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


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Key Takeaways

  • The first human trials of Moderna's COVID-19 vaccine show it can elicit a strong immune response against the virus.
  • The vaccine caused only mild side effects.
  • A 100-microgram dose will move forward to Phase 3 clinical trials later this month.

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.

Trial participants received either 25-microgram, 100-microgram, or 250-microgram doses of Moderna's mRNA-1273 vaccine.

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.

What This Means For You

While we still don't know exactly when a COVID-19 vaccine will be available to the general public, showing that a candidate is both safe and effective in humans is a major step towards progress. Other vaccines should join Moderna soon; AstraZeneca and Johnson & Johnson have also announced plans to begin Phase 3 clinical trials this summer.

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.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Jackson LA, Anderson EJ, Rouphael NG, et al. An mRNA Vaccine against SARS-CoV-2 - Preliminary Report. N Engl J Med. 14 July 2020. doi:10.1056/NEJMoa2022483

  2. Heaton PM. The Covid-19 Vaccine-Development Multiverse. N Engl J Med. 2020. doi:10.1056/NEJMe2025111

  3. National Institutes of Health. A Study to Evaluate Efficacy, Safety, and Immunogenicity of mRNA-1273 Vaccine in Adults Aged 18 Years and Older to Prevent COVID-19. July 14, 2020.

By Anisa Arsenault
Anisa joined the company in 2018 after managing news surrounding fertility, pregnancy, and parenting for The Bump. Her health and wellness articles have appeared in outlets like Prevention and Metro US. At Verywell, she is responsible for the news program, which includes coverage of COVID-19.