An experimental coronavirus vaccine being developed by pharmaceutical giant Pfizer and the German firm BioNTech triggered stronger immune responses in recipients than those seen in people naturally recovering from an infection in a small study published online Wednesday.

The work has not yet been peer-reviewed, and it is still unclear what level of immune response will protect a person from getting sick. But outside scientists praised the company for publishing the data on 45 people, and said the results support moving to a larger clinical trial to test whether the vaccine is safe and effective.

“It’s the first positive data I’ve seen coming out of Operation Warp Speed,” said Peter Jay Hotez, dean for the National School of Tropical Medicine at Baylor College of Medicine, referring to the U.S. government effort to speed up the development, testing and production of multiple coronavirus vaccines. “I’m really happy Pfizer took the initiative to publish it, whereas the others haven’t. I think we need to see more of this.”

The 45 participants were randomly assigned to receive injections of one of three different dose levels of the experimental vaccine, or a placebo. Three weeks after the initial shot, they received a second one.

Hotez said the company shared key data showing the boost in people’s blood levels of antibodies capable of neutralizing the virus after the second shot was given. Those virus-fighting antibodies increased to levels greater than those seen in the blood plasma of patients who have recovered from covid-19, the disease caused by the novel coronavirus.

“What’s most exciting to me is we have a vaccine that offers promise for a path forward to a vaccine that may be able to protect against SARS-COV-2 infection and illness,” said William Gruber, senior vice president of Pfizer Vaccine Clinical Research.

Although scientists still do not know what level of antibody protects people against the virus, the levels found in people who have recovered from the illness are seen as a benchmark for some level of likely immunity.

The biotechnology company Moderna gave a glimpse of its early data through a news release in May and is expected to begin its large clinical trial this month. But the company has yet to share full results from its initial safety study, which are expected to be published by scientists at the National Institutes of Health. Inovio Pharmaceuticals, a company working on a DNA vaccine, also announced positive results via a news release this week. The incomplete data has frustrated scientists trying to evaluate the different approaches.

There was a concerning signal in the Pfizer trial that, scientists said, will have to be closely monitored when the vaccine is tested in a broader population: The majority of people given an intermediate dose of the vaccine developed fevers, chills and other low-grade symptoms that lasted a day after their second shot.

But other vaccines that cause people to feel bad temporarily, such as the shingles vaccine Shingrix, have tremendous value because of their benefits, and the same could be true with a coronavirus vaccine if it is shown to be effective.

“With Shingrix, you miss a day of work. On the other hand, you’re preventing one of the worst pains in medicine, associated with shingles,” said Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia. “I think the same thing is true here: You’re willing to suffer a day or two knowing you have the chance not to be infected.”

Pfizer and its partner, BioNTech, are testing four vaccine candidates, all based on a technology platform that delivers RNA, genetic material that uses the body’s own cells to produce the distinctive spike protein found on the surface of the coronavirus. The results are from one of the candidates, and the company plans to use the data to help inform the design of a 30,000-person trial that could begin at the end of the month, pending regulatory approval.

Pfizer aims to have 100 million doses of a vaccine available by the end of the year and 1.2 billion doses in 2021.