(CNN) Another federal official is making it clear that despite President Trump's predictions, there's hardly any chance a vaccine will be available to Americans by Election Day.
"I don't know any scientist involved in this effort who thinks we will be getting shots into arms any time before Election Day," said the official, who is familiar with Operation Warp Speed, the federal government's effort to develop coronavirus vaccines.
Trump, however, has projected optimism for a quicker timeline.
"[It's] going to be done in a very short period of time -- could even have it during the month of October," the President said at a press briefing Monday. "We'll have the vaccine soon, maybe before a special date. You know what date I'm talking about."
On August 6, Trump said he was "optimistic" a vaccine would be ready by around November 3.
"I believe we'll have the vaccine before the end of the year, certainly, but around that date, yes. I think so," Trump said.
And at a rally last week, he said, "It will be delivered before the end of the year, in my opinion, before the end of the year, but it really might even be delivered before the end of October."
The federal official is not the first to cast skepticism on Trump's forecast.
It's "extremely unlikely, but not impossible" that a Covid-19 vaccine could be authorized for emergency use by the US Food and Drug Administration before the end of October, Moncef Slaoui, the chief adviser to Operation Warp Speed, told NPR last week.
Dr. Larry Corey, who's leading a group set up by the National Institutes of Health to work on coronavirus vaccines, also said he doesn't think there will be a vaccine available by Election Day.
"I would agree with Dr. Slaoui. The chances are very low, very remote," said Corey, who leads the COVID-19 Prevention Network.
In one word, this is why there likely won't be a vaccine available before Election Day: biology.
Here's how the trials work: You take 30,000 people, give half of them a vaccine and half of them a placebo, which is a shot of saline that does nothing. Then those 30,000 people go about their lives, and you wait to see how many in each group become infected and sick with Covid-19, the "endpoint" in medical parlance.
That waiting takes time, especially since the coronavirus vaccines currently being studied in the US are two-dose vaccines with each dose several weeks apart.
Vaccine makers try to target locations and populations that will get them to their endpoint quickly, but it doesn't always work out.
"EVERYTHING will depend on how fast the number of cases accrue and then the number of cases in each group," Dr. Robert Frenck, director of the Vaccine Research Center at Cincinnati Children's Hospital, wrote in an email to CNN. "It REALLY is a numbers game."
The length of that wait depends on how likely the trial participants are to come in contact with the virus in their daily lives. Did the trials recruit people who work from home and wear masks religiously when out in public? If so, it could be a longer wait for them to become infected in significant numbers.
"Who's in the trials - the kind of people who tend to stay at home or the kind of people who attended the Sturgis rally?" said John Moore, an immunologist at Weill Cornell Medicine, referring to a motorcycle rally in South Dakota that led to at least dozens of cases of Covid-19.
Historical precedent, as well as the demographics of the participants in the current coronavirus vaccine trials, suggest more the stay-at-home type.
That does not bode well for bringing the trials to a speedy conclusion.
Typically, those who volunteer for clinical trials tend to be "White, college-educated women," said Frenck, who has been the principal investigator on dozens of vaccine clinical trials, and has served on the Data and Safety Monitoring Board for many others.
All three of those factors are potentially bad news for the coronavirus clinical trials, because data indicates White college-educated women are at lower risk for being exposed to the novel coronavirus.
White people are more likely to be able to work from home, according the US Bureau of Labor Statistics. Also, essential workers are at high risk for contracting Covid-19, and while White people make up 60% of the population, they account for only 55% of essential workers, according to the Economic Policy Institute.
Those factors help explain why White people tend to be less likely than others to contract Covid-19.
While they make up 60% of the US population, White people account for only 41% of US cases for which racial information was available, according to the US Centers for Disease Control and Prevention.
Moderna and Pfizer have not released the education status of their participants, but if it's like other trials, the volunteers are likely to be college-educated.
That's bad news, too.
People with a college education are less likely to be essential workers and more likely to be able to work from home. They're also more likely to wear masks, according to a July Gallup poll.
All of those factors help them stay away from the coronavirus.
As for women -- that's a third potentially bad sign for the trials.
Pfizer and Moderna haven't released a gender breakdown of their trials, but according to the FDA data, 72% of clinical trial participants in 2019 were women.
Not only are women more likely to be able to work from home, they're also more likely to always wear masks in public.
Clinical trials often don't go as fast as researchers would like. But the Covid-19 vaccine trials are unusual because of the urgency. Researchers need the trials to accrue Covid-19 cases in a timely manner so a vaccine can get on the market and life can go back to normal.
Corey, who runs the COVID-19 Prevention Network, noted that Pfizer and Moderna were the first two coronavirus vaccine trials to start, both dosing their first volunteers on July 27. AstraZeneca followed this month, and several more are expected to start later in the fall.
"These are the first trials, and we do not know how efficiently we will achieve the defined endpoints of the trials. Will it be a straight, easily defined line, or more akin to a meandering trail with switchbacks? We are in unchartered waters," Corey said.
Another infectious disease expert said researchers must be concerned that the endpoints will come slowly, given who typically joins trials.
"I can't imagine that anyone is not concerned that the endpoints will be long in coming, just for those reasons," said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center.
He personally knows two college-educated White women who've volunteered for the Covid-19 vaccine trials: his daughter-in-law and his granddaughter.
"They wear masks. They do social distancing. They are meticulous," Schaffner said. "The other day, they came over to our pool and they stayed on one end of the pool and we stayed on the other. They never went inside the house. My wife said to me, 'Well, they're not going to be helpful in the trials, that's for sure.'"