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Booster shots of the Moderna and Pfizer/BioNTech COVID-19 vaccines were 90% effective in preventing hospitalizations from the omicron variant through the last quarter of 2021, data released Friday from the Centers for Disease Control and Prevention shows.

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Three studies published by the CDC, looking at millions of COVID-19 cases, showed that the vaccine boosters provide strong protection against severe disease or hospitalization from the omicron variant across the country.

Omicron is responsible for 99% of the current COVID-19 infections in the United States, according to the CDC.

One of the three reports analyzed data from hundreds of thousands of emergency room visits, urgent care visits and hospitalizations between August 2021 and Jan. 5, 2022. The study showed that a third dose of either the Pfizer/BioNTech or Moderna shots prevented 9 out of 10 hospitalizations and roughly 8 out of 10 trips to an emergency room or urgent care facility.

The CDC and other health officials have encouraged Americans to get a booster of either the Moderna or Pfizer vaccines as evidence mounts that protection from the original vaccine shots has waned.

According to Mark Thompson, a CDC public health researcher and lead author of one of the reports, protection from two doses of those vaccines has declined since omicron became the dominant variant of the virus in November.

“That’s the bad news,” Thompson told The Washington Post. “The good news is that for people who received a third booster dose, the effectiveness of three doses is very high and protects against moderately severe and severe disease. This is among the first data that really shows the actual level of protection in the United States.”

The studies released Friday are similar to ones conducted in Germany and South Africa showing the effectiveness of boosters against serious disease and hospitalizations.

Another study released by the CDC Friday looked at COVID-19 cases and death rates in 25 states from the beginning of April through Dec. 25.

It showed that people who were boosted had the highest protection against COVID-19 infection both when the delta variant was dominant up through when omicron became the most common variant infecting Americans.

The third study looked at people who tested positive for COVID-19 from Dec. 10 to Jan. 1 at more than 4,600 testing sites across the country.

The results showed that three shots of the Pfizer and Moderna vaccines were about 67% effective against omicron-related symptomatic disease compared with people who had not been vaccinated. Two doses, however, offered no significant protection against omicron, the researchers found.

The studies looked not at whether a person who was vaccinated would get a breakthrough case, but whether they would get severely ill from the infection if they did contract the virus.