Moderna COVID-19 Vaccine Shows High Efficacy, Low Risk for Adverse Events

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The mRNA-1273 vaccine, developed by Moderna and the NIH, was evaluated for its efficacy in preventing COVID-19 illness with onset at least 14 days after the second injection.

The lipid nanoparticle-encapsulated mRNA-based vaccine, mRNA-1273, developed by Moderna and the National Institutes of Health was 94.1% effective at preventing infection from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), according to phase 3 trial reports published in The New England Journal of Medicine.

Healthy adults (N=30,420) were enrolled in this study between July 27 and October 23, 2020 at 99 sites in the United States. Investigators randomly assigned participants in a 1:1 ratio to receive either 2 intramuscular doses of the mRNA-1273 vaccine (n=15,210) or placebo (n=15,210) 28 days apart. Participants were monitored for adverse events and symptoms of coronavirus disease 2019 (COVID-19) for a median of 64 (range, 0-97) days.

In the patient population, the mean age was 52.7 years, 53.1% were men, 79.2% were White, 20.5% were Hispanic or Latino, 10.2% were Black, and less than 10% had comorbid conditions that increased risk for severe COVID-19.

Of the participants who did not receive both doses (4%), 153 withdrew consent, and 114 tested positive for SARS-CoV-2 (placebo: n=69; vaccine: n=45).

Injection site adverse effects occurred among 84.2% and 88.6% of the vaccine and 19.8% and 18.8% of the placebo recipients for the first and second doses, respectively. These adverse injection site events were primarily from pain (86.0%) and most were low severity events.

Systemic events, defined as fever, headache, fatigue, myalgia, arthralgia, nausea or vomiting, and chills, occurred among 54.9% and 79.4% of the vaccine and 42.2% and 36.5% of the placebo recipients for the first and second doses, respectively. The systemic events lasted an average of 2.6 days following the first dose and 3.1 days following the second.

For the vaccine and placebo groups, medically attended adverse events were similar (9.7% vs 9.0%) as were serious adverse events (0.6% vs 0.6%). Events determined to be treatment-related were increased among the vaccine recipients (8.2%) compared with the placebo group (4.5%).

Deaths occurred among 2 patients in the vaccine group due to cardiopulmonary arrest and suicide and among 3 patients in the placebo group due to intraabdominal perforation, cardiopulmonary arrest, and severe systemic inflammatory syndrome.

In total, 196 COVID-19 cases were identified, in which 11 were among vaccine (3.3 cases per 1000 person-years [PY]; 95% CI, 1.7-6.0) and 185 among the placebo (56.5 cases per 1000 py; 95% CI, 48.7-65.3) recipients. These rates equated to an efficacy of 94.1% (95% CI, 89.3%-96.8%; P <.001) for the prevention of symptomatic SARS-CoV-2 infection. Key secondary analyses indicated an overall vaccine efficacy of 95.2% and vaccine efficacy was consistent across all subgroups.

Of the 30 patients who had severe COVID-19 infections, all received the placebo. The efficacy for protection against severe COVID-19 infection was 100%.

This study was limited by its short follow-up duration. Participants will continue to be followed for 2 years.

These findings indicated the mRNA-1273 vaccine was highly efficacious for protecting against SARS-CoV-2 with little risk for severe adverse reactions.

Disclosure: Multiple authors declared affiliations with industry. Please refer to the original article for a full list of disclosures.


Baden L R, El Sahly H M, Essink B, et al; for the COVE Study Group. Efficacy and safety of the mRNA-1273 SARS-CoV-2 vaccine. N Engl J Med. Published online December 30, 2020. doi:10.1056/NEJMoa2035389.