Vaccine Efficacy Decreased August 18, 2021 tony Covid-19 Vaccines Moderna and Pfizer received Emergency Use Authorization while showing +90% effectiveness. Those numbers have dropped. Comparison of two highly-effective mRNA vaccines for COVID-19 during periods of Alpha and Delta variant prevalence | medRxiv Pfizer Shot Just 39% Effective Against Delta Infection, But Largely Prevents Severe Illness, Israel Study Suggests (forbes.com)