The first participant in a clinical trial for a vaccine to protect against coronavirus will receive an experimental dose on Monday, according to a US government official.
Public health officials say it will take a year to 18 months to fully validate any potential Covid-19 vaccine.
Scientists at the Kaiser Permanente Washington Health Research Institute in Seattle are carrying out the trial, funded by the National Institutes of Health in the US.
Testing will begin with 45 young, healthy volunteers with different doses of shots co-developed by NIH and Moderna Inc. Officials say there is no chance participants could get infected from the jabs, as they do not contain the virus itself.
The goal of the trial is to check the vaccines show no concerning side effects, setting the stage for larger tests.
Researchers are pursuing different types of vaccines - jabs developed from new technologies that are not only faster to produce than traditional inoculations but might prove more potent.
Coronavirus: Everything you need to know:
Some researchers are even aiming for temporary vaccines - such as jabs that might guard people's health a month or two at a time while longer-lasting protection is developed.
But even if initial safety tests go well, "you’re talking about a year to a year and a half" before any vaccine could be ready for widespread use, according to Dr Anthony Fauci - director of NIH's National Institute of Allergy and Infectious Diseases.
Even that would still be a record-setting pace - but manufacturers know the wait required because it takes additional studies of thousands of people to tell if a vaccine truly protects and does no harm.
US president Donald Trump has been pushing for swift action on a vaccine, saying in recent days that the work is "moving along very quickly" and he hopes to see a vaccine "relatively soon".
In China, scientists have been testing a combination of HIV drugs against Covid-19, as well as an experimental drug named remdesivir that was in development to fight Ebola.
Scientists at the University of Nebraska Medical Centre in the US also began testing remdesivir in some Americans who were found to have Covid-19 after being evacuated from a cruise ship in Japan.
For most people, the coronavirus causes only mild or moderate symptoms, such as fever and cough. For those in high risk categories, however, especially older adults and people with existing health problems, it can cause more severe illness, including pneumonia.
The coronavirus pandemic has so far infected more than 156,000 people, with a death toll of more than 5,800.
The vast majority of people recover, however, with the World Health Organisation advising people with mild illness recover in about two weeks.
It said those with more severe illness may take three weeks to six weeks to recover.