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A small, preliminary study has found a significant drop in how well vaccine-elicited antibodies target the Omicron variant of the coronavirus. But the variant did not completely dodge the immune fighters, the research found.

The results support the hypothesis that the Omicron variant is a larger threat to immunity against Covid-19 than other variants, but experts caution that the implications for real-world protection are limited.

Though the study did not include people who had received booster doses, the research also suggests that people who were previously infected and then vaccinated, or people who’ve had boosters, should maintain greater levels of protection even against Omicron.

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The data, which were posted online Tuesday and have not been peer-reviewed, are some of the first to be released about the impact Omicron has on our immune protection. The results are also, in a way, expected.

The number and specifics of the mutations that the Omicron variant acquired led experts to anticipate a drop in neutralization activity. Essentially, because of the mutations, the neutralizing antibodies generated by the vaccines don’t recognize this version of the virus as well as the original form of the SARS-CoV-2 virus, which is what Covid vaccines currently target.

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In the study, Alex Sigal of the Africa Health Research Institute and a team of colleagues put the Omicron variant up against blood, or sera, from 12 people who had been vaccinated with the Pfizer-BioNTech regimen; six of the people had also previously had Covid-19 during South Africa’s first wave.

The scientists found that, overall, there was a 41-fold reduction in neutralization against Omicron compared to an early form of the coronavirus, a substantial drop but not necessarily a devastating one. And, the researchers noted, “the escape was not complete.”

Notably, five of the participants, all of whom had been previously infected, maintained “relatively high neutralization [levels] with Omicron.”

That suggests, the authors and outside experts said, that previous infection combined with vaccination, or primary vaccination combined with booster doses, can amp up the body’s neutralizing power even against a variant as evolved as Omicron.

“The fact that there is detectable neutralization in sera from those infected and vaccinated is a good sign,” Ali Ellebedy, an immunologist at Washington University in St. Louis, said in an email. “It means that cross-reactive antibodies are there … we just need more of them and that is what the third shot is likely doing.”

Many experts have said their best guess is that Omicron is likely to lead to more breakthrough infections and reinfections, but that for most people, protection against more serious outcomes will be preserved. The new study lends credence to that hypothesis.

A “41-fold reduction is a significant hit and does not bode well for preventing infection,” Larissa Thackray, a virologist at Washington University in St. Louis, wrote in an email, though she added that the data on people who were previously infected “suggest that these people and potentially triple-vaccinated people may fare better.”

Florian Krammer, a vaccinologist at Mount Sinai’s Icahn School of Medicine, interpreted the results similarly.

“There doesn’t seem to be much neutralizing activity left” for people vaccinated with the Pfizer shot, Krammer said, adding that he expected similar results for people who received the Moderna shot as well as people who previously had Covid-19. That meant the protection against infection “will be marginal,” Krammer wrote in a Twitter message.

However, Krammer speculated that protection against severe disease would remain “robust.”

“For people infected and then vaccinated and for people who got booster shots the situation likely looks much better,” he wrote, adding he was most concerned about people who had not been vaccinated at all yet.

These neutralization studies measure only how well these types of antibodies recognize certain variants. Experts note that other layers of the immune system, including T cells, could be less affected by the mutations in the virus, and that they are more important to protecting people against more serious outcomes, even if they can’t halt infections entirely.

The study from Sigal and colleagues is just one of what are expected to be a host of studies looking at the effect of Omicron on our immune protection — and on the different vaccines. Experts noted that they want to see the results from more people than 12 participants, and on Twitter, Sigal noted that this was the first set of data and the results could “be adjusted as we do more experiments.”

Experts are also examining whether Omicron can compete with the highly transmissible and globally dominant Delta variant and if the severity of disease it causes has changed versus other variants.

Angela Rasmussen, a virologist at the University of Saskatchewan’s Vaccine and Infectious Disease Organization, said in a Twitter message the results of the new study were “fairly expected” based on analyses of its mutations. But, she added, “I am relieved that it didn’t exceed expectations for reduced neutralization.”

The new research also indicated that the ACE2 receptor on human cells — which the coronavirus uses to infect our cells — was still required for Omicron to get into cells. And on Twitter, Sigal sounded an optimistic note about what he and his colleagues had found.

“this was better than I expected of Omicron,” he wrote. “The fact that it still needs the ACE2 receptor and that escape is incomplete means its a tractable problem with the tools we got.”

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