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Moderna, the Massachusetts biotechnology company behind a leading effort to create a coronavirus vaccine, announced promising early results Monday from its first human safety tests.

The eagerly awaited data provide a first look at one of the eight vaccines worldwide that have begun human testing. The data have not been published in a scientific journal and are only a preliminary step toward showing the experimental vaccine is safe and effective.

The company’s stock, along with the Dow Jones industrial average, soared on the report that eight participants who received low and medium doses of Moderna’s vaccine had blood levels of virus-fighting antibodies that were similar or greater than those in recovered covid-19 patients. That suggests, but doesn’t prove, that it triggers some level of immunity.

Another 17 people had immune responses at a level similar or greater than recovered patients, but scientists had not yet tested whether their antibodies neutralized the virus.

Moderna’s announcement comes days after one of its directors, Moncef Slaoui, stepped down from the board to become chief scientist for Operation Warp Speed, a White House initiative to speed up vaccine development. Watchdogs called out Slaoui’s apparent conflict of interest. Filings with the Securities and Exchange Commission show Slaoui’s stock options in Moderna are worth more than $10 million with the company’s share price at $66.69. In regular trading Monday, Moderna’s stock soared almost 20 percent to $80.

Moderna also received $483 million from the Biomedical Advanced Research and Development Authority, a federal agency. Moderna has also partnered with a contract development and manufacturing firm, Lonza, and Slaoui stepped down from that company’s board on Monday.

Slaoui is divesting his Moderna stock holdings, effective Tuesday morning, a spokeswoman for the Department of Health and Human Services said Monday afternoon. He will donate the value the Moderna stock gained since Thursday, before the announcement of his position leading Operation Warp Speed, to cancer research.

Companies and countries are racing to create a coronavirus vaccine and taking on financial risk to manufacture doses in large quantities before they even know whether the products are safe and effective. The United States is aiming to have hundreds of millions of doses of a vaccine broadly available by the end of the year.

An effort led by the University of Oxford, in partnership with pharmaceutical giant AstraZeneca, plans to have millions of doses of vaccine available by fall. Last week, those researchers disclosed that more than 1,000 patients had already enrolled in the initial trial of its vaccine, which began in late April.

A Chinese company, Sinovac Biotech, recently published a study in the journal Science showing that its vaccine protected rhesus monkeys against infection and is being tested in 144 people. Two other safety trials are ongoing in the United States, led by Pfizer and Inovio Pharmaceuticals.

Vaccine experts have said these accelerated development timelines are theoretically possible but extremely optimistic — and depend on everything going right. They stress that it will be crucial to gather convincing data that the vaccines are safe and effective before using them broadly — and the new data are promising but preliminary, from the earliest phase one trials that test safety in healthy people.

“I would certainly take this as good news, and it’s certainly worth moving forward” into later-stage trials, said Daniel Salmon, director of the Institute for Vaccine Safety at the Johns Hopkins Bloomberg School of Public Health. “There’s just historically, if you look at vaccine development, lots of vaccines that look good out of Phase 1 that don’t turn out to be good products.”

The data released Monday by Moderna is encouraging but represents only a first step in a long process to bring a vaccine to market. It comes from an interim report on dozens of patients followed over weeks, whereas vaccine approval requires broad testing in thousands of patients followed over many months or years.

Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia, said that the data looked promising and that it made sense to proceed to a large trial this summer.

“The fact that the vaccine elicited neutralizing antibody amounts comparable or higher to those found in convalescent sera [plasma] is very encouraging,” said Arturo Casadevall, chair of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health, who is spearheading the initiative to test plasma as a treatment.

Peter Jay Hotez, who is working on developing a coronavirus vaccine at the Baylor College of Medicine, said it would be important to understand the level of antibodies detected in the patients beyond the information provided in a company news release. He pointed to emerging evidence that many recovered patients do not muster high levels after they recover — and that high levels of antibodies may be needed to neutralize the virus.

The vaccine showed few safety signals, aside from redness at the injection site for one patient and some transient “systemic” symptoms in three patients given the highest dose — which the company will not be using in future trials.

The interim data comes from a clinical trial aimed at showing the safety of its experimental vaccine and helping the company select the correct dose. The company has not yet picked the final dose, or announced the size or length of the large trial that it will start in July, which will be the key one that regulators consider to decide whether the vaccine is safe and effective.

This month, the company received permission from the Food and Drug Administration to begin a 600-person, Phase 2 trial to begin to test the effectiveness of the vaccine.

“We are very, very happy because first the vaccine was generally safe,” Stephane Bancel, chief executive of Moderna, said in an interview. “The piece that was really exciting and was the big question, of course, was can you find antibodies in people in enough quantities” to prevent disease.

Moderna also reported that the vaccine protected mice who were vaccinated and then exposed to the virus, preventing it from multiplying in their lungs.

Moderna’s vaccine uses a genetic material called messenger RNA that codes for the distinctive spike protein that studs the outside of the novel coronavirus. The vaccine delivers the messenger RNA to cells, which then follow the genetic instructions to create the virus protein — allowing the body to learn to recognize and neutralize the pathogen.

Clarification: This story was updated to reflect that in addition to eight people who had antibodies capable of neutralizing the virus, there were 17 others who had antibody levels similar to those found in recovered patients. However, scientists have not yet tested the second group to see whether their antibodies neutralized the virus.