Moderna on Tuesday announced that its vaccine targeting respiratory syncytial virus (RSV) was nearly 84% effective in a clinical trial of older adults, adding another RSV vaccine to the pipeline of candidates for potential FDA approval.

What the data says about Moderna's vaccine

In a trial of around 37,000 people across 22 countries, Moderna found that its vaccine was 83.7% effective at preventing RSV in people ages 60 and older and 82.4% effective at preventing RSV with three or more symptoms.

Patients in the study experienced some adverse events following the shot, including injection-site pain, fatigue, and headache, but no safety concerns were identified related to the vaccine during the trial, Moderna said.

According to Moderna president Stephen Hoge, the vaccine's efficacy appears to hold up for at least six months and could be longer, depending on follow-up analyses. Hoge said Moderna believes its RSV vaccine will be a once-yearly shot ahead of the typical RSV season.

Between 6,000 and 10,000 older adults die from RSV each year, and the disease causes between 60,000 and 120,000 hospitalizations, according to CDC. This year, RSV cases began to spike earlier than normal before slowing down in November.

Moderna joins Pfizer and GSK in developing RSV vaccines for FDA approval. In August, Pfizer announced its vaccine reduced RSV disease with two or more symptoms by 66.7% and reduced severe disease by 85.7%. Meanwhile, GSK announced in October that its RSV vaccine reduced disease by 82.6% and severe disease by 94.6%.

Both Pfizer and GSK have submitted their data to FDA for approval, with a decision expected in May. Moderna said it will submit its data for approval in the first half of this year. Currently, there is no FDA-approved vaccine for RSV.

"Today's results represent an important step forward in preventing lower respiratory disease due to RSV in adults 60 years of age and older," said Moderna CEO Stéphane Bancel. "These data are encouraging and represent the second demonstration of positive phase 3 trial results from our mRNA infectious disease vaccine platform after Spikevax, our COVID-19 vaccine."

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Experts were encouraged by Moderna's results, adding to the potential for the first-ever FDA-approved vaccine for RSV.

"I've been working on RSV for a long time and there's been periodic promises of having an RSV vaccine around the corner, which has never proven to be true," said Larry Anderson, a professor of pediatric infectious disease at Emory University School of Medicine. "To have three vaccines with a good chance to be licensed and this kind of efficacy data is really pretty amazing."

"It's a fantastic result to have high-level efficacy against a disease that causes a lot of morbidity in the elderly, and it seems as though they're finding consistently that they're protecting relatively milder and more severe disease," said Philip Krause, an independent consultant and former FDA official. "And this is a randomized and controlled trial so the quality of the data is likely to be very high."

Now, according to Krause, the important question is: how long does protection from the vaccines last? In essence, all RSV vaccines are booster shots, because people are infected with the disease many times throughout their lives, Krause said. Right now, how long protection from these vaccines lasts is an unknown.

"The real question will be how long will the protection actually last and, is this going to be a vaccine that needs to be given every year or is it a vaccine that can be given less frequently?" Krause said. (Kimball, CNBC, 1/17; Salzman/Kekatos, ABC News, 1/17; Herper, STAT+ [subscription required], 1/17; Loftus, Wall Street Journal, 1/17)