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‘Mild’ Omicron variant was highly lethal, study finds

Air Force Maj. Tonya Toche-Howard, left, a registered nurse assigned to the U.S.  Air Force military medical team deployed to Brockton, Mass., pushes a hospital bed into a patient's room along with a Signature Healthcare Brockton Hospital nurse as part of the COVID-19 response operations in Brockton,  Feb. 17, 2022.Spc. Daniel Thompson/Associated Press

By now anyone who hasn’t had COVID-19 almost surely knows someone who did. And especially for those infected during the recent Omicron wave, chances are the illness was short-lived and not especially devastating.

These everyday experiences make it easy to think COVID-19 has become just like the flu or a cold.

But a recent study of Massachusetts death data shows the opposite to be true: Even the supposedly mild Omicron variant was exceedingly lethal.

Conducted by researchers at Brigham and Women’s Hospital and published in JAMA on May 20, the brief paper tabulated deaths from all causes in Massachusetts during the Delta and Omicron waves. It then compared the number of deaths during each wave with what would be expected in a typical pre-pandemic year.


The number of deaths that exceed what would normally have occurred are called “excess deaths.”

From June 28 through December 5, 2021, when the Delta variant of the coronavirus was dominant, Massachusetts experienced 1,975 excess deaths.

From December 27, 2021, through February 20, 2022, when the Omicron variant was dominant, there were 2,294 excess deaths.

In other words, in a mere eight weeks of Omicron, Massachusetts experienced more excess deaths than in 23 weeks of Delta.

That’s because Omicron was so highly transmissible that many thousands of people caught it, including those with immunity from vaccination or prior infection, said Dr. Jeremy Samuel Faust, the study’s lead author and an emergency physician at the Brigham. So while the percentage of infected people who got seriously ill was lower than in previous waves, so many were infected by Omicron that the sheer number of those killed grew large.

“This experience should be a cautionary tale,” Faust said. It demonstrates that even seemingly mild variants can “deliver a fatal punch to a lot of people.” He predicted that new coronavirus variants, which will inevitably emerge, are going to be very contagious, because the virus has been evolving to spread widely.


“The experience people have – that it’s been mild – works in the virus’s favor. …It’s a huge evolutionary advantage – it fools us,” he said. “At the population level, most people are fine with it. Meanwhile, it causes devastating illness and disability in a subset.”

Could other causes of death, related to the pandemic but not caused by the virus, have led to the increase? There have been reports of deferred care leading to illness and death, as well as higher rates of overdose and suicide related to pandemic stress and isolation.

Faust said these factors may contribute to the toll, but not significantly. The rise in excess deaths coincided with the rise in COVID-19 cases, but did not occur in the periods when people were locked down and afraid to seek care.

“We’re confident that COVID’s driving it mainly, because the numbers line up kind of neatly,” he said.

William Hanage, an epidemiologist at the Harvard T. H. Chan School of Public Health, said the study “helps us see that ‘mild’ is not an appropriate term for a virus as transmissible as Omicron.”

Hanage, who was not involved in the research, said he was “not even slightly” surprised by the high number of deaths during Omicron. “I am relieved it wasn’t worse,” he said in an e-mail. “Omicron probably is somewhat less serious than Delta per infection, but once you have so many infections, that more than makes up for it.”


Both Hanage and Faust saw a good news aspect to the findings: Massachusetts handled the Delta wave very well, vaccinating lots of people and encouraging widespread precautions.

The same can’t be said for Omicron.

“We were able to hold the line against Delta, but we could not hold the line against Omicron,” Faust said, in part because vaccine immunity waned and many people failed to get boosters.

Felice J. Freyer can be reached at Follow her @felicejfreyer.