Updated Covid-19 boosters continue to provide substantial protection even against the rapidly spreading XBB.1.5 subvariant


The updated Covid-19 boosters reduce the risk of a person getting sick from the coronavirus by about half, even against infections caused by the rapidly spreading XBB.1.5 subvariant.

New studies, conducted by researchers at the U.S. Centers for Disease Control and Prevention, are among the first to look at how the bivalent boosters continued to work in the real world as the virus evolved. The data shows that the boosters continue to provide substantial protection against variants currently in circulation.

The near real-time data was collected by the federally funded Enhanced Community Access To Testing program, which administers Covid-19 testing through pharmacies. It includes results for adults undergoing testing at participating pharmacies from December 1 to January 13.

Of the 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 received an updated bivalent booster compared to those who tested positive.

On average, people in the study who had not received a bivalent booster had also not had a dose of a Covid-19 vaccine for more than a year. That’s about the same as the national average, the study authors said. Their protection against disease was probably very minimal, they said.

The study results show that the updated boosters are most effective for younger adults.

For adults aged 18 to 49 years, the boosters reduced the chance of getting a symptomatic infection caused by the BA.5 subvariant by 52%, and reduced the chance of getting an infection caused by XBB or XBB.1.5 by 49%. For adults aged 50 to 64, the new boosters reduce the chance of becoming ill with Covid-19 by 43% for BA.5 and 40% for XBB subvariants. For those 65 years and older, the boosters reduce the likelihood of symptomatic infection by 37% and 43%, respectively, for BA.5 and XBB subvariants.

Ruth Link-Gelles, a senior epidemiologist at the CDC and lead author of the study, said at a news conference Wednesday that these vaccine effectiveness numbers are averages. Because everyone is unique in terms of their underlying health, their previous exposure to the virus, and other factors, these estimates of vaccine effectiveness may not apply at the individual level. She said it’s important to look at them at a population level.

For people wondering if the protection from the bivalent booster they got in September has now worn off, it’s too early to know how decreasing would work with these new dual-species shots, Link-Gelles said.

So far, there’s little evidence of decreasing effectiveness two to three months after people get their injections.

“It is too early, I think, to know how the bivalent vaccine will decline. We know from the older vaccines that we do see protection decreasing over time, especially against symptomatic infection. Just like with general protection, what we’ve seen in the past is that your protection lasts longer for more serious illnesses,” Link-Gelles said.

Researchers don’t have data beyond three months, she said, but based on experience, she would expect protections against serious illness and death to be higher and longer lasting than these results against infections.

“We will continue to monitor it over time in the coming months,” she said.

The study authors said these are just estimates of how well the vaccines protect people against an infection that causes symptoms such as a cough or fever. They probably work even better against more serious consequences such as hospitalization and death.

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

When asked how well the two-strain vaccines work compared to the older single-strain shots, Link-Gelles said it was impossible to know.

“We can’t make a direct, head-to-head comparison between the monovalent and the bivalent vaccines in the US because they were never approved at the same time,” she said. Because protection diminishes over time, compare groups of people who received each type of shot at the same time.

“What this tells us is that people who had the bivalent vaccine were better protected than people who were up to date earlier, had all their monovalent doses and had not received the bivalent vaccine,” Link-Gelles said.

The CDC said it was able to analyze the data and publish it so quickly thanks to the use of a shortcut. Instead of sequencing the genomes of each positive result, the researchers relied on a different marker to distinguish between variants.

The tests used in the study rely on 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 cause one of the test markers to fail. This is called a failure of the S gene target.

In the study, test results showing a failure of the S gene target were considered 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 subline.

As research progressed, XBB.1.5 became a bigger player in the variant mix.

“Later in the study period, most would be XBB.1.5,” said Heather Scobie, an epidemiologist at the CDC.

This gave the researchers confidence that the vaccine effectiveness results reflect how well the vaccines are currently working.

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