For the prevention of coronavirus illness and severe disease, the efficacy of the mRNA-1273 SARS-CoV-2 vaccine was maintained for more than five months. An adequate safety profile and protection against asymptomatic COVID-19 infection was also noted.

A study reported the mRNA-1273's safety and efficacy findings of the final assessment of the blinded phase of trial.

The study recruited people who were at elevated risk for coronavirus disease or its complications. The recruited people (n=30,415) were randomly allocated to receive 2 intramuscular injections of mRNA-1273 (100 μg) or placebo, twenty-eight days apart. Prevention of coronavirus illness with onset at least fourteen days following the 2^nd injection in people who had not previously been infected with SARS-CoV-2 was the primary endpoint with the data cutoff date set as 26^th March 2021.

In total, 15209 people were given the mRNA-1273 vaccine, and 15206 people were given a placebo. More than 96% of people were given both injections. Notably, 2.3% had evidence of coronavirus infection at the baseline. In the blinded phase of trial, the median follow-up was 5.3 months. Vaccine effectiveness to prevent coronavirus illness was 93.2%, with 744 confirmed cases in the placebo group (136.6 per 1000 person-years) and 55 cases in the vaccine group (9.6 per 1000 person-years).

The effectiveness of the vaccine for the prevention of severe disease was 98.2%, with 2 cases in the vaccine arm and 106 in the placebo arm. The effectiveness in the prevention of asymptomatic infection initiating 14 days after the 2nd injection was 63%, with 214 cases in the vaccine arm and 498 in the placebo arm. The effectiveness of the vaccine showed consistency across the racial and ethnic groups, age groups, and people with coexisting conditions, without any safety concerns reported.

mRNA-1273 coronavirus vaccine is effective to prevent SARS-CoV-2 illness and serious disease at extended follow-up, i.e. greater than five months.