Pfizer and BioNTech announced on Thursday that they were developing a version of the coronavirus vaccine targeted at Delta, a highly contagious variant that has spread to nearly 100 countries. The companies expect to start clinical trials with the vaccine in August.
Pfizer and BioNTech also reported promising results from studies of humans receiving a third dose of the original vaccine. A booster given six months after the second dose of the vaccine increases the potency of antibodies to the original virus and the Beta variant by five to ten times, the companies said.
Vaccine efficacy may decline six months after immunization, companies said in a press release, and booster doses may be needed to ward off virus variants.
But the companies’ claims contradict other research, and several experts pushed back against the claim that boosters will be needed.
“There is really no indication of a third booster or a third dose of an mRNA vaccine given the variants that we are circulating at this time,” said Dr. Céline Gounder, an infectious disease specialist at Bellevue Hospital Center in New York. “In fact, many of us question whether you’ll ever need boosters.”
Federal agencies also sounded a dubious note Thursday night. In general, Americans who have been fully vaccinated do not need a booster shot at this point, the FDA and the Centers for Disease Control and Prevention said in a joint statement.
“We are prepared for booster doses if and when science shows they are necessary,” the agencies said.
The Delta variant, first identified in India, is thought to be about 60 percent more contagious than Alpha, the version of the virus that tore through Britain and much of Europe earlier this year, and perhaps twice as contagious as the original coronavirus.
The Delta variant is now driving outbreaks among unvaccinated populations in countries such as Malaysia, Portugal, Indonesia and Australia. Delta is also now the dominant variant in the United States, the CDC reported this week.
Until recently, infections in the United States had the plateau at their lowest levels since early in the pandemic. Admissions and deaths related to the virus continue to decline, but new infections may increase.
It is not yet clear to what extent the variant is responsible; a slower vaccination run and rapid reopens also play roles.
Referring to data from Israel, Pfizer and BioNTech suggested that the effectiveness of their vaccines “in preventing both infection and symptomatic disease has decreased six months after vaccination.” They noted the increase in Delta and other variants and said that “a third dose may be needed within 6 to 12 months after full vaccination.”
Health officials in Israel have estimated that full vaccination with Pfizer-BioNTech offers only 64 percent efficacy against the Delta variant. (Efficacy against the original virus is greater than 90 percent.)
But Israel’s estimates are contradicted by a number of other studies that show that the vaccine is extremely effective in preventing infection – against all variants. A recent study showed, for example, that mRNA vaccines like Pfizers trigger a sustained immune response in the body that can protect against coronavirus for years.
“Pfizer looks opportunistic by hanging a message on the back of very early and undigested data from Israel,” said John Moore, a virologist at Weill Cornell Medicine in New York. “When the time comes to use boosters here, it’s not their decision to make.”
The companies described their plan to develop a new vaccine against Delta as a kind of backup effort should boosters of the original vaccine fail. The new vaccine will target the entire nail protein as opposed to a part, and the first batch has already been produced.
The Delta variant presents challenges for the immune system. In the journal Nature, French researchers on Thursday reported new evidence that the Delta variant may partially bypass the body’s immune response due to changes in the tip protein on the surface that make it more difficult for antibodies to attack.
The team analyzed blood samples from 59 people after receiving the first and second doses of the vaccines. Blood samples from only 10 percent of people immunized with a dose of the AstraZeneca or Pfizer BioNTech vaccines were able to neutralize the Delta and Beta variants in laboratory experiments.
“A single dose of Pfizer or AstraZeneca was either poorly or not at all effective against Beta and Delta variants,” the researchers concluded. Data from Israel and the United Kingdom broadly support this finding, although these studies also suggested that a dose of vaccine was still sufficient to prevent hospitalization or death of the virus.
But another dose increased efficacy to 95 percent. There was no major difference in the levels of antibodies elicited by the two vaccines.
“If you receive two doses of an mRNA vaccine, you are very well protected against serious illness, hospitalization and death with regard to any of the variants,” said Dr. Gounder.
The researchers also looked at blood samples from 103 people who had been infected with coronavirus. Delta was much less sensitive than Alpha to samples from unvaccinated humans in this group, the study found.
One dose of vaccine significantly increased susceptibility, suggesting that people who have recovered from Covid-19 still need to be vaccinated to ward off some variants.
Overall, the results suggest that two doses of vaccine are highly protective against all variants, just as one dose is for people who have recovered from Covid-19 and have some natural immunity.
Some experts also questioned discussions about boosters for Americans, while many are yet to receive a single dose.
“It’s impossible to ignore the global situation,” said Natalie Dean, a biostatistician at Emory University in Atlanta. “It’s hard for me to imagine getting a third dose when there are frontline workers treating Covid patients who have still not been vaccinated.”
Each unvaccinated person offers the virus additional opportunities to mutate into dangerous variants, noted Dr. Gounder.
“If we’re concerned about variants,” she said, “our best protection is to get the rest of the world vaccinated, not to collect more doses to give third doses of mRNA vaccines to people here in the United States.”