Rosenberg ES, Dorabawila V, Easton D, et al. - Overall there are limited population-based data from the United States on the effectiveness of the three coronavirus disease 2019 (Covid-19) vaccines currently authorized by the Food and Drug Administration. Researchers herein examined if waning immunity or the B.1.617.2 (delta) variant of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for the declines in effectiveness of vaccines or there are other causes for the decline.

• Data for 8,690,825 adults in New York State were used to determine the effectiveness of the BNT162b2, mRNA-1273, and Ad26.COV2.S vaccines against laboratory-confirmed Covid-19 and hospitalization with Covid-19 (ie, Covid-19 diagnosed at or after admission).

• Vaccine effectiveness declined against Covid-19 from May through August 2021.

• An inverse correlation appeared between these trends and increasing delta-variant prevalence and among persons 18 to 64 years of age; the trends plateaued during the period in which the prevalence of the delta variant exceeded 85%.

• Simultaneous occurrence of these changes was observed across age, product, and time cohorts, with the largest declines seen among BNT162b2 recipients.

• Combined with the fact that there were synchronous declines in effectiveness among time cohorts, it is suggested that these declines are not primarily due to the waning of immunity.

• In contrast, there remains high effectiveness against hospitalization with Covid-19, with persons 65 years of age or older and those who received Ad26.COV2.S exhibiting lower effectiveness.

• BNT162b2 and mRNA-1273 recipients 65 years of age or older showed modest declines over time (within −6 percentage points).