New study makes Delta the deadliest

May 24, 2022

A new study reported by investigators at the Yale Center for Outcomes Research and Evaluation (CORE) and Harvard shows the Omicron variant of the COVID-19 virus caused more excess deaths in just eight weeks than the Delta variant caused in its entire 23-week outbreak.

The study, published in JAMA, runs counter to the idea that Omicron has caused less harm.

Excess deaths are typically defined as the difference between the observed number of deaths due to all causes during a specific time period compared with the expected number of deaths, according to the Centers for Disease Control and Prevention.

The study used data from Massachusetts — a state of 6.9 million people with high vaccination rates and an increasingly immune population — to compare the Omicron period with the Delta period. It found that in all adult age groups, more deaths occurred during the Omicron period.

“It is important to note that all age groups above 18 in Massachusetts experienced increased excess mortality during the Omicron wave compared to the Delta wave, especially for those older than age 65,” said Chengan Du, co-author of the study and senior statistician at the Yale School of Medicine.

Lead author Dr. Jeremy Faust, emergency physician at Brigham and Women’s Hospital at Harvard Medical School, noted, “This is important because people thought that Omicron was milder. But clearly it was not. We found this across all ages we looked at. And it looks like it was likely true in many other states.”

The researchers said that although the highly contagious Omicron causes relatively milder COVID-19, as some studies have reported, the higher mortality rate in Massachusetts may reflect — despite a moderately lower rate of death from infection — a much higher incidence of infection and thus a higher death rate.

Dr. Harlan M. Krumholz, senior author of the report, director of CORE, and the Harold H. Hines Jr. Professor of Medicine at Yale School of Medicine, characterized the findings as “surprising, striking, and unsettling.”

“In a highly vaccinated state, Omicron exacted a high death toll — and indicates the need to ensure vaccination and strategies to reduce transmission,” he said. “Both are important.”

YaleNews release