SAN FRANCISCO (KRON) — A new CDC study compares how effective each of the three COVID-19 vaccines are in preventing COVID-19 hospitalizations.

At this time, the Moderna vaccine and the Johnson & Johnson vaccine (also known as Janssen) have emergency use authorization in the U.S. for people ages 18 years and older.

The Pfizer vaccine has been granted full FDA approval, and can be administered to people ages 12 years and older. Pfizer and Moderna require two doses, spaced out by a few weeks, while the J&J requires just one dose to be considered effective.

For the study, people with particularly weakened immune systems were excluded.

The CDC reports that 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 J&J vaccine (71%).

Even with the slight differences, the CDC said all three provide substantial protection against COVID-19 hospitalization.

According to the CDC, part of why Moderna had more protection than Pfizer was because they observed vaccine effectiveness declining after 120 days for the Pfizer vaccine, but not the Moderna vaccine. The Moderna vaccine also had higher antibody levels after vaccination.

Researchers estimate that the differences in effectiveness could be due to higher mRNA content in the Moderna vaccine, differences in the timing between doses – which is three weeks for Pfizer versus four weeks for Moderna – or even differences between study participants that were not accounted for.

The CDC says data on vaccine effectiveness over time needs more research. They also did not evaluate vaccine effectiveness with variants. Another limitation was that they did not study antibody response past six weeks post-vaccination.

The study participants included 3,689 adults who are at least 18 years or older and were hospitalized at 21 U.S. hospitals across 18 states between March and August 2021.