Moderna's COVID-19 vaccine was the most effective vaccine against hospitalization in a CDC study released Sept. 17.
The study found Moderna's effectiveness against hospitalization to be 93 percent, Pfizer's to be 88 percent and Johnson & Johnson's to be 71 percent.
The study included 3,689 adults hospitalized at 21 hospitals in 18 states between March 11 and Aug. 15. It did not include immunocompromised people.
"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," the CDC wrote.
Moderna's vaccine was 93 percent effective between 14 and 120 days after a second dose, and dropped slightly to 92 percent after about four months.
Pfizer's vaccine was 91 percent effective between 14 and 120 days after the second dose, and fell to 77 percent after 120 days, leading to its lower overall effectiveness rate.
The CDC said the higher effectiveness of Moderna's vaccine may be due to the fact that Moderna's shot has a higher mRNA content than Pfizer's as well as the difference in timing between doses. Pfizer's doses are given three weeks apart, and Moderna's are given four weeks apart.
Find the full study results here.