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To Fight Delta Variant, Study Says 2 Shots Are Better Than 1

Going out into the world "half-vaxxed" is not enough of a safeguard against the virus mutation now dominating the United States and other parts of the world.

(CN) — Able to spread more readily than past versions of the coronavirus that causes Covid-19, the worrisome delta variant may also be resistant to some of the antibody treatments and even partial vaccination, a new study shows. 

Researchers tested the blood of people previously infected with the virus, as well as those who had received only one dose of either the Pfizer or AstraZeneca vaccine, finding that those protections were not enough to neutralize the variant. After two doses, however, the vaccines were able to knock it out. 

The results were published in the journal Nature

Olivier Schwartz, study author and the head of the virus and immunity unit at France’s Institut Pasteur, said the results help to explain some of the reasons behind the delta variant’s increased transmissibility, especially in partially unvaccinated countries. 

“We show that the increased transmissibility of variant Delta is associated with partial resistance to neutralization by antibodies,” Schwartz wrote in an email to Courthouse News. 

The delta variant has worried health officials because of its ability to spread rapidly, potentially making more people sick. It was first identified in India, and has spread to more than 90 other countries, including the United States. 

Schwartz said the new study points to why the variant is able to get around so reliably. 

“This study describes why vaccination is protective against variant Delta only after two doses of vaccine,” Schwartz said. “A single dose of vaccine does not trigger sufficient amounts of neutralizing antibodies.” 

He and his colleagues examined the blood sera from 103 people who recovered from an infection of SARS-CoV-2, the technical name for the novel coronavirus that spurred the global pandemic. They also tested the blood of 59 people who recently got vaccinated, to see how both groups’ antibodies stood up against the delta variant, compared with earlier alpha and beta variants. 

Compared to the alpha variant, first detected in the United Kingdom, the delta variant was four times less sensitive to the antibodies in the blood of participants who had recovered from Covid-19, up to 12 months after their infection. 

Individuals who got just one shot of the Pfizer or AstraZeneca vaccines were either poorly or not at all protected against either the delta variant or the beta variant, which was first found in South Africa. 

With the second shot, though, the neutralizing response was 95%. Additionally the antibodies produced by the vaccines were three to five times less potent against delta as compared with alpha or beta.

Pfizer announced Thursday that it is developing a booster shot to target the delta variant.

Researchers previously reported that natural immunity — the kind you have after getting sick with the coronavirus and developing antibodies — offers a less robust protection than vaccination does. 

Widespread vaccination, health experts say, also limits the opportunities for new variants to develop. 

“Any time the virus transmits from person to person — the more it replicates, the more it transmits — there’s just more opportunity for it to develop mutations that could give rise to new variants,” said Josh Petrie, research assistant professor in the epidemiology department at the University of Michigan School of Public Health, in a recent interview with Courthouse News.

In addition to illuminating the effectiveness of one vaccine dose and natural immunity against the delta variant, the new findings also showed that the variant was resistant to laboratory-produced monoclonal antibodies, including the treatment bamlanivimab, used as therapies for those infected. 

Those antibodies were unable to bind to the delta variant’s spike protein, preventing them from neutralizing the virus. 

Schwartz said that future studies will help researchers gain an understanding of why the virus is “more contagious” and spreads more efficiently in people who are unvaccinated. 

For now, vaccine distribution efforts should continue to note that it’s crucial for vaccine recipients to finish the job, and get both required shots. 

“It is important to get the two-dose regimen of vaccines to avoid spread of variant Delta,” Schwartz said.

Follow Nina Pullano on Twitter.

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