WASHINGTON — Touting some encouraging early results of its COVID-19 vaccine trial, the U.S. company Moderna said Monday that patients participating have begun producing virus-fighting antibodies.

Moderna, based in Cambridge, Massachusetts, was one of the first companies to receive a green light from the Food and Drug Administration to develop a vaccine for the novel coronavirus.

The company’s first human volunteers entered Phase 1 of vaccine development on Jan. 13 and, in the 63 days that followed, much has been gleaned at a rapid clip, Dr. Tal Zaks, Moderna’s chief medical officer, told reporters and investors in a teleconference call Monday.

Known as mRNA-1273, it is one of roughly 100 vaccines now in development by companies around the world. In Moderna’s study, which involved 45 volunteers, it was determined that injections of the trial vaccine appeared safe and, of equal importance, that the drug successfully stimulated the body’s immune response to fight off the virus.

Dr. Robert Gallo, co-founder and director of the Institute Human Virology at the University of Maryland School of Medicine, said in an interview Monday he was mostly encouraged to hear that the vaccine trial has so far been safe for humans.

“They were efficient, fast and well organized and the safety looks OK, so that’s an advance,” said Gallo, who is also the Homer and Martha Gudelsky distinguished professor in medicine at the University of Maryland.

Gallo tempered his expectations, however, with an acknowledgment of basic physiology.

Typically, he explained, when a foreign protein is inserted into the body, the body does what it is supposed to do in that situation: It creates antibodies to protect itself.

“So if you get an immune response, that’s fine. And if it is safe, that’s fine. But the real tests are what’s coming,” Gallo said. “We don’t know how long these antibodies will last or how protective they will be.”

Dr. Amesh Adalja, a physician and senior scholar at the Johns Hopkins University Center for Health Security, is also cautiously optimistic.

“It’s important to remember it’s a phase 1 clinical trial, which is designed to look at safety, not efficacy, so it is hard to draw any strong conclusions from some of the results that come out of phase one other than safety,” said Adalja, who is certified in emergency medicine and infectious diseases, among other areas.

Moderna broke up its test subjects into groups of 15 people each, giving them an injection of mRNA-1273 in amounts ranging from 25 to 250 micrograms. Zaks explained Monday that each subject received two doses through a shot to the arm roughly 28 days apart.

“The mRNA was generally safe and well-tolerated,” Zaks said. “There was one incidence of redness around the injection site reported in the 100-microgram dose.”

Just three participants taking the highest dosage administered reported more intense side effects to the vaccine including flu-like symptoms such as headache and fatigue. The symptoms subsided within a day.

But thanks to the early successes identified in low doses, Moderna will end its 250-microgram trials.

“The lower dose of 25 micrograms worked, so there is no need to go to ten times that,” Zaks said, noting that this lower dose rate will also be a boon for the supply chain since they fully expect demand to outstrip supply.

Globally the virus has killed more than 315,000 people, according to numbers compiled by Johns Hopkins University. The company expects to launch the second phase of its clinical trial in the first week of June. Another round will begin in July and that test pool will include thousands of people.

Animal tests found when mice received the trial vaccine and were infected again, mRNA-1273 appeared to neutralize Covid-19 from reproducing in the rodent’s lungs.

For humans, all participants ages 18 to 55 across all dosing levels showed improvement by the fifteenth day. By Day 33, the level of antibodies reached the level appearing in convalescent serum from people who recovered from the virus.

Gallo at the University of Maryland, who is also the co-founder and international scientific adviser of the Global Virus Network, said he would be particularly interested to learn what Moderna unearths in future trials.

“What about immune response of those who don’t get better? Please tell me more when there is a correlation with those people who got better,” he said, “because no one has any data that immune response has been corelated with people recovering that is accepted by those in the scientific community yet.”

Adalja, the Johns Hopkins expert, underscored the importance of understanding the duration of antibodies produced by new vaccines.

“We do this with all vaccines,” Adalja said in an interview. “We need to understand how long it is good for before you need a booster shot. For example, a vaccine for yellow fever is good for life, but there are other vaccines where you will need a booster after time.”

With the world having never used this type of vaccine before, the challenges are numerous.

“We have a neutralizing antibody study, and that data is really important, but you also want to see in clinical trials how it fairs in humans and how those individuals who are vaccinated are protected,” Adalja said. “You would see that in phase 3 where people who are in the community that are vaccinated would be exposed to the virus every day.”

Even if phase two proves successful, Moderna must scale up to meet the unprecedented demand for viral treatment needed in every corner of the world.

“We are currently getting manufacturing capacity up as fast as we can,” Moderna CEO Stéphane Bancel said Monday.

Both the U.S. Department of Health and Human Services as well as the National Institutes of Health have been working with Moderna to fast track the vaccine. Representatives for the agencies did not immediately respond to requests for comment.

— By Brandy Buchman, CNS