Pfizer‘s and Moderna‘s omicron boosters reduced the risk of mild illness from the XBB family of subvariants compared to people who did not receive the shot, according to a study from the Centers for Disease Control and Prevention.

The CDC study published Wednesday provides the first estimate of the omicron shots’ real-world effectiveness against the XBB family of subvariants. Some scientists have warned the XBB subvariants could cause another Covid wave because they are so good at evading the antibodies that block infections.

For people ages 18 to 49, the omicron booster reduced the risk of mild illness by about 48% two to three months after receiving the shot. The shots provided 38% protection against mild illness for those ages 50 to 64 and 42% protection for people ages 65 and older, according to the study.

CDC officials, in a call with reporters Wednesday, said the study results are reassuring because people who received the boosters had more protection than those did not. And protection against severe illness should be even higher, they said.

“It cuts your risk of symptomatic infection in about half at the population level,” said Dr. Ruth Link-Gelles, a CDC official and author on the study.

“What we know from past experience is generally that the vaccines protect better against more severe disease,” Link-Gelles said. “So these are estimates for symptomatic infection and we would expect that similar estimates for hospitalization and death would be higher.”

The XBB.1.5 subvariant is quickly rising to dominance in the U.S. and currently makes up about 49% of new Covid cases nationwide. Officials at the World Health Organization have described XBB.1.5 as the most transmissible version of the virus yet, though it doesn’t have any mutations that would suggest it makes people sicker than other subvariants.

XBB.1.5 is very immune evasive and has mutations that allow it to bind better to human cells. But the CDC study found that the omicron boosters provide about as much protection against the XBB family as they do against the BA.5 subvariant and its descendants such as BQ.1 and BQ.1.1. Protection against mild illness from the BA.5 famly was about

“We did not see reduced vaccine protection against symptomatic illness for XBB and XBB.1.5 compared with those other recent BA.5 variants,” said Dr. Brendan Jackson, head of the CDC’s Covid-19 response.

The study compared people who received the new booster with those who received between two and four doses of the original vaccine. The boosters target omicron BA.5 and the original strain of Covid that emerged in Wuhan, China, while the old shots only target the original virus strain.

People who only received the original shots generally got their last dose about 13 months ago. They had very little protection against mild illness due to waning immunity observed with the old vaccines, Link-Gelles said. It’s too early to draw conclusions about how the protection from the omicron boosters holds up over time, she said.

“Even though you may have diminished protection over time against symptomatic infection, you’re likely still protected against more severe disease for a longer period of time,” Link-Gelles said.

This is a developing story please. Check back for updates.