts alive and thereby transmit more."
This is unlikely to be a factor for COVID, where death occurs weeks after the transmission ceases. Direct selection on COVID IFR should be very weak.
This is unlikely to be a factor for COVID, where death occurs weeks after the transmission ceases. Direct selection on COVID IFR should be very weak.
Bruce Levin and Jim Bull wrote a very important paper about virulence evolution back in '94.
To paraphrase and generalize, virulence is often not so much a matter of evolutionary fine-tuning as a matter of "shit happens."
cell.com (paywall)
To paraphrase and generalize, virulence is often not so much a matter of evolutionary fine-tuning as a matter of "shit happens."
cell.com (paywall)
For example, the bacteria that cause meningitis—H. influenzae, N. meningitidis, and S. pneumoniae—are transmitted by respiratory droplets and usually cause little or no pathology in the nasopharynx.
But sometimes they get into the cerebrospinal fluid. They don't transmit from there—the CSF is a transmission dead end for them—but in a small fraction of patients, they get there and cause great harm. For bacterial meningitis, virulence is wholly uncoupled from transmission.
In 2008, Jim Bull wrote another important paper about the difficulty of predicting virulence evolution, with Dieter Ebert. I wrote about it here.
tl;dr relatively early in the process of pathogen adaptation to a new host, anything can happen.
blogs.nature.com
tl;dr relatively early in the process of pathogen adaptation to a new host, anything can happen.
blogs.nature.com
In short, not only do we not yet have good estimates of virulence for omicron, but the theory gives us no strong reason to expect evolution in one direction or another.
As we wait for the dust to settle, we can hope for the best—but we need to be prepared for other outcomes.
As we wait for the dust to settle, we can hope for the best—but we need to be prepared for other outcomes.
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