Moderna Inc.’s Covid-19 vaccine produced antibodies to the coronavirus in all patients tested in an initial safety trial, federal researchers said.

The neutralizing antibody levels produced were equivalent to the upper half of what’s seen in patients who get infected with the virus and recover, according to the results published Tuesday in the New England Journal of Medicine. The Moderna vaccine is one of the farthest along for Covid-19.

While stimulating production of neutralizing antibodies doesn’t prove a vaccine will be effective, it’s considered an important early step in testing. Meanwhile, the side effects reported weren’t severe enough in the majority of patients to preclude further testing, according to the report by researchers from the National Institute of Allergy and Infectious Diseases.

Moderna’s shares gained 16.4% in trading after the market closed. The stock has more than tripled in value so far this year on hopes that its vaccine will gain rapid approval.

More than half of those who got the middle of three doses suffered mild to moderate fatigue, chills, headache and muscle pain. Also, 40% of people in the middle-dose group experienced a fever after the second vaccination. Three of 14 patients given the highest dose of the vaccine experienced severe side effects, but that dose is not being used in larger trials.

“Man, that is a lot of adverse events,” said Tony Moody, a doctor and researcher at the Duke Human Vaccine Institute. He said it would be “unusual” for a vaccine to have this rate of side effects. On the plus side, he said that the antibody levels produced were “really encouraging.”

If researchers are measuring the right thing, the vaccine should work, he said, noting that this can only be proved in large trials.

The initial results are from the first group of 45 patients who received the vaccine, and evaluated three doses of the vaccine that were given in a two-shot regimen. The middle dose from this initial trial will be used in a large final-stage trial of the Moderna vaccine, called mRNA-1273, that is slated to begin on July 27th.

The final stage trial will compare the vaccine to placebo shots in 30,000 healthy people at high risk of coming down with the coronavirus.

One significant limitation of the data is it only includes data from the first 45 patients in the study, all of whom were from age 18 to 55. Results from a second portion of the phase 1 trial that included people older than this — a key demographic for any Covid-19 vaccine, given the high death rate in older patients — are not available yet.

William Haseltine, a former Harvard Medical School researcher who chairs Access Health International, said the levels of neutralizing antibodies produced were “respectable” and possibly protective. But he said “the jury is out” on safety of the vaccine.

Unlike traditional vaccines, which inject a weakened or inactivated virus or a piece of a virus to trigger an immune response, the Moderna product uses genetic material called messenger RNA to cause cells to produce the coronavirus spike protein. The goal is to produce antibodies to the virus that protect against the disease when someone is later exposed to the coronavirus.

How long

The vaccine “clearly worked in that antibodies against the spike
protein were generated, including antibodies that had virus neutralizing capability,” said Paula Cannon, professor of microbiology at Keck School of Medicine of USC. A key question will be how long will the antibodies last before they start to wane, she said.

The initial findings from the Phase 1 trial are largely in line with top-line results Moderna published in a press release in May, but provide more details on the antibody levels produced and side-effects that were seen.

At the time, Moderna was criticized by some scientists for putting out a release describing positive results that temporarily drove up the company’s stock price, but included few numbers that would allow scientists to interpret the data.

The government-sponsored trial was led by Lisa A. Jackson of Kaiser Permanente Washington Health Research Institute in Seattle, the NIAID said in its statement. Emory University in Atlanta also enrolled participants in the trial.