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In the welter of news about the Delta variant spreading around the world, one theme has emerged: This form of the virus that causes Covid-19 is challenging, but vaccination works to protect people against it.

A new study published Thursday in Nature adds new detail about the dominant variant, analyzing how well Delta, in a lab dish, was able to evade monoclonal antibody drugs such as bamlamivimab and natural antibodies made in our bodies after infection or vaccination. Looking at both kinds of antibodies in blood drawn from 162 patients and how they reacted to Delta, researchers from the Institut Pasteur in France found lower protection against the variant than against three other variants also notable for how easily they spread from person to person. 


“This is an important study for confirming the immune evasiveness property of Delta, which is a feature that adds to its enhanced transmissibility, making it the most formidable version of the virus to date,” Eric Topol, director and founder of the Scripps Research Translational Institute, told STAT. “No surprises, but further characterization of the variant, which reinforces why it is so challenging.”

The findings underscore the effectiveness of vaccines against the Delta variant if people get the recommended two doses. They also highlight the added immunity a single dose of vaccine provides people previously infected with another Covid variant.

“All the data shows that people who are fully vaccinated still enjoy robust protection against Delta and every other variant described so far,” said Angela Rasmussen, a virologist and research scientist affiliated with the Georgetown Center for Global Health Science and Security and VIDO-InterVac at the University of Saskatchewan. “Delta remains a threat primarily to unvaccinated or partially vaccinated people.”


Indeed, the study found the lack of protection against Delta compared to two other recent variants — Alpha and Beta — and a reference variant similar to the early virus, was most pronounced in unvaccinated, uninfected people. Delta now accounts for more than half of Covid-19 infections in the United States, the Centers for Disease Control and Prevention estimates. That proportion shoots up in certain states and soars in countries where vaccination levels are lower. 

Antibodies from 56 people who were infected with SARS-CoV-2 in the past did not neutralize Delta very well, but after one dose of either the AstraZeneca, Pfizer/BioNTech, or Moderna vaccines, antibodies from all 56 neutralized all four strains. 

That makes sense, Rasmussen said, because there is a much larger range in both the magnitude and potency of immune responses in infection compared to vaccination. 

Topol said we need to know more about the variant, which first emerged in India and has been surging in England.

“It is concerning about the natural immunity providing less protection, but to date [Public Health England-UK] has not demonstrated that to be a major issue,” Topol said. “Certainly more study about reinfection vulnerability to Delta is vital.”

In uninfected people who had only their first vaccine dose, their antibodies weren’t up to the task until after their second dose, when protection against Delta jumped to 88% of the people studied for AstraZeneca and 100% for Pfizer/BioNTech. 

The data about a steep reduction in neutralization in people with only one shot is consistent with real-world data on Delta from partially vaccinated people, which shows that people with only one shot don’t have much protection against Delta relative to unvaccinated people,” Rasmussen said. 

Monoclonal antibodies were unable to bind to the spike protein of the Delta variant, the study found, which prevented them from neutralizing the virus. Like other variants of concern, Delta harbors changes in the characteristic spike protein that gives the coronavirus its name.

“We show that the increased transmissibility of variant Delta is associated with partial resistance to neutralization by antibodies,” Olivier Schwartz, head of the Virus & Immunity Unit at Institut Pasteur and a study co-author, said in an interview. “It is reassuring to see that a two-dose regimen protects against all known variants so far,” he said, but added “the virus will likely continues to evolve.”

The study’s limitations include its small size, its authors noted. Rasmussen cautioned that the research took place in a lab.

“Evasion of neutralizing antibodies in vitro do not explain what happens in the real world, and also fail to account for the breadth and complexity of the immune response, which is not restricted to neutralizing antibodies,” she said. “Based on what we’ve seen so far in people, vaccines continue to offer strong protection — particularly regarding severe illness and death — against all known variants of concern.”

This story has been updated to add comments from a study co-author.

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