Skip to Main Content

Moderna’s Covid-19 vaccine led patients to produce antibodies that can neutralize the novel coronavirus that causes the disease, though it caused minor side effects in many patients, according to the first published data from an early-stage trial of the experimental shot.

The results were published Tuesday in the New England Journal of Medicine. Moderna had previously released some results in a press release, but many experts said they were not sufficient to draw many conclusions. Even now, many are withholding judgment.


“It certainly is a good beginning,” said Betty Diamond, director at the Feinstein Institutes for Medical Research, who was not involved in the trial. “There are certainly lots of things we don’t know yet right now.”

The study, which was run by the National Institutes of Health, showed that volunteers who received the vaccine made more neutralizing antibodies than have been seen in most patients who have recovered from Covid-19. But a second injection, four weeks after the first, was required before the vaccine produced a dramatic immune response.

“The hallmark of a vaccine is one that can actually mimic natural infection and induce the kind of response that you would get with natural infection. And it looks like, at least in this limited, small number of individuals, that is exactly what’s happening,” said Anthony Fauci, director of the National Institute for Allergy and Infectious Diseases, the NIH branch that conducted the trial. “The data really look quite good,” he added. “There were no serious adverse events.”


The data roughly mirror the results from a similar vaccine being produced by Pfizer and BioNTech, which were released July 1.

Moderna posted a listing on, a government registry, that says it will start a Phase 3 study in 30,000 patients on July 27. Pfizer and BioNTech said they plan to start their own large study by the end of the month. There are 23 vaccines in human clinical trials against the virus, SARS-CoV-2, according to the World Health Organization, with more set to begin testing soon.

In a statement, Moderna CEO Stéphane Bancel called the data “encouraging,” saying they “represent an important step forward” in the development of the vaccine, called mRNA-1273. “We are committed to advancing the clinical development of mRNA-1273 as quickly and safely as possible while investing to scale up manufacturing so that we can help address this global health emergency,” Bancel said.

One big question is whether producing antibodies predicts protection against infection — and how much protection. Another is whether the antibodies will last.

“We don’t know how much [antibody] we need to be protected, so we can’t say” all the participants “achieved a protective level,” Kathryn Edwards, scientific director of the Vanderbilt Vaccine Research Program, said in an email to STAT. “What we can say is that they made antibody that neutralized the virus, which is good.”

The study enrolled 45 healthy volunteers ages 18 to 55, testing three dose levels of Moderna’s vaccine. The trial participants were split roughly 50-50 between men and women. The population was 89% white, 13% Hispanic, 4% Black, 2% Asian, and 2% Native American. More results are expected to be reported later for older patients, who often mount a weaker immune response.

Volunteers got a shot in the arm on day 1 followed by a booster shot four weeks later. At the 100-microgram dose, the one Moderna is advancing into larger trials, all 15 patients experienced side effects, including fatigue, chills, headache, muscle pain, and pain at the site of injection. All side effects were considered mild or moderate.

A higher, 250-microgram dose led to more serious reactions and has been set aside. Although no side effects were severe — meaning that they required hospitalization — they were unpleasant, as was made clear when one volunteer in the study, Ian Haydon, went public with his experience taking the 250-microgram dose.

Researchers and drug companies have been racing with unprecedented speed to create a vaccine against the coronavirus. Moderna began its Phase 1 trial just 66 days after scientists first decoded the genome of SARS-CoV-2.

“It’s amazing just how fast we’ve gotten to this point,” Penny Heaton, CEO of the Bill and Melinda Gates Research Institute in Cambridge, Mass., said in an interview. “It’s like six years of work has been compressed into six months.”

That very speed is a reason for caution, said Paul Offit, chief of the division of infectious diseases at the Children’s Hospital of Philadelphia. He advised companies to be humble. He noted that he had worked on rotavirus vaccines for 25 years, and still the first attempt at one turned out to have a side effect that sent researchers back to the drawing board.

Already, he said, SARS-CoV-2 has done things experts never would have expected, he said. It spreads in hot weather. It causes deadly blood clots. It rarely makes children sick, but sometimes causes a surprising immune disorder. “I can promise you that over the next two years, we’ll learn a lot of things that we wish we’d known now that we are going to learn as we move forward,” Offit said.

Researchers measured the efficacy of Moderna’s vaccine in multiple ways, all of which showed higher average antibody levels than were seen in patients who had recovered from Covid-19. It took time for antibody levels to rise. Only after the booster shot did volunteers compare favorably with recovered patients.

In an editorial in NEJM, Heaton noted that it needs to be confirmed that measurements of such antibodies predict efficacy. “They are the best tools available,” she wrote, “and are supported by findings in nonhuman primates.”

At the 100-microgram dose, patients had neutralizing antibody levels, measured in what are called geometric mean titers, of 231.8 at day 57, compared to 109.2 in patients who recovered from Covid-19.

Experts said that given the multiple types of tests, it is impossible to compare vaccines based on laboratory results between different vaccines. For its vaccine, Pfizer had said that neutralizing antibody levels were 267 at 28 days, but they were 94 (a lower level than Moderna reported) in recovered patients, apparently using a different test. Pfizer has not disclosed data beyond 28 days.

Asked to compare the Moderna and Pfizer vaccine data, Fauci said, “I don’t think you could say anything about one being better than the other. They both induce good responses. Let’s see what happens in the real world.”

Several researchers said that seeing the results increased their hope not so much in Moderna’s vaccine, but in arriving at one or more vaccines that will help reduce the impact of the virus.

“I am cautiously optimistic, based on the data that we’ve seen so far, that amongst the several different vaccine platforms that are being tested, there seem to be encouraging Phase 1 data to suggest that at least one of them is going to work,” said Francis Collins, director of the National Institutes of Health. “And maybe several of them.”

But he warned that such science is not predictable. “Hence the word cautiously is attached to the word optimistic in this situation,” Collins said.

This story has been updated with comments from Anthony Fauci.

Helen Branswell contributed reporting.

  • Read the latest results in the search for a vaccine every few days — looking for hope. One concern that I have not seen addressed, is that recent studies are showing that those previously infected with COVID-19 lose the resulting antibodies relatively quickly. And, more recently, there are some corroborating reports that people can become reinfected with the disease. How does this impact the likely efficacy of a vaccine focused on creating antibodies?

  • I am encouraged by this news and thankful to the many scientists working (at their own risk) on a way to rescue us from Covid. Lets hope that the final vaccine convinces the public that science finds answers by exploring, not guessing, and by being open to the negative as well as the positive results. Meanwhile, let’s hope we can all follow scientific recommendations to let us live as safely as possible until the vaccine is ready.

  • Each time STAT publishes one of these updates, I see the anti-vaxxers mob the comments section with nonsense that may well kill some of those who listen to it. Jacobsen v. Massachusetts (mandatory smallpox inoculation in Cambridge, 1902) notwithstanding, states are unlikely to require Covid shots under penalty of law. None of the scientists deny the fact that vaccination entails risks of rare or long-term adverse effects clinical study fails to discover; we saw this after the swine flu campaign of 1976. But it’s not like they’re planning to inject a totally untested substance. All Covid products in development will have been given to mice, macaques, and three successively larger populations of healthy volunteers before release to the general public, and none of the study groups will have shown evidence of having suffered harm up to that date.

    Our First Amendment exists for good reason. Anti-vaxxers have a right to hold and express their opinions without prior restraint from government agency. But my jaw drops when medical publications carry such opinions. The NY Times certainly doesn’t. Freedom of speech was never meant to guarantee people a megaphone on every platform in the country. Must STAT really entertain Facebook-style drivel to stay in business?

    • The vaccine is useless, since the antibodies fade away quickly (I m using your own words, until this is forbidden to talk about)
      In my humble opinion all this is a monumental hoax to coerce the population to be permanent users of that concoction, of course generating filthy amount of revenue.
      I will take the vaccine when the last congressman, senate and health officer from the top, first of all Mr. Fauci, Kill Gates and company are inoculate with “The Vaccine”. Of course, please don’t use distilled water and make a charade of that.

Comments are closed.