NEW YORK (REUTERS) - Pfizer and partner BioNTech plan to ask US and European regulators within weeks to authorise a booster dose of its Covid-19 vaccine, based on evidence of greater risk of infection six months after inoculation and the spread of the highly contagious Delta variant.

The US Food and Drug Administration (FDA) and the Centres for Disease Control and Prevention (CDC) said, however, in a joint statement that Americans who have been fully vaccinated do not need a booster Covid-19 shot at this time.

Some scientists have also questioned the need for booster shots. The European Medicines Agency did not immediately respond to a request for comment.

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 per cent in June.

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

Data would be submitted to the FDA within the next month, he added.

Pfizer did not release the full set of Israeli data on Thursday (July 8), 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," Dr Dolsten said.

The FDA and CDC, in their joint statement, said: "We are prepared for booster doses if and when the science demonstrates that they are needed."

Pfizer's own data from the United States showed an erosion of the vaccine's efficacy to the mid-80s per cent after six months, Dr Dolsten said, against the variants 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 per cent effective against severe disease.

The vaccine, initially devised by Germany's BioNTech, showed 95 per cent efficacy in preventing symptomatic Covid-19 in a clinical trial the companies ran last year.

Dr Dolsten said early data from the company's own studies shows that a third booster dose generates antibody levels that are fivefold to tenfold 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 some may begin administering them before a potential US authorisation.

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

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 neutralising antibodies," Dr Topol said.

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

BioNTech previously argued that while antibodies are the body's main weapon against an initial infection, the cellular immune response mainly shields against the disease breaking out and worsening.

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

Dr William Schaffner, a vaccine expert at Vanderbilt University Medical Centre, said even if Pfizer succeeds in getting its booster authorised by the FDA, that would be only the first step. The booster would still need to be reviewed and recommended by advisers to the CDC.

"It's not automatic by any means," he said.

Dr Schaffner said that 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.

Because boosters would drive increasing demand for vaccines while much of the world is still unvaccinated, Dr Dolsten said Pfizer is looking at ways to increase production.

It is already targeting production of three billion doses this year and four billion shots next year. Dr Dolsten declined to give a forecast of exactly how many more doses the company could add.

Pfizer and BioNTech said they had designed a new version of the vaccine targeting the Delta variant, with a clinical trial likely to begin in August, but added that the current vaccine version had "the potential" to protect against the variant.

Pfizer expects the Covid-19 vaccine to be a major revenue contributor for years and has forecast sales of US$26 billion (S$35.15 billion) from the shot in 2021.

BioNTech said in May that vaccine supply deals for this year so far are worth €12.4 billion (S$19.9 billion) in revenue, with more contracts on the cards.

Global spending on Covid-19 vaccines and booster shots could total US$157 billion until 2025, according to US health data firm IQVIA Holdings.