Pfizer's and Moderna's omicron booster shots reduced the risk of mild illness from the XBB family of subvariants by about 48% compared to people who did not receive the vaccine, according to a study from the Centers for Disease Control and Prevention.
The CDC study published Wednesday provides the first estimate of the omicron booster 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.
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 who did not. 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 officials 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 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 as a consequence, due to waning immunity observed with the old vaccines, according to CDC officials.
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.
This is a developing story please. Check back for updates.