CDC: Omicron's mutations impaired vaccines' effectiveness

From the end of March through May, when the omicron BA.2 and BA.2.12.1 subvariants were dominant in the US, weekly hospitalization rates increased for all adults — with those over 65 hit the hardest. Even so, the total number of hospitalizations remained much lower than when the delta variant was rampant last fall.

The overall number of hospitalizations is an important point, said Abraar Karan, an infectious disease doctor at Stanford University.

“When you look at who’s hospitalized, it’s much more likely that they will have been vaccinated because so many people are vaccinated now,” Karan said. “The real comparison is how many hospitalizations do we have now versus in the past when people were not vaccinated or not up-to-date with boosters.”

CDC scientists found that vaccines and boosters did a better job of keeping people with delta infections out of the hospital than those with later variants. Effectiveness decreased slightly with the BA.1 variant, then changed significantly with BA.2 — with a much greater share of hospitalized adults who had been vaccinated with at least one booster.