An Overview of the Moderna COVID-19 Vaccine

Moderna's vaccine is the second to receive U.S. approval

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Moderna’s COVID-19 vaccine, called mRNA-1273, was the second vaccine to receive approval from the Food and Drug Administration (FDA).

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Moderna, an American biotechnology company based in Cambridge, Massachusetts, began developing its vaccine in January 2020. The company worked with the National Institutes of Health (NIH) to finalize gene sequencing for the virus and began working on developing the vaccine soon after.

The first batch of the vaccine was developed in early February—just 25 days after the gene was sequenced. The first dose of the vaccine was given in March 2020 in a Phase 1 clinical trial, and those early trials showed promising results. By mid-year, Moderna received funding and fast-track regulatory allowances to help move development along.

While a vaccine initially wasn’t expected for 18 months or more, the new developments meant Moderna could deliver millions of doses much sooner. In August 2020, the U.S. government ordered 100 million doses of the vaccine—even though it hadn’t even been approved for use yet, and in November, results of the Phase 1 clinical trial were published.

In December, results from Phase 3 clinical trials were published and the U.S. ordered an additional 100 million doses. In February 2021, the U.S. ordered another 100 million doses, bringing the total to 300 million doses.

When Was Moderna's Vaccine Officially Approved?

American health officials granted Moderna’s vaccine an emergency use authorization (EUA) on December 18, 2020, for adults 18 and older, making it the second vaccine to receive the designation from U.S. health officials in one week. On January 31, 2022, the FDA granted the vaccine full approval for those 18 and older. The approved vaccine is marketed under the name Spikevax.

How It Works

The Moderna vaccine is an mRNA vaccine, similar to the one developed by Pfizer. The technology behind these vaccines has been around for about three decades and shown promise in preventing notoriously difficult-to-prevent diseases—even cancer. Unlike conventional vaccines that use inactivated or live viruses, mRNA vaccines are non-infectious and contain no parts of the virus they fight. This makes mRNA vaccines safer for certain populations.

An mRNA (messenger ribonucleic acid) vaccine contains a single-stranded molecule that complements a strand of your DNA. These strands have a special coating that can protect mRNA from chemicals in the body that can break it down, and help it enter muscle cells.

Rather than exposing the body to a small amount of virus to create an immune response, mRNA causes the vaccine recipient to make the protein of interest. This includes:

  • Helping the cell make part of the spike protein that makes COVID-19 so potent
  • Teaching immune cells to recognize and fight similar spike proteins

According to the Centers for Disease Control and Prevention (CDC), mRNA vaccines teach our cells how to make a protein—or even just a piece of a protein. This triggers an immune response.

Moderna’s vaccine was initially tested in adults aged 18 to 55. The Phase 1 trial investigated three different doses—25, 100, and 250 micrograms—given twice, 28 days apart. By the Phase 3 trial, Moderna zeroed in on a two-dose formula of 100 micrograms each, given in two doses about four weeks apart.

How Was Moderna's Vaccine Tested?

Moderna's Phase 3 trial tested a two-dose vaccine (100 micrograms each, given 28 days apart) in adults ages 18 and older. The trial had more than 30,000 participants, including about 7,500 adults ages 65 and older and more than 5,000 adults ages 18 to 64 with risk factors for severe illness from COVID-19.

How Effective Is It?

Researchers that conducted the trials were able to test how effective the vaccine is in creating antibodies and neutralizing responses with tests that use bodily fluids like blood or urine to see whether antibodies to a particular virus can be found. These tests include:

  • ELISA (enzyme-linked immunosorbent assay) test
  • PsVNA (pseudotyped lentivirus reporter single-round-of-infection neutralization assay)
  • PRNT (plaque-reduction neutralization test)

Antibody levels from these tests were compared to those found in people who had recovered from COVID-19 infections. The study investigated responses by other immune cells, like T-cells, too.

After the first round of vaccinations in the 100-microgram and 250-microgram formulas in the Phase 1 trial, antibody responses were similar to the median level of immunity in those who had recovered from and had natural immunity after a COVID-19 infection.

Other immunity tests, like the PsVNA, showed little response after the first dose. However, after the second doses, antibody and other immune responses were strong among the vaccination group—reaching levels that were close to those with the highest natural immune responses after recovering from COVID-19.

Immune responses developed within about two weeks of the first dose, according to the trial report, but were much stronger after the second dose.

Exactly How Effective Is It?

Moderna’s vaccine has been shown in trials to be very effective—preventing COVID-19 in 94.1% of the people enrolled in the Phase 3 clinical trial. Additionally, the vaccine seems to have broad efficacy, with success rates consistent across various age, race, ethnicity, and gender groups.

The CDC also tracked almost 4,000 healthcare personnel, first responders, and other frontline workers under real-world conditions and concluded that the mRNA vaccines (Pfizer-BioNTech and Moderna) were 90% effective at least two weeks after the second dose.

The lifespan of these vaccines isn't clear yet, but research suggests that the Moderna vaccine offers strong antibody protection for at least six months following the second dose.

The leaders of the Phase 3 trial say they will keep collecting blood samples from the study group to check for immunity throughout the first year after vaccination. In the past, viruses in the same virus families don't generate long-lived immune responses, the trial report notes. 

Lab studies of the Moderna vaccine also suggest protection against variants, including B.1.1.7 first found in the United Kingdom, yet there may be a significant drop in protection for B.1.351 first found in South Africa. On March 10, 2021, Moderna announced the launch of a booster study targeting the B.1.351 variant. In March the company also began a Phase 1 trial to test a potentially refrigerator-stable mRNA vaccine candidate for COVID-19.

Limited studies also suggest the vaccine provides protection against the Delta (B.1.617.2) variant. One new study (which has not yet been peer-reviewed) found one dose of the Moderna vaccine to be 72% effective.

How Many Doses Are Available?

By July 2022, the U.S. government had purchased 500 million Moderna COVID-19 vaccine doses and 66 million doses of Moderna's COVID-19 booster vaccine. As of March 2023, slightly more than 250 million vaccine doses and 19 million booster doses had been administered.

While the vaccine itself is free to the public, vaccine providers who administer the vaccine may seek reimbursement for administrative costs from public and private insurance plans, as well as through the Health Resources and Services Administration's COVID-19 Uninsured Program for people with no public or private health insurance. No one receiving a vaccine can be charged any fees.

Who Can Get the Moderna Vaccine?

Everyone 6 months and older is now eligible for the Moderna vaccine, according to the CDC.

State and local health departments are coordinating efforts to distribute doses of the vaccines as they become available. The vaccine is typically available in healthcare centers and retail locations like pharmacies that administer other vaccines. Many states also have mass vaccination sites.

The Moderna vaccine is given in two doses that are 4 to 8 weeks apart. However, according to the CDC, an eight-week interval may be considered for some people ages 6 months through 64 years—especially males ages 12 through 39 years—to help reduce the risk of myocarditis and pericarditis associated with the mRNA COVID-19 vaccines.

In addition, the FDA granted EUA for updated bivalent boosters of the Moderna and Pfizer COVID-19 vaccines. Updated boosters, also known as bivalent boosters, target the most recent Omicron subvariants, known as BA.4 and BA.5, in addition to the original SARS-CoV-2.

Updated boosters are available for people ages 6 months and up and should be given at least two months after the last primary series dose or booster. Children under 6 years can get the Moderna booster only if they also received the Moderna primary series.

Immunocompromised people have a higher risk of being hospitalized, becoming severely ill, or dying from COVID-19. Therefore, the CDC recommends that some immunocompromised people receive an extra primary dose of either the Pfizer or Moderna vaccine and an updated bivalent booster.

When Will Children Be Able to Get the Vaccine?

Moderna began recruiting participants for a clinical trial in 12- to 17-year-olds in December 2020 and fully enrolled 3,724 participants. In June 2021, the company filed for an emergency use authorization for use in adolescents.

After reviewing data from the company's clinical trials, in June 2022, the FDA expanded the EUA for Moderna's vaccine to include children ages 6 months to 17 years. By December 2022, Moderna's bivalent booster received EUA from the FDA.

Side Effects and Adverse Events

Adults who took part in Moderna's Phase 3 trial were asked to record any reactions to the vaccine, either to the area where the vaccine was injected or that they experienced overall. Pain at the site where the vaccine was injected and overall tiredness were the most common side effects in the Phase 3 trial.

Systemic effects are reactions that affect the entire body, and headache and fatigue topped that list and were most common in the vaccine group. Less than 20% of the people who were vaccinated report a fever after the second dose.

Local effects, on the other hand, affect only a small, limited area. The most common effect in this category was pain at the injection site.

The severity of reactions increased with the second dose of the vaccine. Few reactions were rated as severe with the first dose of the vaccine, but several were rated as severe after the second dose. The most common reactions reported by participants after the second dose of the vaccine were:

  • Pain at the injection site
  • Fatigue
  • Headache
  • Muscle aches
  • Chills
  • Joint pain

Nausea or vomiting, fever, red or swollen areas at the injection site, and swollen or enlarged lymph nodes were also reported. 

The percentage of serious adverse reactions was the same for the placebo and vaccine groups (0.6%) and did not indicate any safety concerns. There was also no evidence of vaccine-associated enhanced respiratory disease in the trials. Previous vaccines for coronavirus in veterinary use and animal models have led to respiratory problems related to vaccination.

The most common side effects reported in clinical trials that followed children ages 6 months to 5 years were pain, redness and swelling at the injection site, fever, and swollen lymph nodes in the armpit (or groin) on the same side of the body where the shot was given.

In clinical trials that followed individuals ages 6 to 17 who received the vaccine, the most commonly reported side effects were pain, redness, and swelling at the injection site, fatigue, headache, muscle pain, chills, joint pain, swollen lymph nodes in the armpit on the same side where the shot was given, nausea and vomiting and fever.

More observation will be done on the study groups as time goes on, and more studies are planned on special populations.

Although it's rare, serious allergic reactions can occur after receiving the Moderna vaccine. The Vaccine Adverse Event Reporting System (VAERS) reported that there were 10 cases of anaphylaxis, a life-threatening allergic reaction, after about 4.04 million first doses of the Moderna vaccine (2.5 cases per million doses). Most cases occurred within 15 minutes of vaccination and no deaths were reported.

Risk of Myocarditis and Pericarditis

On June 25, 2021, the FDA announced an update to the Moderna and Pfizer COVID-19 vaccine fact sheets for healthcare providers and vaccine recipients and caregivers about the rare risk of developing myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the lining outside the heart).

The CDC reported a rate of 12.6 cases per million second doses of Moderna's or Pfizer's vaccine within three weeks following vaccination among 12- to 39-year-olds. Cases have been predominately reported in male adolescents and young adults age 16 and older, with symptoms typically occurring within a few days after receiving the vaccine. Most people who developed these conditions have responded well to medication and rest.

The CDC continues to recommend the COVID-19 vaccine for those 12 years and older given the suggested increased risks of these conditions. However, the agency states that an eight-week interval between doses of the mRNA vaccine may be optimal for certain people ages 6 months through 64 years—particularly for males ages 12 to 39 years—to help reduce this risk.

Warning Symptoms

Seek immediate medical attention if you experience any of the following symptoms after vaccination:

  • Chest pain
  • Shortness of breath
  • Feelings of having a fast-beating, fluttering, or pounding heart

Funding and Development

Moderna worked with the National Institutes of Health and the U.S. Department of Health and Human Services to develop its vaccine. The company received about $1.6 billion in federal funding through the Biomedical Advanced Research and Development Authority (BARDA)—a U.S. government program working to speed up the manufacture and distribution of vaccines to fight COVID-19. Each time the U.S. government exercised its option for an additional 100 million doses from Moderna it cost around $1.65 billion, bringing the total cost to about $5.75 billion.


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.

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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.
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By Rachael Zimlich, BSN, RN
Rachael is a freelance healthcare writer and critical care nurse based near Cleveland, Ohio.