COVID-19 vaccine effectiveness waned as omicron mutated, CDC finds

COVID-19 vaccines were more effective during the winter omicron surge, when BA.1 accounted for most cases, than they were this spring, when BA.2/BA.2.12.1 prevailed, according to a CDC study.  

The results bolster the case for vaccine-makers to tweak their formulas and issue omicron-targeted doses, which are expected to debut this fall pending FDA and CDC approval. 

Before patients received any boosters, vaccine effectiveness was 61 percent between December 2021 and March 2022 — when omicron BA.1 accounted for more than 75 percent of cases. Between March and June, when BA.2/BA.2.12.1 was dominant, that same measure fell to 24 percent, according to the study. 

To determine efficacy, the researchers evaluated emergency care visits and hospitalization rates among those with Pfizer and Moderna's two-dose vaccines and subsequent booster shots. 

Boosters helped bridge the gap but still shrunk in efficacy over time. Among patients with a booster when BA.1 made up the majority of cases, vaccine efficacy was 92 percent. When BA.2.12.1 surpassed BA.1, vaccine efficacy was 69 percent among those with a booster dose.

When fourth booster doses for adults over 50 rolled out in late March, the study found they also improved vaccine efficacy up to 80 percent.

The overall decrease in efficacy isn't too surprising, since the vaccines were more effective during delta's reign before omicron entered the scene, according to the study. But as omicron subvariant BA.5 now sweeps the nation and COVID-19-related hospital admissions rise, eyes are on vaccine manufacturers to reverse this trend.


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