The updated boosters are cutting the risk that a person will get sick from Covid-19 by about half, even against infections caused by the rapidly spreading XBB.1.5 subvariant.
The studies, conducted by researchers at the US Centers for Disease Control and Prevention, are one of the first looks at how the bivalent boosters have continued to work in the real world as the virus has evolved. The data shows that the boosters are continuing to offer substantial protection against currently circulating variants.
The near real-time data was collected by Increased Community Access To Testing program, which administers Covid-19 tests through pharmacies. It includes results for adults receiving tests at participating pharmacies from December 1, 2022 to January 13, 2023.
Out of nearly 30,000 test results included in the analysis, more than 13,000 (47%), were positive for Covid-19.
More people who tested negative had gotten an updated, bivalent booster compared with those who tested positive.
The study results show that the updated boosters are most effective for younger adults.
For adults between the ages of 18 and 49, the boosters cut the odds of getting a symptomatic infection caused by the BA.5 subvariant by 52%, and it cut the odds of getting an infection caused by XBB or XBB.1.5 by 49%. For adults ages 50 to 64, the new boosters cut the odds of getting sick with Covid-19 by 43% for BA.5 and 40% for XBB subvariants. For those age 65 and older, the boosters cut the odds of an infection with symptoms by 37% and 43% for BA.5 and XBB subvariants, respectively.
The study authors saw little evidence of waning effectiveness two to three months after people got their shots.
The study authors said that these are just estimates of how well the vaccines are protecting people against an infection that brings on symptoms like cough or fever. They are probably working even better against more severe outcomes like hospitalization and death.
“What we know from past experience is generally that the vaccines protect better against more severe disease. So these are estimates for symptomatic infection and we would expect that similar estimates for hospitalization and death would be higher,” lead study author Ruth Link-Gelles, a senior epidemiologist at the CDC, said in a news briefing on Wednesday.
Link-Gelles cautioned that these vaccine effectiveness numbers are averages. Because everyone is unique in terms of their underlying health, their past exposure to the virus, and other factors, these estimates of vaccine effectiveness may not apply on an individual level. She said it’s important to think of them on population level.
To hasten the study results to the public, the researchers used a shortcut to estimate which Covid-19 infections were caused by the BA.5 subvariant and which were caused by the newer XBB recombinant subvariant and its XBB.1.5 sublineage.
Test results use a series of probes, or markers, to identify a positive case. Some variants of the virus that causes Covid-19 have mutations in their spike protein that causes one of the test markers to fail. This is called an S-gene target failure.
In the study, test results that showed an S-gene target failure were considered to be an infection caused by a BA.5 subvariant. Those that were S-gene target positive, were considered to be caused by the XBB or XBB.1.5 sublineage.