Booster shots of the Pfizer-BioNTech and Moderna COVID-19 mRNA vaccines protected against hospitalisation during this winter’s Omicron surge, but lose a substantial amount of effectiveness after about four months, according to a new study published Friday (February 11) by the Centers for Disease Control and Prevention.

Researchers found that mRNA vaccines remained highly effective against both moderate and severe cases of COVID-19 for about two months after a third dose, but the boosters’ effectiveness dropped substantially by four months. The study suggests that those at high risk of severe disease from SARS-CoV-2 infection may need additional booster shots.

In the study, researchers measured how effective COVID-19 mRNA vaccines—either Pfizer-BioNTech or Moderna—were at preventing both hospitalization and visits to emergency departments or urgent care facilities. Using data from 10 US states gathered between August 26, 2021 and January 22, 2022, the researchers analyzed 241,204 visits to emergency departments and urgent care facilities as well as 93,408 hospitalizations among adults older than 18 who had received two or three doses of the vaccine.

They found that booster shots were 91% effective at preventing hospitalization against any variant for two months. After four months, however, this number declined to 78%.

In the same time frame after the booster, protection against emergency department and urgent care visits dropped from 87% to 66%. This number dropped further, to just 31%, after five months. But, according to The New York Times, most people received their booster shot within four months of the study’s timeframe, so the data are limited.

Without a booster shot, protection against hospitalization within two months of a second mRNA shot was 71%, falling to 54% after five months.

Preliminary data previously collected by the CDC and researchers in Israel suggested that protection from boosters wanes over time, according to The Washington Post. However, the new analysis represents the first real-world data in the United States to confirm these findings. 

The data include dates during which the Delta and Omicron variants accounted for more than 50% of cases in the country, reports the Times. The Omicron variant became the dominant variant around December 20, according to The Wall Street Journal. 

The researchers focused on a population of people who sought medical care for COVID-19 symptoms, which may have skewed the study population toward overrepresenting older or immunocompromised people, since these groups are more likely to be hospitalized than younger, healthy individuals. This narrow focus may have made the boosters seem less effective than they really are at protecting against hospitalization among the general population, reports the Post.

Other studies have shown that over time, vaccines may lose some of their ability to prevent severe illness in adults over 65 years old but remain effective in younger people in good health, reports the Times. Federal health officials still need to figure out who is at high risk after receiving three doses in order to determine who needs additional boosters. 

“There may be the need for yet again another boost—in this case, a fourth-dose boost for an individual receiving the mRNA—that could be based on age, as well as underlying conditions,” National Institute of Allergy and Infectious Diseases Director Anthony Fauci said in a White House press briefing last week.