Harvard Medical School Researchers Find Omicron Subvariant Has Lower Mortality Rate than Previous Strains

A Harvard-led team of researchers found the Omicron BA.2 subvariant — the Covid-19 strain currently dominant in the United States — appears to have a lower mortality rate than previous strains of the virus, per an article published by the Journal of the American Medical Association last month.

The study, published on Oct. 25, was led by Harvard Medical School instructor Zachary H. Strasser and included researchers from HMS, Massachusetts General Hospital, and Minerva University. The study found that the Omicron BA.2 subvariant has a mortality rate of 0.3 percent — less than half of the Delta variant’s 0.7 percent mortality rate.

“The results suggest that the BA.2 lineage has become intrinsically less severe than that of the original Omicron variant and the Delta variant,” the authors wrote in the paper.

To compare the mortality rates of different Covid-19 strains, the researchers used entropy balance — a statistical method which allowed the team to control for various patient conditions, such as vaccination or prior infection.

“We want to compare patients that have been treated in the same way so that we can really understand the severity difference,” Strasser said.

The researchers also considered the impacts of potential confounding variables — such as patient comorbidities and prior infections — according to Shawn N. Murphy, a professor at HMS who co-authored the study.

“People have all been vaccinated now, they have had prior infections, and we did adjust for all those different factors and treatments,” Murphy said.

Though the results of the study suggest a decreasing pattern in the strength of Covid-19, Strasser said the trajectory of the virus remains unclear.

“We're just looking at really three strains, but at least among these three strains, it seems like the variants are becoming weaker,” Strasser said. “But that doesn't necessarily mean that the trend is going to continue.”

Strasser cautioned against making broad inferences about the virus from this study.

“There might be an inclination to look at this data and say that this means that Covid is going away or that it's becoming less severe, but really it's just three data points, three variants,” Strasser added. “It's good news, and hopefully it continues, but it doesn't necessarily mean that it has to.”

Strasser said his team hopes to conduct future studies using electronic health record data to examine the long-term effects of Covid-19, particularly on patients who suffer from severe cases of the virus.

“One of the advantages of the electronic health record is that you can capture longitudinal data,” he said. “That is advantageous when you're thinking about something like long Covid and wanting to look at what happens to a patient over time.”

“We're going to be focusing more on long Covid going forward,” Strasser said.