Centers for Disease Control and Prevention

The Biden administration this week announced that it will offer vaccine booster shots next month to address concerns over waning vaccine protection amid surging delta cases.

But the underlying causes remain murky.

Researchers who revisited a cohort of more than 4,000 U.S. frontline workers found the effectiveness of COVID-19 vaccines dropped from 91% to 66% since the delta variant rose to prominence.

Other studies report different numbers, but the reduction remains consistent and sizable.

Research clearly shows vaccines guard against severe illness and prevent hospitalizations.

What remains unclear is whether the drop-off in protection stems from vaccine efficacy waning over time, the delta variant excelling at transmission, or behavioral changes contributing to greater spread.