Americans who got their COVID-19 bivalent booster vaccine are not fully protected against getting sick, according to a new report.
The Centers for Disease Control and Prevention released a report that said the updated boosters were just 48% effective at preventing symptomatic infection from the XBB variant of the disease.
The World Health Organization has a 50% threshold to deem a vaccine effective — meaning the bivalent shots fall just below their benchmark when targeting the dominant XBB strain.
However, the CDC noted that the main function of the vaccine is to prevent hospitalization and death, rather than transmission and minor illness from the virus — and the booster is still expected to provide protection against severe illness.
Indeed, a 2022 study published in the New England Journal of Medicine concluded that those who recently received a booster and were previously infected from the virus — as opposed to those who skipped vaccines and relied on natural immunity — benefited from the strongest protection against further illness from COVID.
Researchers at the CDC compiled data from a government COVID testing initiative conducted between December 1, 2022 through January 13, 2023 — when XBB and XBB.1.5 were the dominant strains — and accounted for symptomatic infection, which is defined by one or more symptoms.
They determined whether the tested people were vaccinated and compared it to overall population vaccine data, finding that the people who received the booster were 48% more protected than those who did not get it.
Only 15% of the US population has received the bivalent booster, according to the CDC.
The bivalent booster was the fourth or fifth dose for most participants and was 40-50% effective from two weeks to three months after receiving it in almost all age groups.
Effectiveness against Omicron and its subvariants is just slightly lower than the vaccine’s 52% effectiveness against the BA.5 variant.
Some scientists even called the highly mutated variant the most vaccine-resistant strain ever.
The CDC found that about half of those who received the booster went on to test positive for COVID. However, the updated booster vaccine reduced the risk of death from COVID by more than twofold compared to previously vaccinated people without the bivalent booster, and by nearly 13-fold for those who have never been vaccinated for COVID.
Overall, the shot cuts the risk of symptomatic infection by about half of a population, but people will see different results and benefits based on risk factors, said Ruth Link-Gelles, author of the study.
In 2020, the original Pfizer and Moderna COVID-19 vaccines reduced symptomatic infections by 95%, as seen in clinical trials.
But as COVID continued to mutate and new variants were formed, the shots began to lose their strength.
The vaccines fell to 36% effective at preventing illness by the time the original Omicron variant became the main strain in late 2021.
Omicron and its subvariants led the vaccine producers to make updated shots to specifically target the new dominant variant. The bivalent booster shots were designed to boost immunity against Omicron.
The Omicron boosters contained the variant’s genetic code as well as a piece of the original strain.
As of mid-January, the XBB.1.5 sub-variant made up 49% of the CDC sequenced cases — and it’s expected that it has risen since.
Despite the lower effectiveness rate, the CDC said that hospitalizations and deaths were still limited, and the vaccine rollout has been deemed a success.