Moderna COVID-19 Vaccine Produces an Immune Response—Here's What That Means

Results from Phase 1 clinical trials show Moderna's COVID-19 vaccine is able to produce an immune response against the novel coronavirus. The results, published on July 14 in The New England Journal of Medicine, indicate the company's vaccine induced anti–SARS-CoV-2 immune responses in all 45 participants.

Moderna is the first of the U.S. vaccine candidates to publish results of human trials in a peer-reviewed journal.

What Is an Immune Response?

An immune response against SARS-CoV-2, the virus that causes COVID-19, is what researchers hope to see. It's an indicator that the vaccine is doing its job.

"An immune response is the reaction of an organism's immune system to molecules called antigens—usually antigens from microscopic pathogens like viruses and bacteria," says Andy Miller, MD, an infectious disease specialist and Verywell Health Medical Expert Board Member. "Immune responses help us fight pathogens, and can provide immunity so that we might be protected the next time we are exposed to the same pathogen." 

In response to the antigens introduced by Moderna's vaccine, the body should, in theory, create protective antibodies against SARS-CoV-2, building up immunity to the virus.

After two doses, that's exactly what happened.

While no participants had neutralizing antibody levels before receiving the vaccine, after the first vaccination, roughly half did. After a second vaccination was administered 28 days later, neutralizing antibodies were found in all participants.

These neutralizing antibody levels also increased over time. Among participants who received 100-microgram doses of the vaccine—the dosage level that's moving on to the next phase of clinical trials—levels increased from a mean of 23.7 "geometric titers" at day 15 to 231.8 at day 57. Researchers compared these levels against neutralizing antibodies found in people recovered from COVID-19, which averaged only 109.2 geometric titers.

Evaluating Safety

Proving that a vaccine can elicit an immune response is only half the battle. Scientists must also show it's safe.

The Phase 1 clinical trials identified only mild reactions, including:

• Fatigue
• Chills
• Headache
• Myalgia (muscle pain)
• Pain at the injection site

These reactions were more common after the second injection, and more common with higher dosages. Three participants receiving the 250-microgram dose reported more serious side effects, including a 103.28°F fever, prompting researchers to drop it from consideration for Phase 2 and 3 clinical trials.

Next Steps

Based on its safety and effectiveness, researchers will move forward with the 100-microgram dose of the vaccine.

“These Phase 1 data demonstrate that vaccination with mRNA-1273 elicits a robust immune response across all dose levels and clearly support the choice of 100 micrograms in a prime and boost regimen as the optimal dose for the Phase 3 study,” Tal Zaks, MD, PhD. Chief Medical Officer of Moderna said in a press release. “We look forward to beginning our Phase 3 study of mRNA-1273 this month to demonstrate our vaccine’s ability to significantly reduce the risk of COVID-19 disease.”

A Phase 2 trial of 600 adults comparing 100-microgram and 50-microgram doses of the vaccine is already underway. With support from the National Institutes of Health (NIH), Moderna plans to begin Phase 3 trials of the 100-microgram dose on July 27 with 30,000 participants.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.