Covid-19 Variants Keep Getting More Contagious. Here’s Why.

As the Omicron BA.5 subvariant is the latest to take over and dash the hopes of a summer Covid-19 lull, researchers have attempted to quantify and explain its edge over earlier versions of the virus. The BA.5 subvariant is estimated to be some 70% more transmissible than BA.2, which took over the landscape in the spring, said Trevor Bedford, a computational biologist at the Fred Hutchinson Cancer Center. That estimate is based on how quickly the BA.5 variant displaced its predecessor, he said. Comparing the BA.5 subvariant to earlier versions of the virus, such as Delta or even BA.1, however, is more of a challenge, Dr. Bedford and other infectious-disease experts said. Covid-19 vaccinations and infections have changed our immune defenses over time, making head-to-head matchups between past and present variants harder, including for characteristics like intrinsic severity of the disease.