SAN FRANCISCO (KRON) – A study from the Centers for Disease Control and Prevention released Friday compares how effective each of the three COVID-19 vaccines are in preventing hospitalization from the virus.

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The CDC reports that effectiveness was higher for the Moderna vaccine (93%) than the Pfizer-BioNTech vaccine (88%) and the J&J vaccine (71%).

For the study, which ran from March 11 to August 15, 2021, researchers used data from 3,600 adults hospitalized in 18 states. People with particularly weakened immune systems were excluded.

Beyond 120 days after inoculation (median average of 143 days), researchers found that the effectiveness of the Pfizer vaccine fell to 77 percent while Moderna maintained its effectiveness. The Moderna vaccine also had higher antibody levels after vaccination.

Even with the differences, the CDC said all three provide substantial protection against COVID-19 hospitalization and serious illness. Hours after the CDC released its study on the Pfizer vaccine, a Food and Drug Administration panel of experts voted against booster shots for the general public.

At this time, the Moderna vaccine and the Johnson & Johnson vaccine 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.

Researchers speculated that the differences in lasting effectiveness could be due to higher mRNA content in the Moderna vaccine, or 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.