Infection risk from COVID variant XBB.1.5 cut in half by new booster – San Francisco Chronicle

The updated COVID-19 boosters cut the risk of getting sick from newer omicron subvariants, including the fast-spreading XBB.1.5 strain, by about half.

In the first study of the shots’ real-world effectiveness, researchers at the U.S. Centers for Disease Control and Prevention found that the bivalent vaccines manufactured by Pfizer and Moderna continue to protect against the coronavirus despite its evolving evasiveness.

“Today we have additional evidence to show that these updated vaccines are protecting people against the latest COVID-19 variants,” Dr. Brendan Jackson, head of the CDC’s COVID-19 response, told reporters in a briefing on Wednesday.

The updated vaccine boosters, which became available last fall, were designed to target the original SARS-CoV-2 virus and the BA.4 and BA.5 omicron variants, which are no longer dominant. But in the review of about 20,000 COVID-19 cases between Dec. 1 through Jan. 13, when the XBB and XBB.1.5 variants were gaining dominance, researchers found the updated shots helped prevent illness in about half of the people who had previously received two to four doses of the original COVID-19 vaccine, the CDC said.

The article, published in the CDC’s Morbidity and Mortality Weekly Report, showed that among adults aged 18 to 49, the updated vaccine was 48% effective against symptomic infection by XBB/XBB.1.5, compared to 52% against BA.5. For those aged 50 to 64, the effectiveness was 40% compared to 43% against earlier variants.

Among those 65 and older, effectiveness fell to 43% against XBB/XBB.1.5 and 37% against BA.5. Early data show the bivalent shots also reduced the risk of death from COVID-19 by more than twofold in previously vaccinated people and by 13-fold compared with unvaccinated people.

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

The Centers for Disease Control and Prevention estimates that the immune evasive XBB.1.5 omicron subvariant drove nearly half of all coronavirus infections in the U.S. last week. On Friday, the agency estimated that the latest virus strain was detected in 49.1% of COVID-19 cases sequenced in the week ending Jan. 21, up from 37.2% of cases for the week ending Jan. 14.

XBB.1.5 has officially overtaken BQ.1.1., which made up 26.9% of circulating variants last week, and BQ.1, which made up 13.3%. Its parent XBB variant was detected in 3.3% of cases. XBB.1.5’s prevalence varies by region. The strain, which public health experts call the most transmissible COVID-19 subvariant so far, was detected in about 85% of infections in the Northeast but just 24% of cases on the West Coast.

Scientists have been squabbling over the efficacy of the new shots against newer variants since they first became available.

Two papers and a commentary from prominent scientists published in the New England Journal of Medicine earlier this month offered evidence that the quickly developed bivalent COVID vaccine boosters from Pfizer and Moderna might not be much better than the original monovalent mRNA boosters at preventing coronavirus infection.

“Boosting with new bivalent mRNA vaccines targeting both the BA.4/BA.5 variant and the D614G strain did not elicit a discernibly superior virus-neutralizing peak antibody response as compared with boosting with the original monovalent vaccines,” a research team led by David Ho at Columbia University wrote of their pseudovirus study.

Another study found that both the monovalent and bivalent boosters “markedly increased antibody responses” but did a better job neutralizing ancestral strains of the coronavirus — for which the vaccines were originally developed — than the more recent BA.5 omicron descendant. All told, wrote the team at the Dan Barouch Lab at Harvard’s Beth Israel Deaconess Medical Center, “the median BA.5 neutralizing antibody titer was similar after monovalent and bivalent mRNA boosting, with a modest trend favoring the bivalent booster by a factor of 1.3.”

The researchers also found that neither monovalent nor bivalent boosters substantially augmented immune response from the foundational T-cells that fight off infection in tandem with antibodies.

Paul Offit, a member of the vaccine advisory group to the FDA and booster skeptic, wrote an analysis of the Ho and Barouch studies in which he argued that given their limited incremental benefit, bivalent boosters might best be aimed at high-risk individuals.

“Although boosting with a bivalent vaccine is likely to have a similar effect as boosting with a monovalent vaccine, booster dosing is probably best reserved for the people most likely to need protection against severe disease — specifically, older adults, people with multiple coexisting conditions that put them at high risk for serious illness, and those who are immunocompromised.”

Eric Topol of Scripps Research in San Diego replied sharply in a Substack essay that despite these findings there are still benefits to promoting bivalent shots, which he asserts outperform monovalent doses in preventing serious illness, including crucially from more recent coronavirus strains such as BQ and XBB.

“Bivalent boosters work well to prevent severe COVID, as manifest by reduction of hospitalizations and deaths. They are not a panacea, by any means — their efficacy against infections is limited and of short duration, which has been the case for shots since the omicron variant came along in late 2021.” He added, “We’re lucky in the United States to have a big supply and it’s frankly disappointing to see the divisiveness, cherry-picking of data, and detractors that defy the body of evidence that has now accumulated. The bivalent booster impact of broadening our immune response has exceeded expectations.”

About 50 million Americans, or a little over 15% of the population, have received the updated shots compared to nearly 270 million, or 81% of the population, who received at least one dose of the original vaccine.

The CDC officials hope the new study puts the issue to rest.

“So bottom line, from this first MMWR is that 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,” said Jackson during the briefing. “People who received the updated or bivalent COVID vaccines had higher levels of protection than those who received only the previous vaccines with only the monovalents.”

Aidin Vaziri is a San Francisco Chronicle staff writer. Email: