A coronavirus vaccine being tested in Seattle and Atlanta triggered strong immune responses in a small group of volunteers, according to preliminary results published Tuesday in the New England Journal of Medicine.

The results are encouraging, but researchers caution that much more work needs to be done to figure out whether the vaccine will actually protect people from COVID-19.

“No matter how you slice it, this is good news,” Dr. Anthony Fauci, the U.S. government’s top infectious disease expert, told The Associated Press.

After two doses of the vaccine, volunteers produced more neutralizing antibodies — the type that inactivate the virus — than the average levels seen in the blood of people who survived COVID-19.

The first volunteers, injected in Seattle on March 16, were also the first people in the United States to participate in human trials of any vaccine against the novel coronavirus. Since then, the field has exploded, with more than 20 experimental vaccines now in human trials and more than 100 in preclinical stages.

Dr. Lisa Jackson, a senior investigator at Kaiser Permanente Washington Health Research Institute in Seattle and lead author of the new report, said the speed of progress has been amazing.

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The Seattle trial was launched just 66 days after the virus’s genome was sequenced, and four months later the first results are out. “Processes that usually take years or even decades are now being accomplished in a matter of months,” Jackson said in an interview.

But first may not necessarily be best, cautioned Mark Slifka, an immunologist and vaccine expert at Oregon Health & Science University who was not involved in the study. While the initial results look promising, they also raise some red flags — and the question of how long immunity will last is still unanswered, he said.

The research so far has focused solely on determining if the vaccine is safe and whether it induces a robust immune response.

The results reported Tuesday cover the first 57 days post-injection and are from 45 healthy volunteers 18 to 55 years old. The participants, who were enrolled in Seattle and at Emory University in Atlanta, received an initial shot of low, intermediate, or high dosage, followed by a booster a month later.

The first shots didn’t induce much of a reaction, Jackson said. But after the second shot, the volunteers mounted a strong immune response. In general, responses were proportional to the dose.

More than half of participants reported side effects, including fatigue, chills, headaches and pain at the injection site. Most were relatively minor and fleeting, but three participants in the highest-dose group described their side effects as severe. Jackson said the highest dose is being eliminated from subsequent trails.

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The high rate of side effects is concerning, Slifka said. At the preferred, intermediate dose — which will be the focus of follow-up trials — all participants reported side effects and about 40% had a fever over 100 degrees.

“These are healthy young people,” he said. It’s possible side effects could be even more pronounced in the elderly and people with underlying health problems, who are most vulnerable to the virus.

The two-shot regimen could also be problematic, Slifka said, imposing a long waiting period between the first shot and reliable protection.

A cohort of older people was later enrolled in the trials, and those results should be available soon, Jackson said. Fauci told the AP the final testing will also include older adults, along with people with chronic health conditions and Black and Latino populations.

The experimental vaccine is being developed by Moderna Inc. and the National Institute of Allergy and Infectious Diseases. It is based on a new approach that uses bits of genetic material from a virus to invoke immunity.

The vaccine injections contain the RNA code for the spike protein the virus uses to attach to and invade human cells. After injection, the RNA insinuates itself into the person’s cells, which then produce the viral protein. If the vaccine works as planned, the immune system will recognize the viral protein and mount an attack.

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Any effective vaccine needs to confer long-lasting immunity, and it’s not clear yet whether RNA vaccines can deliver on that count, Slifka said. Tests of a similar RNA vaccine against influenza showed immunity waned within a few months.

Jackson herself remains cautious, because so much remains unknown about the novel coronavirus — including whether people can be reinfected and what level of immunity is needed for lasting protection.

“It’s not a slam dunk,” she said. “I’m humbled by decades of studying flu vaccines, and their efficacy is only about 50 percent.”

A phase 2 trial, which delves more deeply into the safety of the vaccine and the immune response it provokes, is already underway. A phase 3 trial, designed to find out if the vaccine can prevent infection, is set to begin later this month with 30,000 volunteers.

Seattle-area residents who are interested in participating in the Moderna vaccine trials can find out more and register at corona.kpwashingtonresearch.org.

The COVID-19 Prevention Trials Network, based at the Fred Hutchinson Cancer Research Center, has a volunteer screening registry of people nationwide who are interested in participating in a broader range of trials of vaccines and other preventive treatments at various research facilities in Seattle and around the country: coronaviruspreventionnetwork.org.

Information from The Associated Press is included in this report.