Updated Covid-19 booster shots targeting the omicron variant offered similar protection to the original versions of the vaccines, according to two new preprint studies—suggesting there may not be a significant benefit to one vaccine over another.

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How do the bivalent boosters compare to the original shots?

In one study, researchers from Columbia University and the University of Michigan compared the levels of neutralizing antibodies in the blood samples of 21 people who received the updated bivalent booster and 19 people who received the original monovalent vaccine as a booster.

According to the researchers, individuals who received the updated boosters "had similar neutralizing antibody titers as those receiving a fourth monovalent mRNA vaccine" three to five weeks post-vaccination. Although the updated boosters generated 20% higher antibody levels against BA.4 and BA.5, this difference was not believed to be clinically significant.

In a second study, researchers from Beth Israel Deaconess Medical Center compared the blood samples of 15 people who got the original vaccine boosters and 18 people who received the updated boosters. Individuals who received the updated booster had 30% higher antibody levels, but the researchers said this difference was "modest and nonsignificant."

In addition, the researchers also examined participants' T-cell response, which helps generate long-term immunity against viruses, but neither vaccine significantly increased T-cell response.

"Both the monovalent and bivalent boosters work," said Dan Barouch, who runs Beth Israel's Center for Virology and Vaccine Research and led the second study. "But there is no evidence that the bivalent booster works better than the monovalent booster against BA.5."

Similarly, David Ho, a virologist who leads Columbia's Aaron Diamond AIDS Research Center and the first study's senior author, said that "[s]o far we don't see the benefit" of the updated boosters over the original Covid-19 vaccines.

However, Ho noted a difference in effectiveness between the two vaccines could emerge over time or that a second booster shot with the new bivalent vaccine could be needed to increase antibody levels more significantly.

According to Michael Worobey, a professor at the University of Arizona who was not involved with either study, the limited difference between the new and old vaccines was likely due to immune imprinting.

In a phenomenon known as original antigenic sin, people's immune systems generally carry the "imprint" of previous infections or vaccinations, which helps their bodies recognize a virus in the future. This can affect an individual's future immune response to a virus, as well as how well they respond to an updated vaccine.

Most of the vaccines people have received were targeted against the original coronavirus strain, which means their immune systems may generate antibodies based on that versus newer variants targeted by the updated boosters.

"Your body is on a hair trigger to create more antibodies of what it has a good memory of," Worobey said.

So far, Pfizer and Moderna have both declined to comment on either study and said they plan to release their own analyses of their respective boosters in the coming weeks.

The updated boosters still offer significant protection

According to Eric Topol, director of the Scripps Research Translational Institute who was not involved with the studies, these results "can be considered a disappointment" due to expectations that the updated boosters would offer increased protection, but this does not mean that people should avoid getting a booster at all.

"A booster is a booster until proven otherwise and we are in great need of getting more of them in the US," Topol said.

Similarly, Rob Stein, NPR's health correspondent said that "[e]ven if the new boosters aren't any better than the original vaccine, they still look like they're at least just as good at helping restore some of the immunity that has faded since people got their last shots or infections, and that could be lifesaving, especially for those who are most vulnerable, like the elderly."

Currently, CDC data shows that only around 20 million people, or less than 10% of eligible Americans, have received an updated booster since it was first authorized. Health officials have continued to encourage people to get a booster, especially with a potential winter surge coming.

On Tuesday, President Joe Biden publicly received an updated booster, which he said is "incredibly effective, but the truth is not enough people are getting it."

"We've got to change that so we can all have a safe and healthy holiday season," he added. (Langreth, Bloomberg, 10/25; Twenter, Becker's Hospital Review, 10/25; Saltzman/Cross, Boston Globe, 10/25; Goodman, CNN, 10/26; Stein et al., NPR, 10/25; Vazquez, CNN, 10/25)