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 written 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 the latest estimate from the Centers for Disease Control and Prevention — full vaccination offers a much better firewall against infection than partial vaccination.
“Please, get vaccinated. It will protect you against the surging of the delta variant,” Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said Thursday at a White House coronavirus briefing.
The studies on the delta variant emerge as concerns grow globally about a resurgence of the coronavirus. With infections increasing in Japan, that country’s prime minister Thursday declared a state of emergency in Tokyo, and organizers of the Tokyo Olympic Games moved to bar spectators from all events in and around the city.
Data released this week by Israel’s Ministry of Health showed vaccine efficacy against the coronavirus waning in that highly vaccinated country in tandem with the spread of the delta variant. The Israeli data showed lower protection against symptomatic illness, only 64 percent, although vaccinated people retain protection against severe illness.
Pfizer and its partner BioNTech released a statement Thursday evening saying they would seek federal regulatory approval for a booster shot following studies that found such a shot caused disease-blocking antibodies to increase 5 to 10 times higher than after the two-shot regimen. The companies also said they plan to start clinical trial research using a reformulated vaccine designed to thwart the delta variant.
“We continue to believe that it is likely, based on the totality of the data we have to date, that a third dose may be needed within 6 to 12 months after full vaccination,” the statement said. “While protection against severe disease remained high across the full 6 months, the observed decline in efficacy against symptomatic disease over time and the continued emergence of variants are key factors driving our belief that a booster dose will likely be necessary to maintain highest levels of protection.”
The emergence of the delta variant is neither entirely surprising — virologists have warned that SARS-CoV-2 will continue to mutate — nor welcome, coming as many nations that have been mired in pandemic mode for a year and a half are trying to return to normal activity. England, for example, plans to lift all restrictions by July 19 despite a surge in delta infections, which accounts for an estimated 95 percent of new cases. Britain’s health secretary, Sajid Javid, told BBC Radio that coronavirus cases could reach 100,000 a day this summer.
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 research on Pfizer is presumed to apply to the similar Moderna vaccine. The one-shot Johnson & Johnson vaccine regimen, by contrast, offers roughly the same protection against severe disease after one shot but is somewhat less effective at preventing mild to moderate symptomatic cases, according to earlier clinical trial data.
Britain 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.
Gandhi has warned against overreaction to the delta variant’s new dominance. She has said there is no evidence the delta variant is more deadly for an individual who becomes infected, a view echoed by Paul A. Offit, a pediatrician and vaccine expert at Children’s Hospital of Philadelphia.
“There’s no good evidence that this variant causes more serious disease,” Offit said. But he expressed concerns that the large number of people who are unvaccinated — including, because they are not yet eligible, children under 12 — could fuel a winter wave of infections.
“It’s a winter virus. I think it’s going to surge this winter,” he said. “We have a lot of people in America who are not vaccinated, and they are going to be fertile ground for this virus.”
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. Numerous studies in recent months suggest that the vaccines can hold the line 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 suggest the delta variant, though slippery, struggles to evade vaccine-induced immunity. The researchers looked at blood samples from more than 100 people, most of them unvaccinated and having recovered from an infection. The study also looked at people who had been given a single dose or two doses of a vaccine.
The analysis found that antibodies from a natural infection were less able to neutralize the delta variant, particularly a year after infection. But among samples from people fully vaccinated, 95 percent retained sufficient antibodies to thwart the virus, said Olivier Schwartz, lead author of the Nature study and head of the Virus and Immunity Unit at the Pasteur Institute in Paris.
He said this should be seen as “good news,” adding that future research will determine how long the post-vaccine 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. Compared with the original strain of the virus, one of the delta lineages was 6.8 times less susceptible to neutralization by antibody-loaded serum taken from vaccinated people and 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 people probably still had “protective immunity” from either sublineage of the delta variant.
This type of research has become critically important as the delta variant, 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 percent 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.
“Although we expected the delta variant to become the dominant strain in the United States, this rapid rise is troubling,” CDC Director Rochelle Walensky said in the White House briefing. “We know that the delta variant has increased transmissibility, and it is currently surging in pockets of the country with low vaccination rates.”
She said preliminary data from several states show that 99.5 percent of deaths from covid-19 in recent months have been among people who were unvaccinated.
In response to a reporter’s question, Walensky said that people who are fully vaccinated and experience symptoms of upper respiratory infections — runny nose, cough, sore throat — should get tested for the coronavirus. That has been the CDC guidance since May, but may not be widely known.
The delta variant 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 think 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 professor in the department of immunology and microbiology 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 wrote 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.”
It is increasingly clear that the virus — the precise origin of which remains unclear and the subject of fraught political and scientific debate — was not exquisitely adapted to human beings when it began to spread person to person. All viruses mutate, and SARS-CoV-2 does not mutate particularly fast. But as it spread around the world, 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 variant, in turn, is believed to be 60 percent more transmissible than the alpha variant, according to the study in Nature.
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.”
Lindsey Bever and Carolyn Y. Johnson contributed to this report.