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Protection from booster doses of the COVID-19 vaccines developed by Pfizer-BioNTech and Moderna wane over time, though data shows that they continued to protect well against hospitalization and death as the delta and omicron variants fueled infections nationwide, according to a study published Friday by the Centers for Disease Control and Prevention.

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Researchers said in the study, published in the CDC’s Morbidity and Morality Weekly Report, that the data underscored the importance of booster doses to prevent severe outcomes from COVID-19, while also reinforcing the need to consider whether additional booster doses will be needed to keep protection high.

Amid the spike in COVID-19 infections spurred by the advent of the omicron variant, officials said data showed that within two months of people receiving booster vaccine shots, the vaccines were 87% effective at protecting against emergency department or urgent care visits and 91% effective at preventing hospitalizations. However, officials noted that four months after a person received their booster dose, effectiveness dropped to 66% for emergency department or urgent care visits and 78% for hospitalizations.

>> Read the full study released Friday

The study examined data from 10 states on 93,000 hospitalizations and 241,000 emergency department and urgent care visits from August 2021 to January 2022. It found that vaccine effectiveness was always higher after a person got a vaccine booster, when compared to after a person got a second dose of Pfizer-BioNTech or Moderna vaccine.

The data provides the first look at real-world data on vaccine booster efficacy amid the delta and omicron waves in the U.S., according to The Washington Post. Officials noted in the study published Friday that a similar drop in protection has been seen in Israel and in preliminary reports from the CDC.

On Wednesday, Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, said that officials continue to closely follow data about the efficacy of booster doses. He added that officials are unlikely to take a blanket approach to recommending additional booster shots.

>> Related: CDC: Boosters 90% effective in preventing severe COVID-19, hospitalizations

“I think you should be appreciative of the fact that when you’re talking about any decisions that will be made – and I’m not anticipating any of that now – but that has to be put into the context of whom you’re talking about,” he said during a COVID-19 White House response team news briefing. “For example, there may be the need for yet again another boost – in this case, a fourth-dose boost for an individual receiving the mRNA (vaccines, which are those developed by Pfizer-BioNTech and Moderna) – that could be based on age, as well as underlying conditions.”

The CDC recommends that most people get booster vaccine doses five months after receiving the vaccines developed by Pfizer-BioNTech or Moderna, or two months after receiving Johnson & Johnson’s Janssen vaccine. Officials emphasized Friday that boosters “ensure optimal protection against hospitalizations and severe outcomes” from COVID-19.

>> Related: Coronavirus: Up to date or fully vaccinated? What’s the difference?

As of Friday morning, nearly 76% of the U.S. population – 251.7 million people – has gotten at least one dose of any of the available COVID-19 vaccines, CDC data shows. Just over 64% of Americans, or 213.5 million people, have been fully vaccinated, and nearly 43% of those who have been fully vaccinated have gotten booster shots, according to the CDC.

Officials have confirmed more than 77.6 million COVID-19 infections and reported over 918,000 deaths nationwide, according to numbers compiled by Johns Hopkins University.

More than 408.2 million COVID-19 cases have been reported worldwide, resulting in 5.8 million deaths, according to the university.