Pfizer Chief Executive Officer Albert Bourla said he was disappointed that the company's work was politicized during this week's presidential debate.

Bourla issued the statement as he tried to reassure U.S. staff that the company will not be pushed to rush the COVID-19 vaccine.

According to a staff letter that Bourla wrote, he told employees on Thursday that the company is "moving at the speed of science" and not on any political timing.

"The only pressure we feel - and it weighs heavy - are the billions of people, millions of businesses and hundreds of government officials that are depending on us," Bourla noted.

Top federal health officials had repeatedly said that a vaccine is unlikely to be widely available until 2021. President Donald Trump, on the other hand, maintained that a vaccine would be ready before election day.

On Tuesday's debate with former Vice President Joe Biden, Trump said that he had talked with the companies whose COVID-19 vaccines are furthest along in testing.

Trump noted that he has spoken to Pfizer and all the other people he had to talk with, including Moderna, Johnson & Johnson and others. He said these companies could "go faster than that by a lot."

"It's become very political," Trump noted.

Pfizer said it expects to have data from its ongoing late-stage by October that could show whether its potential vaccine is safe and effective.

In his letter, Bourla said he hopes to have a hundred million doses delivered by the end of the year. Those doses could not be publicly distributed until the Food and Drug Administration (FDA) reviews Pfizer's data.

The FDA would then decide whether to issue an emergency use authorization. With the emergency use authorization, the distribution of the vaccines would be allowed on a limited basis.

Initial shots are expected to go to medical and other frontline workers, nursing homes, and people most at risk of catching or becoming seriously ill from the virus.

Moderna's chief executive, Stéphane Bancel, announced on Wednesday that Moderna would not be ready to apply for emergency use authorization with the FDA for its potential vaccine until Nov. 25 at the earliest.

Johnson & Johnson has already started the late-stage and final patient study of its vaccine last week. AstraZeneca, on the other hand, has its U.S. study on hold while the FDA reviews a potential safety problem. AstraZeneca has a candidate in late-stage trials around the world.

Bourla and top executive of eight other companies developing COVID-19 vaccines and treatments vowed in early September not to seek an emergency use authorization for their products until they were proven safe and effective.

Meanwhile, a team of experts have said that a successful COVID-19 vaccine will not conquer the spread of the virus alone.

A report from a multidisciplinary group convened by the Royal Society called Delve said there are serious challenges in procuring a vaccine. These challenges include manufacturing and storage, how effective the vaccines are, and problems with public trust.

Prof Nilay Shah, the head of the department of chemical engineering at Imperial College London, said that COVID-19 vaccines would have to answer questions about effectivity and passing regulations. Shah is also an author of the report.

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