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SciCheck’s COVID-19/Vaccination Project

A Guide to Moderna’s COVID-19 Vaccine

This article is available in both English and Español

Moderna’s mRNA vaccine is the second COVID-19 vaccine authorized for use in the U.S. Here, we give a rundown of basic facts about the vaccine and an overview of how it works.

Quick Summary

Vaccine name: mRNA-1273

Design type: mRNA

Dose number: 2 doses, 28 days apart

Efficacy: 94.1% efficacy in preventing symptomatic COVID-19 in adults two weeks or more after receipt of the second dose.

Safety: No serious safety concerns reported other than some rare allergic reactions and a likely, but rare, association with heart inflammation. The shots, however, commonly elicit reactions in people that can include injection site pain, fatigue, headache, muscle or joint pain, and chills.

Expected timeline: First doses were administered beginning on Dec. 21 to prioritized populations, following the Food and Drug Administration’s emergency use authorization on Dec. 18.

Operation Warp Speed involvement: The U.S. government has devoted more than $4 billion to the development, testing and manufacturing of the vaccine. The government has contracted to buy 300 million doses, with delivery of 100 million doses each by the end of March, June and July.

Storage considerations: -20°C (-4°F) for long-term storage; after thawing, vaccine vials should be stable at refrigerator temperatures for 30 days.

Just a week after the FDA authorized the first COVID-19 vaccine on an emergency use basis — Pfizer/BioNTech’s mRNA shot — the agency greenlighted a second, from the Cambridge, Massachusetts, biotech company Moderna.

Like its predecessor, the Moderna vaccine also uses an mRNA design, but it differs on the specifics, including the amount of time between doses, the amount of genetic material in each shot and the precise lipid nanoparticles that surround and protect the mRNA. That difference in fat formulation is thought to be why the Moderna vaccine doesn’t need to be kept as cold — a feature that should make it easier to distribute.

After releasing positive interim results from its phase 3 trial on Nov. 16, Moderna announced on Nov. 30 that its vaccine had an efficacy of 94.1% in preventing disease in adults. The company submitted its emergency use authorization, or EUA, application to the FDA the same day.

On Dec. 17, an external panel advising the FDA agreed the vaccine’s benefits outweighed the risks in a 20-to-0 vote, with one abstention, and the following day the agency authorized the vaccine for emergency use in people 18 years of age and older.

While the United Kingdom was the first to authorize the Pfizer/BioNTech vaccine, the U.S. was the first country to permit the use of the Moderna vaccine, following substantial R&D and manufacturing support from the government and a collaboration with the National Institutes of Health.

Moderna Q&A

What is in the vaccine and how does it work?

As with most other COVID-19 vaccine designs, the Moderna mRNA vaccine works by triggering an immune response against the SARS-CoV-2 virus’s spike protein, which sits on the surface of the virus and is what the virus uses to enter cells.

The vaccine is made of modified messenger RNA, or mRNA, wrapped in a special blend of fatty molecules known as lipid nanoparticles. The mRNA provides instructions for cells to make their own spike proteins, prompting the body to generate protective antibodies and activate other immune cells known as T cells. The lipids help deliver the RNA into cells and prevent it from being degraded too quickly.

Whereas the Pfizer/BioNTech vaccine includes 30 micrograms of mRNA in each dose, Moderna’s shot has 100 micrograms; the two vaccines also use a different mix of lipids. These differences may explain why Moderna’s vaccine appears to produce more expected side effects than its competitor.

As the Centers for Disease Control and Prevention has explained, there is no way to catch COVID-19 from this type of vaccine because the vaccine is not made of a virus. And because the mRNA from the vaccine doesn’t enter the nucleus — the part of the cell that houses DNA — it “does not affect or interact with a person’s DNA,” contrary to some online rumors.

How was the vaccine developed?

The Moderna vaccine was developed in collaboration with the National Institutes of Health. Researchers at the National Institute of Allergy and Infectious Diseases’ Vaccine Research Center were already working with scientists at Moderna on an investigational vaccine to protect against MERS, another disease caused by a coronavirus. As soon as the genetic sequence of SARS-CoV-2 became available in January, the team was able to apply that knowledge to design a COVID-19 vaccine targeting the virus’s spike protein.

In mid-March, the NIH began testing three different vaccine doses in a phase 1 trial involving 45 adults 55 years of age or younger. Two doses of the inoculation triggered the expected immune responses, including neutralizing antibody responses similar to those recovering from COVID-19, without raising any serious safety concerns. The trial was later expanded to include 40 older adults. 

The company launched a phase 2 trial with 600 volunteers in May to confirm the dose and continue to assess safety.

Meanwhile, a preclinical study published in July showed vaccinated monkeys that were challenged with virus rapidly cleared the pathogen from their bodies. A study published the following month found that in mice, the vaccine successfully prevented infection in the lung and nose.

A phase 3 randomized controlled trial began in late July that included just over 30,000 volunteers across 99 sites in the U.S. Enrollment was slowed in September to increase minority participation. Because the trial only included people over the age of 18, Moderna began enrolling 3,000 teens as young as 12 in a combined phase 2/3 trial in mid-December, with results expected in the spring of 2021.

How effective is the vaccine?

Moderna announced in a Nov. 30 press release that its vaccine was 94.1% effective in preventing COVID-19 disease in adults starting two weeks after the second dose. 

As we’ve explained before, that roughly means that a vaccinated person has a 94% reduced risk of becoming sick, compared with a similar person who wasn’t immunized — although the true effectiveness of the vaccine outside of a trial may be lower. (A study by the Centers for Disease Control and Prevention, published in late March 2021, found the Pfizer/BioNTech and Moderna vaccines were 90% effective in real-world conditions, two weeks after the second dose, and 80% effective two weeks after the first dose.)

The efficacy figure is based on observing a total of 196 symptomatic COVID-19 cases, 185 of which were in the placebo group versus 11 in the vaccine group.

According to the FDA’s summary of the company’s data, the efficacy was a bit lower in adults 65 years of age or older (86.4%), although earlier interim results did not suggest any differences in efficacy across age, race and ethnicity, or gender.

The vaccine also protects against severe cases of COVID-19. In the trial, all 30 cases of severe COVID-19, including one death, occurred in the placebo group. Moderna, however, did reveal during its Dec. 17 meeting with the FDA advisory committee that there was one severe COVID-19 case in the vaccine group that had not yet been fully assessed.

It is not yet known how long the vaccine affords protection against disease or whether the vaccine can prevent infection or keep someone from transmitting the virus to others. However, the CDC has said that “a growing body of evidence” suggests those who are fully vaccinated are “potentially less likely to transmit SARS-CoV-2 to others.” 

How safe is the vaccine?

No serious safety concerns were reported in the trials. As with the Pfizer/BioNTech vaccine, however, vaccine recipients have frequently experienced temporary side effects, including pain at the site of injection, swelling of the lymph nodes in the vaccinated arm, fatigue, headache, muscle or joint pain, and chills.

According to the FDA’s summary, these responses are more likely after the second dose and tend to be more severe in younger adults compared with those 65 years of age or older.

While scientists say this is a sign that the shots are working as they should, these reactions can be unpleasant. 

Serious adverse events, which refer to potentially life-threatening or disabling events, occurred in 1% of participants in both the vaccine and placebo groups in the phase 3 trial. Only three such events were considered by the FDA to be likely due to the vaccine: one case of uncontrollable nausea and vomiting and two cases of facial swelling, both of which occurred in women who had had cosmetic dermal filler injections in the past six months.

A third case of facial swelling — not considered severe — also involved a woman who had previously had a dermal filler injection in her lips and had experienced a similar reaction following an influenza shot.

Similar to the Pfizer/BioNTech vaccine, there was a slightly higher number of cases of Bell’s palsy, a form of temporary facial paralysis, in the vaccine group. With three instances out of more than 15,000 vaccine recipients and one in a similarly sized placebo group, the FDA was unable to determine whether the condition was triggered by the vaccine or not, but suggests ongoing surveillance.

At least a few serious allergic reactions — expected with any vaccine — have been reported with the Moderna vaccine. As of Jan. 18, there have been 2.5 cases of anaphylaxis per million doses of the Moderna vaccine, according to the CDC. None of these cases has been fatal.

At this time, the only people the FDA says should not receive the Moderna vaccine are those who have had a serious allergic reaction to a previous dose of the vaccine or one of its ingredients. The CDC advises skipping the immunization if you have had an allergic reaction to polyethylene glycol, a form of which is in the vaccine, or polysorbate, which is similar to polyethylene glycol. The agency also recommends consulting with your physician if you have ever had an immediate allergic reaction to other vaccines or injectable therapies.

The Moderna vaccine may also be linked to an increased risk of rare heart inflammation in young people. In late June, the CDC’s Advisory Committee on Immunization Practices concluded there is a “likely association” between COVID-19 mRNA vaccination and myocarditis, or inflammation of the heart muscle, and pericarditis, or inflammation of the lining around the heart, in teens and young adults.

As of July 6, the agency says there have been 971 preliminary reports of either condition in people 30 years old or younger following immunization with any COVID-19 vaccine. The bulk of the reports, which are through the Vaccine Adverse Event Reporting System and do not necessarily mean the vaccine caused the problem, are with the Pfizer and Moderna vaccines and are more common among males and after the second dose.

Health officials have emphasized that the potential vaccine-related myocarditis and pericarditis cases are rare and the benefits of vaccination still outweigh the risks. Early evidence, the committee said, suggests these myocarditis cases are less severe than typical ones. The CDC has also noted that most patients who were treated “responded well to treatment and rest and quickly felt better.”

Editor’s Note: This story will be updated as necessary.

Editor’s note: SciCheck’s COVID-19/Vaccination Project is made possible by a grant from the Robert Wood Johnson Foundation. The foundation has no control over our editorial decisions, and the views expressed in our articles do not necessarily reflect the views of the foundation. The goal of the project is to increase exposure to accurate information about COVID-19 and vaccines, while decreasing the impact of misinformation.