A coronavirus vaccine won’t be widely available for a long time, several top pharmaceutical executives said Wednesday.

Early stage clinical trials likely won’t start until the end of the year or in 2021, they said, cautioning that vaccines for previous viral outbreaks, such as SARS and Zika, never made it “over the finish line.”

“It does take longer for vaccines. They are more complex, the manufacturing is more difficult, and because you're going to be delivering them to otherwise healthy people for prevention, you have to make sure they're very, very, very safe,” Dr. Julie Gerberding, executive vice president at Merck and former director of the US Centers for Disease Control and Prevention, said at a media briefing.

“So that's why we are cautioning people not to overpromise on the timeline of the vaccine availability,” she said, adding that preventing the future spread of coronavirus will take “more than a year and potentially longer than that," Gerberding said.

President Trump is pushing drug makers to develop vaccines swiftly, asking pharmaceutical executives on Monday if it could ready in the next few months. His comments came days after Dr. Tony Fauci, director of the National Institute of Allergy and Infectious Diseases, said a vaccine would hopefully be ready if the virus returns next year.

Johnson & Johnson will likely start phase one trials by the end of this year and begin larger-scale trials next year, said Paul Stoffels, the drug maker’s chief scientific officer.

Some outbreaks, such as Zika, disappear while vaccines are still in the clinical trial stage, Stoffels said.

Big money: Drug makers will likely get federal funding to help develop coronavirus vaccines, testing and treatment. The House of Representative on Wednesday passed a $7.8 billion spending package that includes more than $3 billion for such efforts.