Early results from Moderna coronavirus vaccine trial help to spark a big rally on Wall St.

The Cambridge biotech reported that it has the ‘potential’ to prevent infection from the virus.

Inside a Moderna lab in Cambridge.David L. Ryan/Globe Staff/file

A closely watched experimental vaccine for COVID-19 developed by Cambridge biotech Moderna produced antibodies in eight healthy volunteers similar to those found in people who recovered from the disease, suggesting that the vaccine “has the potential to prevent” infection, according to a company executive.

The encouraging results from the first phase of a clinical trial buoyed investors, boosting Moderna’s stock by nearly 20 percent and helping to spark a major rally on Wall Street, with the Dow Jones industrial average closing up just shy of 4 percent.

More than 100 experimental vaccines are in the works as drug firms, academic laboratories, and governments around the globe scramble to develop a way to prevent COVID-19, which has killed more than 90,000 people in the United States and 315,000 worldwide. The potential financial reward for the first company that markets a successful vaccine could be worth billions of dollars.

But some vaccine experts expressed skepticism about Moderna’s announcement, which came two hours before the market opened Monday morning.

Dr. Peter Hotez, co-director of the Texas Children’s Hospital Center for Vaccine Development, said updates on COVID-19 vaccine efforts generally feature an online preview of the study, which Moderna didn’t provide. Although these so-called preprint studies aren’t peer-reviewed ― like those published in a scientific journal ― Hotez said they give scientists an opportunity to scrutinize back-up data.

“You cannot do science by press release," said Hotez, who is working on two other potential COVID-19 vaccines, including a partnership with PATH, a global nonprofit organization dedicated to improving public health. “They didn’t back it up with any data, and the statements they did make about it simply confused rather than illuminated.”

Experts say multiple vaccines are probably needed to inoculate people worldwide because no single drug maker has the ability manufacture enough doses by itself.

Moderna’s vaccine candidate was the first to enter clinical trials, although at least seven others have followed.

The eight subjects in the Moderna trial received two doses of the vaccine and developed neutralizing antibodies to the virus at levels comparable to those in people who recovered, the company said.

The antibodies — extracted from the blood and tested in cells in the lab ― stopped the virus from replicating, the fundamental requirement for an effective vaccine, according to Moderna.

Scientists suspect that antibodies found in the blood of people who fight off COVID-19 may make them immune to reinfection, but that has not been proven.

There were no serious side effects from the vaccine in volunteers, although one had redness around the injection site in the shoulder, Moderna said.

A few dozen volunteers participated in the first phase of the study, which took place at Kaiser Permanente Washington Health Research Institute in Seattle, but Moderna said data were available for only a small portion of them.

Moderna Therapeutics headquarters in Cambridge. Blake Nissen for the Boston Globe/file

The data "substantiate our belief that mRNA-1273 has the potential to prevent COVID-19 disease and advance our ability to select a dose for pivotal trials,” said Dr. Tal Zaks, Moderna’s chief medical officer.

Alan Carr, an analyst at Needham & Co, said the results were “pretty encouraging.” The vaccine, he said, appeared to produce a significant immune response and caused no worrisome safety concerns.

“Based on these data, we believe the vaccine is likely to be found effective for prevention of infection" in the final phase of a clinical trial, he said in a note to investors.

Unlike Hotez, the Texas vaccine developer, Carr wasn’t troubled by the absence of an accompanying pre-print study. He said the National Institutes of Health will probably publish the backup data soon.

Hotez also said he didn’t know what to make of Moderna’s statement that the vaccine developed neutralizing antibodies at levels similar to those in people who recovered from infection.

He noted that a recent pre-print by scientists from the Laboratory of Molecular Immunology at Rockefeller University in New York found that plasma obtained from individuals who recover from COVID-19 without hospitalization “do not contain high levels of neutralizing activity.”

Moderna was founded 10 years ago by Flagship Pioneering, a Cambridge venture capital firm, and went public in December 2018. Like many biotechs, it has no approved products to sell, yet it has a market value of more than $24 billion. Moderna’s share price has more than tripled this year, based largely on investors’ hopes for the coronavirus vaccine.

After trading closed Monday, Moderna announced that it plans to sell $1.25 billion worth of stock to fund manufacturing and distribution of its vaccine candidate, if it can receive regulatory approvals.

The biotech’s experimental vaccine uses custom-built messenger RNA ― the genetic material that directs cells to do something ― to trigger an immune response. Moderna developed the vaccine candidate with investigators from the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health. NIAID conducted the first phase of the study.

The Biomedical Advanced Research and Development Authority, a federal agency, has pledged up to $483 million to help the biotech develop the vaccine.

The company announced on May 7 that the Food and Drug Administration had cleared its application to proceed to a mid-stage clinical trial, an indication that the first phase was likely going well.

The mid-stage trial will involve about 600 people and is expected to begin shortly. Assuming that it’s successful, the last stage of the study is expected to start in July.

Stephane Bancel, the French billionaire who is chief executive of Moderna, has repeatedly said that even if his firm’s vaccine wins FDA approval, it wouldn’t be commercially available until sometime next year at the earliest.

But that would still set a record, public health experts say. Vaccines typically take 10 to 15 years of research and testing before they can be deployed. Nonetheless, President Trump has predicted that a vaccine will be available by the end of the year.

Like most experimental drugs that enter clinical trials, vaccine candidates generally fizzle out before they get to market. At least 90 percent never win approval from the FDA, according to public health experts.