Thread by @mlipsitch

the following thread is purely my personal opinion as a @CCDD_HSPH @HarvardEpi epidemiologist. I would welcome feedback though I am trying to take some breaks so don't promise to reply. Thanks in advance to anyone who provides critique or further info

Impact of #Omicron obviously depends on severity. Number of cases is growing exceptionally fast and several countries have seen such growth continue for quite a while (weeks, which is a lot of time with doubling times of small number of days

AFAIK there are two publicly available assessments of severity. Kudos to @Imperial_IDE and Discovery Health for rapidly putting out reports imperial.ac.uk/mrc-global-inf… and discovery.co.za/corporate/news… (site down as I write, but previously had a link to a detailed webinar

Below queries about what was done are not criticisms of these groups who got info out fast, but hopes for more clarity about how to interpret their findings.

SA Discovery finds in adults a 29% lower case-hospitalization ratio and a 77% lower hospitalization-ICU ratio, for (I think) an 82% lower case-ICU ratio for Omicron vs Delta.

Slightly higher case-hospital ratio for children w Omicron vs Delta. Not sure if the CIR can be calculated from the hospitalization and ICU/Hospitalization ratio because I don't understand the risk adjustment algorithm. Could be very important to understand how that was done.

But ~80% reduction in ICU risk per case would be very good news, if correct (see caveats below for issues that may contribute to bias)

2. Imperial finds in logistic regression for that hospitalization has an OR of 0.95 (0.61-1.47) for predicting SGTF, a proxy for Omicron. I cant tell if this is multivariable regression, adjusting for everything, but almost any single approach would be hard to interpret.

Would love to understand better what they did. Notably the point estimate of 0.71 relative risk of hospitalization from SA is within their 95% CI for the OR in England. Both analyses taken at face value suggest at most a modest reduction in hospitalization risk.

(though unfortunately no uncertainty estimates in the South Africa analysis). A modest reduction in hospitalization risk could be swamped by severalfold more cases as several models predict.

Complicating analysis could be
1) hospitalization may not have occurred yet, preferentially in the Omicrons, making the results too optimistic (applies to both analyses)

2) If SA is correct, the real reduction in severity is at the highest-severity end, which is not considered in the Imperial report (i.e. ICU)

3) Both may (I suspect do, at least for England) include those admitted for non-COVID causes and thus make hospitalization rates look higher (because Omicron is contagious enough that many people in hospital have it unrelated to their admission)

Including nonCOVID admissions who test positive for SARS-CoV-2 would make severity among hospitalized look lower as suggested by this account from SA samrc.ac.za/news/tshwane-d… .

Bottom line we are far from having evidence that severity is low enough to make this not a worry at the societal level, especially with already-stretched hospitals. Evidence that vaccines, especially when boosted, offer protection against severe disease is more consistent.

Already learning from the responses, thank you. Two sets of data suggesting modest contribution of "incidental" SARS-CoV-2 diagnoses among hospitalizations, one in Tshwane, Gauteng, SA

https://twitter.com/enjoyingthewind/status/1472329401585045518?s=20

And one in London, UK

https://twitter.com/cripipper21/status/1472328298025869314?s=20

which shares this

https://twitter.com/chrischirp/status/1471900266811244547?s=20

. A constant proportion of hospital admissions over time that are incidental SARS-2 findings vs. admitted for COVID is indeed consistent with similar severity per infection to the past variants, as @chrischirp notes.

This presentation from yesterday is illuminating ht to a responder whom I can't find any more, twitter fail...

Several lines of evidence here suggesting a lower hospitalization proportion than in previous waves, limited to first 25 days. Caveats 1) this is growing faster, so higher proportion of recent cases makes bias (missing not-yet-hospitalized) worse even when compare first 25 days

2) the proportion vaccinated is going up, as is the proportion previously infected, so a more immune population. So hard to compare but declining in-hospital severity measures here as in Discovery Health are hopeful signs.