evidence: In presenting their rationale for the decision on Aug. 18, health officials pointed to multiple recent studies conducted in the U.S. that have suggested a decline in the real-world effectiveness of the Pfizer/BioNTech or Moderna vaccines in preventing any kind of lab-confirmed infection. [...] An unpublished study of patients in the Mayo Clinic Health System compared the two mRNA vaccines and found the Pfizer/BioNTech vaccine began with an effectiveness of 76% against any kind of PCR-confirmed infection in January that fell to 42% in July. The drop for Moderna was smaller, from 86% to 76%. [...] Notably, however, officials said there was not much evidence that the vaccines were working less well in protecting against severe disease. The New York study, for example, found that vaccine protection against hospitalization remained at 90% or higher for each week analyzed. And another MMWR report, which drew on hospital information from 18 states, found the Pfizer/BioNTech and Moderna vaccines stayed 84% to 86% effective in preventing hospitalization for 24 weeks.