Omicron caused more excess mortality than Delta in Massachusetts.
While the average Omicron illness may be somewhat milder, that factor was overwhelmed by Omicron’s contagiousness.
Friday, my colleagues and I published research in the Journal of the American Medical Association (JAMA), reporting that the Omicron Covid-19 variant caused more excess mortality in Massachusetts than the Delta variant did.
While this goes against the perceived “milder” nature of Omicron, the math makes sense. Even if each Omicron infection was 50% as lethal as earlier variants (and that’s probably overstating its lesser virulence), if Omicron is, say, four times more contagious than its predecessors, its rapid spread would mean that a higher number of deaths would be expected to occur.
Think about it: 50% as lethal x 400% as contagious = 200% the mortality in a given time frame. That’s exactly what happened in Massachusetts and in other places (we have not yet published data from other states) during the Omicron wave.
We call this result the “mortality product.” We didn’t mean to coin a phrase, but the JAMA reviewers pointed out that this term had not been used before, and a quick Google search confirmed that—at least as far as I know.
Why does that matter? For one thing, it means we can’t shrug off somewhat milder but highly contagious variants. Omicron infected lots and lots of people. But it punished the unvaccinated and the unboosted older and immunocompromised populations, in particular. Another reason a variant with a high mortality product (but a relatively lower rate of severe disease and/or death rate) matters is hospital capacity. A variant that is contagious enough to cause many severe cases at the same time can stretch the healthcare system’s overall capacity. That’s exactly what we saw during the Omicron wave. Hospitals were crushed with demand, likely reflecting a combination of Covid-19 care, other medical problems, and, importantly, the combination of those (i.e., Covid-19 infections causing dangerous exacerbations in chronic but typically manageable conditions).
The data also show that excess mortality occurred in all adult age groups, not just among the oldest. Over 1,100 more Massachusetts residents under the age of 65 died than usual during the Delta and Omicron periods, which is a 14% relative increase. That’s a lot. If other states had similar outcomes, this would correspond to over 52,000 excess deaths in non-geriatric adults during Delta and Omicron combined, which is staggering. We’ll know more soon and will update you.
Another finding that our team uncovered (but did not publish in JAMA) is that the excess mortality during both the Delta wave and the Omicron wave was highly correlated to the rates of unvaccinated people in each community. In Massachusetts counties with the lowest vaccination rates, the most excess mortality occurred. In counties with the highest vaccination rates, the least excess mortality occurred. That finding also held up with boosters during the Omicron wave (the Delta wave largely played out before many people had been boosted), even though a relatively small fraction of residents had been boosted by the time Omicron erupted here in Massachusetts.
Massachusetts counties with higher vaccination rates had lower overall mortality during Delta and Omicron. The same held up for boosters, even though a small number of residents had been boosted when Omicron hit. That's probably because older people were boosted at higher rates. Data: My research collaborative at Harvard and Yale: Graphs: Dr. Kristen Panthagani.
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So we now know that Omicron was, as we said in Inside Medicine, not mild enough. Going forward, we need to take each variant as it comes. It’s likely that future variants will be as contagious as the recent ones. Evolution favors that. Evolution does not favor much of a change in the disease severity for any particular variant—at least not at the margins. As the population gains immunity, it may appear that some people have milder illnesses with each new variant. That is an illusion. Too many people appear to have fallen for that mirage during Omicron; it did find and kill an alarming number of people. Excess mortality helps us track that. Let’s not keep getting fooled by this virus.
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Acknowledgements: I would like to thank my co-authors on the JAMA study: Dr. Chengan Du, Dr. Chenxue Liang, Dr. Katherine Dickerson Mayes, Benjamin Renton, Dr. Kristen Panthagani, Dr. Shu-Xia Li, Dr. Zhenqiu Lin, and Dr. Harlan M. Krumholz and the Massachusetts Department of Health for providing the raw data.