In Gauteng, South Africa’s Omicron epicenter, the wave seems to be cresting. But why? dwallacewells spoke with trvrb about how to make sense of this phenomenon
A technician inspects scans of platelets on a monitor during COVID-19 antibody neutralization testing in a laboratory at the African Health Research Institute in Durban, South Africa, on December 15. Photo: Waldo Swiegers/Bloomberg via Getty Images In Gauteng, South Africa’s Omicron epicenter, the wave seems to be cresting. In other parts of the country, too, the terrifyingly fast rise of the new variant appears already to be slowing and even receding.
Yeah, it’s a really good question. I’ve been looking into this a bit. The basic idea is that we can measure Rt, and there’s a very simple equation that will convert Rt into your population attack rate: how many people will be infected in the entire epidemic wave. That projection is quite linear. With Delta and the Delta Rt of 1.5 — when it was coming in — I was able to convert that to an attack rate that ultimately matched what we saw.
Rather than continue to spread outward from the initial case, in other words, the fact of natural social networks and limited niches means that chains of transmission can’t continue indefinitely. One of the fundamentals of this dynamical modeling field is that epidemics crash not when they’ve infected everyone but when your number of secondary infections is less than one, when Rt falls below one. So even if we have an initial Rt of two, the numbers are quite big, but you don’t infect the whole population. You infect almost 80 percent of the susceptible population. With an Rt of three you may infect 90 percent of the susceptible population.
And in fact, that has been the dynamic in the U.K. and many other countries, which are often a bit better vaccinated than the U.S., but not in a different ballpark vaccination level. There, they’ve seen this quite dramatic decoupling, even through Delta — when there has been an uptick in severe cases and death, it has been accompanied by much higher caseloads. Why haven’t we seen that in the U.S.?
For over-65’s, we can see the big peak last winter — but not as bad as in Delta, thanks to vaccination. Whereas if we look in the 35 to 44 category, there’s just a much larger fraction of 35 to 44 year olds who are getting COVID in September and fewer of them vaccinated so you can still get this big jump in mortality. But about your central question of why given what we know about how strong the age-mortality effect is, I would still have expected CFR to change much more dramatically.
Because of reinfection, you mean, and how effective Omicron is at getting to those who’ve already been sick.
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