evidence: - The study found Pfizer effectiveness dropped to 42% when Delta became the dominant strain. [...] Pfizer's coronavirus vaccine may not be as good at preventing infection with the Delta variant as Moderna's, according to a new preprint from the Mayo Clinic and analytics company nference. [...] During that month in Minnesota, Pfizer's effectiveness against COVID-19 infection dropped to 42% while Moderna's dropped less dramatically to 76% effectiveness.