Moderna Inc.’s COVID-19 vaccine produced antibodies to the coronavirus in all patients tested in an initial safety trial, federal researchers said, clearing an important milestone as the U.S. continues to grapple with a surge in new infections.

The U.S. biotech firm’s breakthrough is one of the most promising developments yet in the race to develop a vaccine against the virus that has paralyzed the world’s economy. Yet the road to a successful shot is filled with hurdles, and some patients in the trial experienced severe side effects.

“The good news is that this vaccine induced antibodies,” said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, “not just any kind of antibodies, but neutralizing antibodies.”

Fauci, in a telephone interview, called the Moderna data “really quite promising.” Though he said the side effects seen were not alarming and were typical of those experienced with other vaccines, some experts sounded a note of caution.

More than half of participants who got the middle of three doses administered in the trial suffered mild to moderate fatigue, chills, headache and muscle pain. Also, 40 percent of people in the middle-dose group experienced a fever after the second vaccination. Three of 14 patients given the highest dose experienced severe side effects, but that dose is not being used in larger trials.

“Man, that is a lot of adverse events,” said Tony Moody, a doctor and researcher at the Duke Human Vaccine Institute. He said it would be “unusual” for a vaccine to have this rate of side effects.

On the plus side, Moody said that the antibody levels produced were “really encouraging.” The neutralizing antibody levels in the trial produced were equivalent to the upper half of what’s seen in patients who get infected with the virus and recover, according to the results published Tuesday in the New England Journal of Medicine.

Moderna’s shares gained about 16 percent in U.S. pre-market trading Wednesday. The stock has more than tripled in value this year on hopes that the company’s vaccine will gain rapid approval. The vaccine will move into a much larger late-stage trial later this month that’s likely to determine whether it’s fit for commercial use.

Although stimulating production of neutralizing antibodies doesn’t prove a vaccine will be effective, it’s considered an important early step in testing. The side effects reported weren’t severe enough in the majority of patients to preclude further testing, according to the report by researchers from the NIAID.

The vaccine news came as the pandemic continued thriving throughout the U.S. Cases nationwide increased Tuesday to 3.4 million, according to data collected by Johns Hopkins University and Bloomberg News. More than 136,117 Americans have died.

While some states that suffered this spring have managed to quell the contagion, fierce hot spots are breaking out in the Sun Belt.

Moderna’s initial results are from the first group of 45 patients who received the vaccine, called mRNA-1273. It evaluated three doses that were given in a two-shot regimen. The middle one was selected for use in the large final-stage study that is slated to begin on July 27.