Pfizer plans to ask US regulators to authorize a booster dose of its COVID-19 vaccine within the next month, the drugmaker's top scientist said on Thursday, based on evidence of greater risk of reinfection six months after inoculation and the spread of the highly contagious Delta variant.

Pfizer chief scientific officer Mikael Dolsten said the recently reported dip in the vaccine's effectiveness in Israel was mostly due to infections in people who had been vaccinated in January or February. The country's health ministry said vaccine effectiveness in preventing both infection and symptomatic disease fell to 64 percent in June.

"The Pfizer vaccine is highly active against the Delta variant," Dolsten said in an interview. But after six months, he said, "there likely is the risk of reinfection as antibodies, as predicted, wane."

Pfizer did not release the full set of Israeli data on Thursday, but said it would be published soon.

"It's a small data set, but I think the trend is accurate: six months out, given that Delta is the most contagious variant we have seen, it can cause infections and mild disease," Dolsten said.

Pfizer's own data from the United States showed an erosion of the vaccine efficacy to the mid-80s after six months, Dolsten said, against the variants that were circulating there in the spring.

He stressed that data from Israel and Britain suggests that even with waning antibody levels, the vaccine remains around 95 percent effective against severe disease.

The vaccine, developed with German partner BioNTech SE , showed 95 percent efficacy in preventing symptomatic COVID-19 in a clinical trial the companies ran last year.

Dolsten said that early data from the company's own studies shows that a third booster dose generates antibody levels that are five to 10-fold higher than after the second dose, suggesting that a third dose will offer promising protection.

He said multiple countries in Europe and elsewhere have already approached Pfizer to discuss booster doses, and that some may begin administering them before a potential U.S. authorization.

Dolsten said he believes that booster shots are particularly important in older age groups.

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Pfizer expects the COVID-19 vaccine to be a major revenue contributor for years, and has forecast sales of US$26 billion from the shot in 2021. Global spending on COVID-19 vaccines and booster shots could total US$157 billion through 2025, according to US health data firm IQVIA Holdings.

Dr Eric Topol, a professor of molecular medicine and director of the Scripps Research Translational Institute in La Jolla, California, said basing the decision on waning antibody protection ignores the role of important other parts of the immune response, including memory B cells, which can make antibodies on demand when challenged by the virus.

"You need better studies to be able to assert that. It isn't just neutralizing antibodies," Topol said.

Pfizer plans to launch soon a placebo-controlled efficacy trial of the booster with 10,000 participants. That study will run throughout the fall, Dolsten said, meaning it will not be completed ahead of the company's filing with the Food and Drug Administration.

Dr William Schaffner, a vaccine expert at Vanderbilt University medical Center, said even if Pfizer succeeds in getting its booster approved for use by the FDA, that is only the first step. The booster would still need to be reviewed and recommended by advisers to the US Centers for Disease Control and Prevention.

"It's not automatic by any means," he said. Schaffner said realistically, most of the public health bandwidth in the United States is still focused on encouraging Americans to get their first and second doses of the vaccine.

The FDA declined to comment on Pfizer's plans.

Because boosters will drive increasing demand for vaccines as much of the world is still unvaccinated, Dolsten said Pfizer is looking at ways to boost production.

It is already targeting production of 3 billion doses this year and 4 billion doses next year. Dolsten declined to give a forecast of exactly how many more doses the company could add, but said "we can step up billion after billion in '22."

Pfizer has previously said people will likely need a booster dose. But some scientists have questioned when, or whether, they will be needed.

Dolsten also said Pfizer and BioNTech are designing a new version of the vaccine targeting the Delta variant, but said the companies do not believe that the current version will need to be replaced in order to combat the variant.