CNN  — 

Eighteen months might sound like a long time, but in vaccine years, it’s a blink. That’s the long end of the Trump administration’s time window for developing a coronavirus vaccine, and some leaders in the field say this is too fast – and could come at the expense of safety.

The estimated time made headlines last month, when Trump remarked at a televised Cabinet Room meeting with pharmaceutical executives that a vaccine could be ready in “three to four months.” There, in front of TV cameras, Dr. Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases (NIAID), poured cold water on Trump’s estimate, saying it would be more like a year to a year and a half.

Ever since, that estimate of 12-18 months has become gospel, its appearance in media stories ubiquitous. But medical experts and scientists with firsthand experience developing vaccines are skeptical.

“Tony Fauci is saying a year to 18 months – I think that’s optimistic,” said Dr. Peter Hotez, a leading expert on infectious disease and vaccine development at Baylor College of Medicine. “Maybe if all the stars align, but probably longer.”

Dr. Paul Offit, the co-inventor of the successful rotavirus vaccine, put it more bluntly.

“When Dr. Fauci said 12 to 18 months, I thought that was ridiculously optimistic,” he told CNN. “And I’m sure he did, too.”

Vaccines development typically measured in years, not months

As the number of U.S. coronavirus deaths surges past 3,000, the pressure on the scientific community to find a vaccine is immense. In just a few weeks, the virus has jammed the gears of a robust economy and destroyed 3.3 million jobs. Fear is off the charts, and with that comes the pressure to find a fix yesterday.

On March 16, two weeks after the Cabinet meeting, the first federally funded trial for the novel coronavirus – officially known as SARS-CoV-2 – kicked off at the Kaiser Permanente Washington Health Research Institute in Seattle. On Friday, it expanded to Emory University in Atlanta. Forty-five volunteers in the Seattle and Atlanta communities are participating in the first phase of the trial, which Fauci said was “launched in record speed.” (Although several vaccines are in development, only one other clinical trial is underway, in China.)

The problem is, experts say, the oft-stated timetable is ambitious at best.

“I don’t think it’s ever been done at an industrial scale in 18 months,” said Dr. Amesh Adalja, a senior scholar focused on emerging infectious disease at the Center for Health Security at Johns Hopkins University. “Vaccine development is usually measured in years, not months.”

Vaccine trials typically start with testing in animals before launching into a three-phase process. The first phase involves injecting the vaccine into a small group of people to assess safety and monitor their immune response. The second ramps up the number of people – often into the hundreds, and often including more members of at-risk groups – for a randomized trial. If the results are promising, the trial moves to phase-three test for efficacy and safety with thousands or tens of thousands of people, according to the US Centers for Disease Control and Prevention.

Dr. Emily Erbelding, an infectious disease expert at NIAID – which is part of the National Institutes of Health – said the typical vaccine takes between eight and 10 years to develop. While she is careful not to contradict her boss’s timeline – although she did say “18 months would be about as fast as I think we can go” – she acknowledged that the accelerated pace will involve “not looking at all the data.”

“Because we are in a race here to beat back this epidemic and a vaccine is very important, people might be willing to take a chance on just going quickly into phase two,” Erbelding told CNN. “So the 18 months would rely upon speeding things up.”

A researcher works on a vaccine against the new coronavirus at the Copenhagen University research lab in Denmark on March 23, 2020.

Volunteers in each phase need to be monitored for safety, Erbelding said. “Usually, you want to follow their immune response for at least a year,” she said.

But that is not what will be happening in the current study in Seattle and Atlanta, where researchers will test animals and humans in parallel, as opposed to sequentially, according to Stat, a health news website produced by Boston Globe Media.

CNN reached out to Kaiser about the potential tradeoffs associated with a quickened timeline, but researchers at the organization were unavailable to comment.

Walt Orenstein, a professor of medicine at Emory and the former director of the US National Immunization Program, said the tradeoff is a difficult balancing act.

“If you want every ‘t’ crossed and ‘i’ dotted, how many more people will die or suffer from Covid-19?” he said. “It’s not an easy decision, it is a breakneck speed for moving things.”