New laboratory research on the swiftly spreading delta variant of the coronavirus is highlighting the threats posed by viral mutations, adding urgency to calls to accelerate vaccination efforts across the planet.

A peer-reviewed report from scientists in France, published Thursday in the journal Nature, found that the delta variant has mutations that allow it to evade some of the neutralizing antibodies produced by vaccines or by a natural infection. A single shot of a two-dose vaccine “barely” offers any protection.

But the experiments found that fully vaccinated people – with the recommended regimen of two shots of the Pfizer-BioNTech or AstraZeneca vaccine – should retain significant protection against the delta variant. That echoes another report authored by a collaboration of scientists in the United States and published Wednesday in the New England Journal of Medicine.

The bottom line is that, in a time when the delta variant is rapidly gaining traction – it now accounts for a majority of new infections in the United States, according to a new estimate from the Centers for Disease Control and Prevention – full vaccination offers a much better firewall against infection than partial vaccination.

The studies on the delta variant emerge as concerns grow globally about a resurgence of the coronavirus. On Thursday, citing the virus, the Japanese prime minister declared a state of emergency, and organizers of the Tokyo Olympic Games moved to bar spectators from all events held in and around Tokyo.

The first two authorized vaccines in the United States – shots from Pfizer and Moderna that use a technology called messenger RNA, known commonly as mRNA – are designed as a two-shot program. The Johnson & Johnson vaccine, by contrast, offers roughly the same protection against severe disease after just one shot although is somewhat less effective at preventing mild to moderate symptomatic cases, according to clinical trial data.

The United Kingdom adopted a strategy of giving people a first dose of a vaccine and delaying the second, intending to broaden the reach of the limited supply. But that has led to breakthrough infections driven by the delta variant, said Monica Gandhi, an infectious-diseases doctor at the University of California at San Francisco who was not involved in either research study.

The new research from France published in Nature “really verifies the need for the full two-dose vaccine regimen to get full effectiveness of the vaccine against the delta,” she said.

The coronavirus, SARS-CoV-2, continues to mutate as it circulates through the human population. The variants are more transmissible than the earliest strain that emerged in Wuhan, China, although there is limited and less compelling evidence that they are more likely to cause severe disease in any individual.

So far, the vaccines have held the line, by and large, against the onrushing swarm of variants. Experiments in the lab and real-world data show the vaccines are particularly effective at preventing severe illness.

The results of the research in France represent “good news,” said Olivier Schwartz, lead author of the Nature study and head of the Virus and Immunity Unit at the Pasteur Institute in Paris. Schwartz added that future research will determine how long the neutralizing antibody response lasts against the delta.

The New England Journal study was also based on laboratory experiments. It looked at two slightly different lineages of the delta variant. One was 6.8 times less susceptible to neutralization by antibody-loaded serum taken from patients who had recovered from covid-19, the disease caused by the virus. The other lineage – the one that is spreading most rapidly – was 2.9 times less susceptible.

Despite this erosion of neutralization, the researchers concluded that fully vaccinated persons probably still had “protective immunity” from either sublineage of the delta.

This type of research has become critically important as the delta, which emerged in India, outcompetes other strains of the virus. Data posted this week by the CDC showed that, as of July 3, the delta variant represented an estimated 51.7% of new infections nationally, five times the prevalence of just four weeks earlier.

In some regions it is particularly dominant. As of June 19, the delta variant was seen in 72 percent of new cases in Iowa, Kansas, Missouri and Nebraska, collectively, the CDC data show.

The delta is part of a growing list of coronavirus “variants of concern,” a designation given to variants no longer merely “of interest.”

The World Health Organization switched to Greek letters earlier this year to help people differentiate variants that had been known as the B.1.1.7 (alpha), the B.1.351 (beta), the B.1.617.2 (delta) and so on. Virologists believe it is possible that continued mutations in the virus will lead to a shortfall of Greek letters.

“If we continue to let the virus run loose, then yes, I’m certain we’ll run out of letters and future variants will likely be worse than the current crop – and yes, we’ll likely have several circulating at the same time,” Kristian Andersen, a virologist at Scripps Research Institute, said in an email.

Each variant contains a unique suite of mutations. Some of those mutations enhance transmissibility. Some make the virus more evasive when faced with antibodies and other immune system cells.

As virologists try to understand these microscopic processes, the world is conducting an experiment on a grand scale, with most of the planet still unvaccinated and the virus circulating with limited obstacles. The evidence is clear: The long war against the coronavirus depends on the thoroughness and speed of the global vaccination effort.

“We need to vaccinate the world NOW with an all-out effort led by the United States,” Andersen said in the email.

Michael Worobey, a University of Arizona virologist, agrees the virus has not run out of moves: “We may never see the end of new variants. The virus is likely to become a fixture of human infectious disease, just like influenza viruses.”

Research on the delta variant has largely emerged from laboratory experiments that expose the virus to blood from patients who have survived covid-19 or have vaccine-induced immunity. Simultaneously, a real-world experiment is afoot: The virus is circulating in parts of the United States and in many places globally where vaccination rates are low or, in some cases, virtually nonexistent.

It is increasingly clear that the virus – the precise origin of which is unclear and the subject of fraught political and scientific debate – was not exquisitely adapted to human beings when it first began to spread person to person. All viruses mutate, and SARS-CoV-2 does not mutate particularly fast. But as it spread across the planet, it had abundant opportunities to evolve.

One strain, with a mutation called D614G, emerged early in the pandemic and boosted the virus’s ability to infect people by roughly 20 percent. That mutation is now seen in almost every sample of SARS-CoV-2.

The alpha variant is roughly 50 percent more transmissible than the D614G strain. The delta is half-again more transmissible than the alpha, according to preliminary research.

For now, the evolution of the coronavirus has been driven largely by mutations that enhance its ability to bind to cells or grow in those cells. A secondary strategy of evading antibodies has been seen in several variants, and may become more significant over time as immunity builds in the human population, said Jesse Bloom, a virologist at the Fred Hutchinson Cancer Research Center in Seattle.

“We’ve seen the virus evolving to get better and better at human transmission. I would expect that process will sort of plateau,” Bloom said. “But I don’t think the evolution will stop. Because I think there is a sort of endless potential for the virus to get mutations to escape from antibodies.”T

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