Data show Moderna’s vaccine continues to provide strong protection six months later

The efficacy rate topped 90%, the company said, with 95% being protected from severe COVID-19

“Moderna will make as many updates to our COVID-19 vaccine as necessary until the pandemic is under control,” said CEO Stephane Bancel.Adam Glanzman/Bloomberg

Moderna’s two-shot vaccine prevented more than 90 percent of COVID-19 cases in updated trial data released Tuesday by the Cambridge biotech, slightly lower than the 94.1 percent efficacy rate trumpeted when the vaccine was cleared for use in December, but still impressive.

In addition, the vaccine prevented 95 percent of severe cases in trial participants, according to the study, which gauged the efficacy of the vaccine at a median of six months after 30,000 volunteers received two shots. Half of the volunteers received the vaccine, and half were given a placebo.

All of the trial sites were in the United States and before the emergence of worrisome viral variants in Europe, Africa, and South America.

Moderna shared the updated data in a news release the day before the company hosts an online “Vaccines Day” for investors. Executives are expected to discuss Moderna’s pipeline of vaccines against other diseases. As with the coronavirus vaccine, Moderna wants to harness messenger RNA to prevent those illnesses. The custom-made messenger molecules instruct cells to stimulate the body’s immune system.

The company also said it has begun testing two tweaked versions of its COVID-19 vaccine on laboratory mice in the hope the shots will act against a variant called B.1.351. That variant, which first emerged in South Africa and has been detected in the United States, has caused alarm because it appears to blunt the effectiveness of the vaccine cleared for emergency use on Dec. 18 by the Food and Drug Administration.

One of the revised vaccines is tailor-made for that variant. The other combines the variant-specific vaccine and the authorized vaccine. Both raised the levels of antibodies against the variant in the mice. Moderna has begun testing the same vaccines on human volunteers in a clinical trial.

“The new [animal] data on our variant-specific vaccine candidates give us confidence that we can proactively address emerging variants,” said chief executive Stephane Bancel. “Moderna will make as many updates to our COVID-19 vaccine as necessary until the pandemic is under control.”

So far, the company has delivered 117 million doses to the federal government and is on target to deliver another 83 million by the end of May, said Colleen Hussey, a spokeswoman. Moderna plans to deliver another 100 million by the end of July, for a total of 300 million.

The government last year bankrolled the costs of developing the vaccine as part of the Operation Warp Speed program and pledged to buy at least 100 million doses for $2.48 billion.