Pfizer and BioNTech announced Thursday that they are developing a version of the coronavirus vaccine that will target Delta, a highly contagious variant that has spread to nearly 100 countries. The companies expect to begin clinical trials of the vaccine in August.

Pfizer and BioNTech also reported promising results from studies of people who received a third dose of the original vaccine. A booster shot six months after the second dose of the vaccine increases the effectiveness of the antibodies against the original virus and beta variant by five to ten-fold, the companies say.

The vaccine’s effectiveness could decline six months after immunization, the companies said in a press release, and booster doses may be needed to fight off virus variants.

The data were neither published nor peer-reviewed. The vaccine manufacturers said they expected to submit their results to the Food and Drug Administration in the coming weeks, a step toward approval for booster shots.

But the companies’ claims contradict other research, and several experts dismissed the claim that boosters are needed.

“Given the variants currently circulating, there is really no evidence of a third booster or a third dose of an mRNA vaccine,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center in New York. “In fact, many of us wonder if you’ll ever need boosters.”

Federal authorities also sounded dubious on Thursday night. In general, Americans who have been fully vaccinated currently do not need a booster vaccination, the FDA and the Centers for Disease Control and Prevention said in a joint statement.

“We are prepared for booster doses when science shows they are needed,” the authorities said.

The Delta variant, first identified in India, is believed to be about 60 percent more contagious than Alpha, the version of the virus that ripped through the UK and much of Europe earlier this year, and perhaps twice as contagious as the original coronavirus.

The delta variant is now causing outbreaks among unvaccinated populations in countries like Malaysia, Portugal, Indonesia and Australia. In the USA, too, Delta is now the dominant variant, the CDC reported this week.

Until recently, infections in the US were at their lowest level since the pandemic began. Hospital stays and deaths related to the virus have continued to decline, but new infections could increase.

It is not yet clear to what extent the variant is responsible for this; A slower vaccination campaign and quick reopenings also play a role.

Citing data from Israel, Pfizer and BioNTech suggest that the effectiveness of their vaccine “in preventing infections and symptomatic illnesses decreased six months after vaccination.” Given the surge in Delta and other variants, the companies said “a third dose may be required within 6 to 12 months of full vaccination”.

Updated

July 8, 2021, 6:10 p.m. ET

Health officials in Israel have estimated that full vaccination with the Pfizer BioNTech is only 64 percent effective against the Delta variant. (It is more than 90 percent effective against the original virus.)

But Israel’s estimates have been disproved by a number of other studies which found the vaccine to be very effective at preventing infection – against all variants. For example, a recent study showed that mRNA vaccines like those from Pfizer trigger a sustained immune response in the body that can protect against the coronavirus for years.

“Pfizer is looking opportunistic by putting an announcement 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, the decision is not up to you.”

The companies described their plan to develop a new vaccine against Delta as a kind of backup measure in case the original vaccine replenishment fails. The new vaccine targets all of the spike protein, not a portion, and the first batch has already been made.

The delta variant poses challenges for the immune system. In the journal Nature on Thursday, French researchers reported new evidence that the delta variant can partially bypass the body’s immune response, as changes to the spike protein on its surface make it difficult for antibodies to attack.

The team analyzed blood samples from 59 people after they received the first and second doses of the vaccine. Blood samples from just 10 percent of those immunized with a dose of the AstraZeneca or Pfizer-BioNTech vaccines were able to neutralize the Delta and Beta variants in laboratory tests.

“A single dose of Pfizer or AstraZeneca was either poorly effective or not effective at all against beta and delta variants,” the researchers concluded. Data from Israel and the UK largely support this finding, although those studies also suggested that one dose of vaccine was still enough to prevent hospitalization or death from the virus.

But a second dose increased the effectiveness to 95 percent. There was not much difference in the levels of antibodies produced by the two vaccines.

“When you receive two doses of an mRNA vaccine, you are very well protected against serious illness, hospitalization and death for each of the variants,” said Dr. Gounder.

The researchers also looked at blood samples from 103 people infected with the coronavirus. Delta was much less sensitive than Alpha to samples from unvaccinated individuals in this group, the study found.

One dose of vaccine increased sensitivity significantly, suggesting that people who have recovered from Covid-19 may still need to be vaccinated to fight off some variants.

Taken together, the results suggest that two doses of the vaccine provide strong protection against all variants, as does one dose for people who have recovered from Covid-19 and have some natural immunity.

Some experts also questioned discussions about boosters for Americans while much of the world has not yet received a single dose.

“It’s impossible to ignore the global situation,” said Natalie Dean, biostatistician at Emory University in Atlanta. “I find it hard to imagine getting a third dose when there are front line workers treating Covid patients who have not yet been vaccinated.”

Every unvaccinated person offers the virus additional opportunities to mutate into dangerous variants, said Dr. Gounder feast.

“If we are concerned about variants,” she said, “our best protection is to get the rest of the world vaccinated, and not to hoard more doses to give people here in the US third doses of mRNA vaccines. “