WASHINGTON (NYTIMES) - Moncef Slaoui, the chief adviser for the White House vaccine programme, said on Thursday (Sept 3) that it was "extremely unlikely but not impossible" that a vaccine could be available by the end of October.

In an interview with NPR, Mr Slaoui, the chief scientific adviser of the Trump administration's coronavirus vaccine and treatment initiative, called Operation Warp Speed, explained that the Centres for Disease Control and Prevention's guidance to states to prepare for a vaccine as early as late October - a notification Mr Slaoui said he had learnt of through the news media - was "the right thing to do" in case a vaccine was ready by that time.

"It would be irresponsible not to be ready if that was the case," he said.

However, he described that as a "very, very low chance".

That message ran counter to the optimistic assertions in recent days from the White House that a vaccine could be ready for distribution before Election Day on Nov 3.

At the Republican National Convention, President Donald Trump said a vaccine could be ready "before the end of the year or maybe even sooner".

And he and others have tried to project confidence in a quick victory.

Mr Slaoui confirmed that the two main candidates, referred to as Vaccine A and Vaccine B, were being developed by Pfizer and Moderna.

He said that there was "no intent" to introduce a vaccine before clinical trials were completed, and that trials would be completed only when an independent safety monitoring board, separate from the government, affirmed the effectiveness of the vaccine.

The interviewer, Ms Mary Louise Kelly, raised the timing of a possible vaccine given in the documents that the CDC recently sent to public health officials, and asked directly whether the delivery of the vaccine was being motivated by political concerns.

"For us, there is absolutely nothing to do with politics," Mr Slaoui responded, saying that those involved were working as hard as they could because so many people were dying every day.

"Many of us may or may not be supportive of this administration. It's irrelevant, frankly."

Though he continued to express doubt that a vaccine would be ready by the end of October, Mr Slaoui said, "I firmly believe that we will have a vaccine available before the end of the year and it will be available in quantities that can immunise patients, subjects at the highest risk", including the elderly and those who are working in jobs with high exposure to the virus.

He estimated that there would be enough vaccine by the end of the year to immunise "probably between 20 and 25 million people".

He said that manufacturing would be ramped up so that there would be enough doses for the US population "by the middle of 2021".