Adenovirus vaccine (Johnson & Johnson):
Results from the ongoing, pre-authorization clinical trial among nearly 40,000 people, some of whom had stable pre-existing medical conditions or HIV infection and a small percentage of whom had previous SARS-CoV-2 infection, found that the vaccine was 72% effective at preventing moderate and severe COVID-19 at 28 days after vaccination in the U.S. and 64% effective in South Africa, where a COVID-19 variant is much more common — overall, its efficacy against for severe/critical disease was 85% and 100% in preventing hospitalizations (FDA Fact Sheet for Administering Janssen COVID-19 Vaccine, 2021FDA Vaccines and Related Biological Products Advisory Committee). (The Moderna vaccine was 100% effective at preventing severe disease, and there is insufficient data for the Pfizer vaccine for preventing severe disease; however, it is difficult to compare results with the J&J vaccine, as prevalence of the South African variant was much lower during mRNA vaccine trials than the J&J trial.)