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

 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.  The specific traits that help a variant thrive and take over have shifted, too, as our immunological landscape has changed. Earlier in the pandemic, when most people had yet to encounter the virus, variants that figured out how to increase their innate infectiousness had the advantage. Now, after most people have been vaccinated, infected or both, variants that can also evade or suppress our existing immune responses tend to have the edge.

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