New data from the Centers for Disease Control and Prevention (CDC) indicates that the updated bivalent COVID-19 vaccines do not lose significant efficacy in preventing symptomatic coronavirus cases caused by the XBB.1.5 omicron subvariant when compared to previous strains.

Data released in the CDC’s most recent Morbidity and Mortality Weekly Report (MMWR) shows, through a comparison between people who had and had not received the updated shot, that the bivalent booster dose was 52 percent effective at preventing symptomatic coronavirus cases caused by the BA.5 omicron subvariant and 48 percent effective at preventing symptomatic cases caused by XBB and XBB.1.5 subvariant infections.

The study observed adults between the ages of 18 and 49.

“We did not see reduced vaccine protection against symptomatic illness for XBB and XBB.1.5 compared with those other recent BA.5 variants. Quite reassuring,” CDC spokesperson Benjamin Haynes said in a press briefing Wednesday. “People who received the updated or bivalent COVID vaccines had higher levels of production than those who received only the previous vaccines.”

According to Ruth Link-Gelles, CDC epidemiologist and lead author of the MMWR, the findings can be interpreted to mean that vaccines currently cut the risk of symptomatic cases in about half the population.

“It’s hard to interpret it as like an individual’s risk because every individual is different,” Link-Gelles said. “Their immune system is different. Their past history of prior infection is different. They may have underlying conditions that put them at more or less risk of COVID 19 disease. And so for all those reasons, I would think about it more as a population number.”

The CDC also found that the updated boosters provided extra protection against symptomatic coronavirus cases caused by XBB.1.5 for at least three months after vaccination.

To make these determinations, the CDC analyzed national testing data from Dec. 1 to Jan. 13 and looked for symptomatic cases among adults who were not immunocompromised. For the purposes of the CDC’s study, symptomatic cases were defined as having one or more COVID-19 symptoms.

According to the most recent CDC data, 50 percent of COVID-19 cases in the U.S. are due to XBB.1.5. This omicron subvariant is believed to be more transmissible than previous mutations, though it is yet to be determined if it causes more severe illness.

A small minority of the U.S. population — about 15 percent — has received a bivalent booster dose. The shot has been authorized for people as young as five. The CDC officials noted on Wednesday that most people in the U.S. have not had a COVID-19 vaccination in quite some time.

“Those that did not get the bivalent or updated booster, they were about 13 months out from their last dose,” said Link-Gelles. “Nationally, we know that that’s about average. Most people in the US that have not had a bivalent vaccine are a year or more out from their last dose.”

Health authorities including White House COVID-19 Response Coordinator Ashish Jha have previously cautioned that people who have not been recently vaccinated or recovered from an infection likely have very little protection against infection from XBB.1.5.