The acting CDC director delayed publication of a study in the agency’s Morbidity and Mortality Weekly Report (MMWR) that was expected to quantify COVID-19 vaccine effectiveness against severe illness. HHS spokesperson Andrew Nixon said Dr. Jay Bhattacharya flagged concerns about the observational methodology used to calculate vaccine effectiveness. A preliminary summary reviewed by NBC News suggested the 2025–26 vaccine formulation halved the likelihood of severe illness among adults. The episode has intensified concerns among CDC staff and outside experts about potential political interference in federal vaccine evidence and methodology decisions. The delayed MMWR release comes amid broader policy shifts discussed by officials in the administration, including criticism of “gold-standard science” practices and a stated intent to align agency outputs with administration priorities.