The Omicron BA.2 subvariant is less severe than the Delta variant and to an even greater extent than the original Omicron variant, according to a research team led by investigators at the US-based Massachusetts General Hospital.
Of 1,02,315 confirmed COVID-19 cases in the US from March 3, 2020 to June 20 this year, there were 20,770 labelled as Delta variants, 52,605 labelled as Omicron B.1.1.529 variants, the original Omicron variant, and 28,940 labelled as Omicron BA.2 subvariants, researchers found.
Mortality rates were 0.7 per cent for Delta, 0.4 per cent for the original Omicron variant and 0.3 per cent for Omicron BA.2.
The researchers said after adjustments, the odds of death were more than two times higher for the Delta and the original Omicron variant compared with Omicron BA.2.
Patients with Delta and original Omicron variants were also more likely to need hospitalisation, invasive ventilation and intensive care admissions, according to the study, which has been published in the Journal of the American Medical Association Network Open.
These findings suggest that the severity of SARS-Cov-2 may be diminishing, according to the study.
To provide an accurate assessment of the severity of SARS-Cov-2 variants above and beyond previous studies, the researchers used a method called entropy balancing to account for potential confounding factors such as prior infections, vaccinations, treatments and comorbidities.
The team applied this method to data leveraged from the Mass General Brigham's electronic health record system that is linked to a COVID-19 vaccine registry, the researchers said.
"While the SARS-CoV-2 virus always has the potential to mutate to a more deadly form, when you look at the recent trajectory of Delta, Omicron BA.1, to Omicron BA.2, the virus seems to be getting intrinsically less severe. Hopefully this trend will continue," the study's lead author, Zachary Strasser, said.
"We can continue to use our analytics system and method to assess many other questions such as which vaccinations have the most impact on preventing long Covid, or whether certain treatments reduce the likelihood of long Covid," Strasser said.
The Omicron variant of concern was the dominant one earlier in the year, circulating globally and accounting for nearly all sequences reported to GISAID, an initiative which promotes rapid sharing of data from all influenza viruses and the coronavirus causing COVID-19.
Omicron is made up of several sublineages, the most common ones reported at that time being BA.1, BA.1.1 and BA.2.
