A head-to-head study of all three authorized coronavirus vaccines in the United States finds the Moderna vaccine is slightly more effective than Pfizer's in real-life use in keeping people out of the hospital, and Johnson & Johnson's Janssen vaccine comes in third, but still providing 71% protection.

Pfizer's vaccine provided 88% protection against hospitalization, and Moderna's was 93% effective.

The US Centers for Disease Control and Prevention led a nationwide study of vaccination involving more than 3,600 adults hospitalized for Covid-19 between March and August.

"Among U.S. adults without immunocompromising conditions, vaccine effectiveness against Covid-19 hospitalization during March 11--August 15, 2021, was higher for the Moderna vaccine (93%) than the Pfizer-BioNTech vaccine (88%) and the Janssen vaccine (71%)," the team wrote in the CDC's weekly report on death and disease, the MMWR.

"Although these real-world data suggest some variation in levels of protection by vaccine, all FDA-approved or authorized Covid-19 vaccines provide substantial protection against Covid-19 hospitalization."

They found that the biggest difference between the vaccine made by Moderna and Pfizer/BioNtech's vaccine was driven by a decline that started about four months after people were fully vaccinated with Pfizer's vaccine.

"Differences in vaccine effectiveness between the Moderna and Pfizer-BioNTech vaccine might be due to higher mRNA content in the Moderna vaccine, differences in timing between doses (3 weeks for Pfizer-BioNTech versus 4 weeks for Moderna), or possible differences between groups that received each vaccine that were not accounted for in the analysis," the team wrote.

"A single dose of the Janssen viral vector vaccine had comparatively lower anti-SARS-CoV-2 antibody response and vaccine effectiveness against Covid-19 hospitalizations," they added.

"Understanding differences in vaccine effectiveness by vaccine product can guide individual choices and policy recommendations regarding vaccine boosters. All FDA-approved or authorized Covid-19 vaccines provide substantial protection against Covid-19 hospitalization."

CDC worked with researchers across the country to study 3,689 patients at 21 hospitals in 18 states for the study. They also looked at antibodies in the blood of 100 healthy volunteers after they'd been vaccinated with one of the three available vaccines.

CNN