Three studies released Friday provide more evidence that a COVID-19 vaccine can protect against omicron variants, at least in people who receive booster shots.

Health officials say this is the first large study in the U.S. to examine the vaccine’s protective effects on omicron.

The papers echo previous research — including in Germany, South Africa and the UK — showing that existing vaccines are less effective against omicron than earlier versions of the coronavirus, but boosters also significantly improved protection.

The first study looked at hospital admissions, emergency room and urgent care center visits in 10 states between August and this month.

It found that the vaccine was most effective after three doses of the Pfizer or Moderna vaccine in preventing emergency department and urgent care visits related to COVID-19. Protection dropped from 94% during delta waves to 82% during omicron waves. Protection from just two doses was low, especially six months after the second dose.

The second study focused on COVID-19 cases and death rates in 25 states from early April to Christmas. Those who received the boost had the highest protection against coronavirus infection, both during periods of delta dominance and during periods of omicron takeover.

Not a Modern Healthcare subscriber? Register today.

Both articles are published online by the Centers for Disease Control and Prevention.

A third study, also led by CDC researchers, was published in JAMA.It looked at people who tested positive for COVID-19 at more than 4,600 U.S. testing sites from Dec. 10 to Jan. 1

The three-dose Pfizer and Moderna vaccines were about 67 percent effective against omicron-related symptomatic disease compared with unvaccinated people. However, the researchers found that the two doses had no apparent protective effect on omicron.

“This really shows the importance of booster doses,” said CDC’s Emma Accorsi, one of the study’s authors.