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People who have received two doses of the Pfizer-BioNTech Covid-19 vaccine are likely susceptible to infection from the Omicron variant of the coronavirus, but a third shot restores antibody activity against the virus, the companies said Wednesday.

Their findings are based on lab experiments using the blood of people who have received the vaccine. In a press release, the companies reported a 25-fold reduction on average in the levels of neutralizing antibody activity against the Omicron variant in people who had received two doses of their Covid-19 vaccine. But they also found that blood, or sera, from people who had received boosters “neutralized the Omicron variant to levels that are comparable to those observed” for the original form of the coronavirus that the vaccine was based on, with neutralizing antibody levels increased 25-fold by a third shot.

The results from Pfizer echo those reported on Tuesday by a team of scientists in South Africa.

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Also Wednesday, scientists in Germany released preliminary data looking at how different vaccines and vaccine combinations fared against Omicron. The results showed drops on similar scales in neutralizing power against Omicron versus the Delta variant. Data from other neutralization studies are expected to be released soon, the result of a fast and furious global rally to understand the latest variant threat.

These studies provide a measure only of how well neutralizing antibodies generated from vaccination or an earlier infection continue to recognize new variants and block them from infecting cells. Experts caution that there are limits to what can be gleaned from these lab experiments and that they cannot be used to definitively say how Omicron may lower vaccine effectiveness in the real world. Vaccines and infections trigger different layers of immune protection beyond neutralizing antibodies, including T cells, which are fighters more important to guarding people from serious outcomes than to blocking infection.

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Though more research is to come, experts say that the initial results from neutralization studies indicate that two doses of the most common vaccines might not be able to put up much of a fight against infection from the Omicron variant. (No data are available yet on the one-shot Johnson & Johnson vaccine.) Still, many experts believe that protection against severe disease would be preserved for most people who have received the primary vaccine regimen, though the neutralization studies don’t shed much light on that aspect. At the same time, the data suggest that someone who has recovered from an infection and been vaccinated, or has had a booster shot on top of the primary series, will mount a response that should restore overall protection against Omicron.

Pfizer and BioNTech drew a similar conclusion in their press release. Two doses of their shot “may not be sufficient to protect against infection with the Omicron variant,” the companies said. But they said that other components of immune protection, including vaccine-elicited T cells, are not as affected by Omicron’s mutations, and that “the companies believe that vaccinated individuals may still be protected against severe forms of the disease.”

Pfizer and other vaccine manufacturers are starting to develop Omicron-specific vaccines should they be needed.

The World Health Organization has for months argued that countries should pause booster shots for most people until a greater share of people around the world had received their first and second doses — a message that fell on deaf ears in wealthy nations that pushed forward with booster campaigns.

Asked Wednesday if the group’s thinking on boosters had changed in the face of Omicron, Soumya Swaminathan, WHO’s chief scientist, said that the data available so far were from a small number of participants and that there was wide variability in the results, from a fourfold to over a 40-fold reduction in neutralizing antibody activity against the variant. She also noted that the real-world implications of these lab studies have yet to be determined.

“We really haven’t got information on the effectiveness,” Swaminathan said. “So I think it’s premature to conclude that this reduction in neutralizing activity would result in a significant reduction in vaccine effectiveness. We do not know that.”

She added: “What we really need now is a coordinated research effort, and not jumping to conclusions study by study.”

The different results from the neutralization studies can be explained in a variety of ways. While the studies report average drops in neutralization, the researchers note that there is wide variability in individual participants’ responses to Omicron, and that most of the studies are based on a relatively small number of patient samples. The results might also reflect the timing of when the sera was collected from participants — meaning how long after vaccination or infection — as well as different experimental methods.

The Pfizer-BioNTech study relied not on a live version of the Omicron variant like the South African study, but what’s called a pseudovirus. Essentially, scientists engineer a version of the SARS-CoV-2 spike protein — complete with Omicron’s mutations — and plug it into a backbone of another virus. It’s an easier and faster way to study an evolving virus than obtaining and growing an actual sample.

Even as neutralization data pour in, experts are also going to be studying how vaccines stand up to Omicron in the real world — a process that will take longer. They will track, for example, whether breakthrough infections increase with Omicron and whether vaccine protection wanes faster in the presence of the variant. They will also monitor whether more breakthrough infections — which overwhelmingly have only caused mild outcomes so far — lead to more serious outcomes like hospitalization or death, a sign that boosters or even Omicron-specific vaccines might be needed to combat the variant.

In the United States, only about 1 in 4 adults who are fully vaccinated have received a booster shot, according to federal data, though health authorities have reported seeing an increase in interest since Omicron emerged. Among people 65 and older, who are more vulnerable to worse outcomes from Covid-19, 48.1% of fully vaccinated people have received a booster.

Helen Branswell contributed reporting. 

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