evidence: Pfizer CEO Albert Bourla was confident in June about the ability of his company’s vaccine to protect against the highly contagious delta variant, as it marched across the globe and filled U.S. hospitals with patients. [...] Just weeks later, Pfizer said it would seek authorization for a booster shot, after early trial results showed a third dose potentially increased protection. At the end of July, Pfizer and BioNTech announced findings that four to six months after a second dose, their vaccine’s efficacy dropped to about 84%. [...] Yet the effectiveness of the Pfizer vaccine against variants is still under debate. This month a new preprint study by the Mayo Clinic found that the product’s effectiveness against infection dropped to 42% from January to July ― as the delta variant’s prevalence markedly increased.