Mild or bitter
There’s still discussion about whether the Omicron covid variant is milder than Delta. We don’t really know yet, but this post is about why that’s not even the question.
First, Omicron is still scary: people do end up in hospital; people do die; even a ‘mild’ case can still really suck; and we have literally no idea what proportion of people will get Long Covid. If it’s milder, it’s still very much in the Do Not Want category.
Second, we do know that the proportion of people who get hospitalised will probably be lower than with Delta, and that isn’t the answer to ‘mild or not?’ The primary facts about Omicron are that (a) the vaccine is definitely much less effective at preventing infection, but (b) the vaccine is probably still somewhat effective at preventing severe disease. Suppose, to give us something to work with, an Omicron infection was exactly as likely to cause hospitalisation for an infection in vaccinated individual as Delta, and was exactly as likely to cause hospitalisation for an infection in an unvaccinated individual as Delta.
If you (Dear Reader) are vaccinated or otherwise immune, you’re more likely to be hospitalised by Omicron because you’re more likely to be infected. The vaccine protection is less, even with a third dose, and the prevalence will be higher so you’re more likely to be exposed. If you aren’t vaccinated, you’re more likely to be hospitalised by Omicron because you’re more likely to be infected: the communal vaccine protection is less so the prevalence will be higher and you’re more likely to be exposed. So, in that sense Omicron is worse: you are more likely to get sick, more likely to be hospitalised, probably more likely to die than if Omicron hadn’t come along.
On the other hand, the fraction of cases who end up in hospital is likely to be lower than we were seeing with Delta. That’s because we will have a larger fraction of cases in vaccinated people, and these are less likely to end up in hospital. The number in hospital will go up, but by a smaller multiple than the total number of infections.
So, if the question about a milder variant is “will the fraction of people with serious disease go down?” the answer is probably “yes”. If the question is “will the number of people with serious disease go down?” the answer is probably “no”. If the question is “should I relax because it’s not serious?”, the answer is “holy fuck no”.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »
Just to complete your statements:
“If you (Dear Reader) are vaccinated or otherwise immune, you’re more likely to be hospitalised by Omicron”
– More likely /than/ before (that is, by Delta), not more likely /than/ unvaccinated people
“If the question is: will the number of people with serious disease go down? the answer is probably no”
– So, if the question is: will the stress on ICUs go up? the answer is probably yes.
2 years ago
Just a cue:
https://www.mcgill.ca/oss/article/covid-19-pseudoscience/are-triple-vaccinated-people-more-likely-be-infected-omicron-variant-unvaccinated
As far as his remarks about the Danish data go, he is said to have implied that triple-vaccinated people are more likely to be infected with Omicron than the unvaccinated. He has clarified his remarks and says what he meant is that if you are triple vaccinated and still get infected, then there is a greater chance that the infection will be due to Omicron than Delta. If you are unvaccinated, then the Delta variant is more likely to be the cause of an infection. He correctly states that this does not mean you are more likely to be infected if you are triple vaccinated.
The data that prompted the whole discussion shows 8% Omicron infections among the triple vaccinated and 1.2% among unvaccinated. This actually makes sense. Since the vaccine works much better against the Delta variant than against Omicron, you would expect that whatever infections arise among the triple vaccinated are more likely to be Omicron because they are better protected against Delta than against Omicron.
The important question, however, is not the relative Omicron/Delta distribution among the vaccinated versus the unvaccinated, but the rate of infection among the triple vaccinated versus the unvaccinated no matter whether the infection is due to Omicron or Delta. The Danish data show that infections are about four times more prevalent among the unvaccinated.
2 years ago