Surprisingly, the U.S. currently has about 9 new COVID-19 cases per 100K population, same as it’s been for three weeks now.
Data source for this and other graphs of new case counts: Calculated from The New York Times. (2021). Coronavirus (Covid-19) Data in the United States. Retrieved 4/7/2022, from https://github.com/nytimes/covid-19-data.” The NY Times U.S. tracking page may be found at https://www.nytimes.com/interactive/2020/us/coronavirus-us-cases.html
Cases continue to increase in the Northeast. If there’s any rhyme or reason to what’s happening in the rest of the country, it’s beyond me. Basically, we’re on hold, waiting to see what, if anything, happens next.
Map courtesy of datawrappe.de
Best guess, this is what endemic COVID-19 looks like in the U.S.
And I’m already starting to get a sense of how this does and does not differ from (e.g.) endemic flu.
Contemplating the recent Gridiron Club super-spreader event. How does flu spread, compared to Omicron?
And so, if we’re going to have to live with this level of Omicron in circulation, what are the implications compared to (e.g.) living with flu, as we have done all of our lives?
First, this brings the discussion right back to the start of the pandemic. Does flu spread only via droplets (fairly large particles that rapidly fall to the ground), or via aerosols (tiny particles that are airborne and can travel significant distances). With droplet transmission, the six-foot social distancing rule keeps you safe. Not so with aerosols.
Evidence suggests that flu spreads via both droplets and aerosols, but if I had to guess, I’d say the consensus is that droplet spread is more important for flu than it is for COVID. The CDC still maintains that flu is spread via droplets, period. You can read that on this CDC web page, where there is no mention of aerosol transmission. But the CDC said that about COVID-19, too, until enough scientists twisted their arms that they grudgingly changed their language to include aerosol transmission. Other research pretty strongly suggests that flu, like COVID-19, may also spread via aerosols (reference). And still other research suggests that airborne transmission of flu via aerosols is pretty common (“at least half”, reference).
With flu, as with COVID-19, people begin to be infectious before onset of symptoms, and remain infections for many days afterward (reference CDC). That said, the mix of symptomatic and asymptomatic days is different. It appears that flu is only infectious for one day prior to symptom onset, whereas in the typical case of COVID I think it was two to three days. Also, only about 16 percent of flu cases are asymptomatic (I can’t find the reference for that), versus perhaps 40 percent of COVID (from an old statement by Dr. Fauci).
Flu appears much hard to spread, on average, than Omicrion. The “R-nought” for the current strain of Omicron has been estimated to be somewhere around 22. (That is, absent any immunity or protections, the average infected individual would have gone on to infect 22 others). Seasonal flu, by contrast, has a typical R-nought of about 1.3.
Flu has super-spreader events (reference), but they appear to be far more rare than COVID-19 super-spreader events.
In general, the tendency to spread via large clusters of infections is the “overdispersion” factor or K factor. The K for COVID-19 has been estimated to be as low as 0.1 (reference). Perhaps 80 percent of infected individuals never spread the disease, and most spread occurs from large clusters attributable to a handful of individuals. For flu, by contrast, the K is closer to one. Spread in that case is far more uniform, much closer to the case where each person spreads it to just one or two others.
Interestingly, that high K factor for COVID-19 — the propensity for super-spreader events — should change (and apparently has changed) as the pandemic progresses. Once you start getting a lot of immunity in the population, massive superspreader events become less likely. That’s the gist of the discussion in this NEJM article: “Overdispersion was thought to be an unstable trait that would disappear, with transmission becoming more uniform and higher overall. That transition appears to have occurred as newer variants take over.”
As any parent will tell you, kids play a huge role in spreading seasonal flu and colds. As this article in The Lancet put it, “Children and adolescents contribute more to the transmission of common cold betacoronaviruses and influenza than they do to the emerging betacoronaviruses.” For COVID-19, by contrast, they played a far smaller role. The initial studies of the Wuhan outbreak found no child-to-parent transmission whatsoever.
Finally, with vaccination and a booster, it’s not clear that the rate of severe outcomes from Omicron infection is worse than for typical flu. At least, not for the vaccinated and boostered individual. (This is all based on earlier analysis, e.g., Post #1430). CDC data show a roughly 1.4 percent case hospitalization rate for flu. By contrast, our current case hospitalization rate for Omicron is around 6 percent. But much of that is for the unvaccinated. By calculation, the case hospitalization rate for the vaccinated and boostered should be about 18% of the raw average, or about (6% x 18% =) 1.1%. This slurs over the impact of vaccination on flu hospitalizations. But it shows that the rates for the vaccinated/boostered population are certainly in the same ballpark.
This, of course, does not include the risks of any other intermediate-level outcomes from COVID, including loss of sense of smell and taste, or any of the “long COVID” symptoms, including lasting cardiovascular and neurological damage.
That said, if I focus on the acute, severe outcomes, the risk from a case of flu and a case of Omicron are not hugely different, for the vaccinated and boostered population.
Here’s how I sum all that up.
Your risk of catching flu is a lot more obvious than your risk of catching COVID-19. It’s more likely to be one-to-one transmission. The person transmitting it is more likely to appear to be sick. You provide more protection to yourself with that six-foot social distancing rule. You are more likely to catch it from your kids, and less likely to catch it in some massive infection event at some communal gathering.
For Omicron, by contrast, the risk is higher that you’re going to catch it via long-distance (aerosol) transmission from some asymptomatic stranger. You aren’t likely to get it from your kids, and you don’t have to be within six feet of somebody to get a good dose. And it’s far more likely that the person who gives you a dose of COVID doesn’t know they have it and doesn’t appear under the weather.
And so, at the end of the day, I think that large gatherings of closely-packed strangers are going to bear a risk of COVID-19 infection that does not occur — or does not occur to the same degree — with normal seasonal flu. With flu, maybe it’s mostly good enough to avoid those who are sniffling and sneezing in that kind of situation. With Omicron, you’re still going to face risk of infection even if you do that.
All other things equal, if flu and COVID had identical incidence rates, the apparently-healthy person sitting six feet behind you, drinking a beer and cheering on the team, represents a far higher risk to you for of Omicron infection that of seasonal flu infection. And, accordingly, I think that a cautious person is going to have more reason to avoid large indoor crowds than in the past, when flu was really the only worry.
I think that’s a permanent change, and there’s no obvious way to mitigate it. It’s just a consequence of a disease that will, on average, mess you up at least as badly as seasonal flu, but gets transmitted in stealthier ways, at greater distances, and in larger clusters.
