Pfizer’s experimental vaccine for a respiratory virus called RSV was nearly 86% effective in preventing severe illness in a late-stage clinical trial of older adults, the company announced in a release Thursday.

The vaccine, called RSVpreF, was also found to be about 67% effective in preventing milder illness from the virus and caused no serious safety concerns, the company said.

The results were based on an early analysis of a phase 3 trial of 37,000 adults ages 60 and older, according to Pfizer. The protein-based vaccine is administered in a single dose.

Experts say the findings are significant as there are currently no approved vaccines to prevent RSV infections, which are responsible for 177,000 hospitalizations and 14,000 deaths in older adults each year, according to the Centers for Disease Control and Prevention

An RSV vaccine for this population “makes sense,” said Dr. Ofer Levy, the director of the Precision Vaccines Program at Boston Children’s Hospital. “There is a serious unmet need to protect older adults against viral respiratory disease, which can cause severe illness or even death in this age group.”

Levy noted that while Pfizer’s results look promising, he would still like to see more data on the vaccine. The company’s results were announced in a news release, and it has not yet been peer-reviewed.

RSV, or respiratory syncytial virus, occurs each year in most regions of the United States during fall, winter and spring. It usually causes mild, cold-like symptoms in adults but can sometimes lead to life-threatening illness.

The virus can be especially dangerous for older adults with other medical conditions, such as chronic lung illness or heart disease, with infection severity sometimes rivaling that of influenza, said Dr. William Schaffner, an infectious diseases expert at Vanderbilt University Medical Center.

Synagis, a monoclonal antibody injection, has been approved for use in the U.S. to prevent severe illness, but it’s only available for certain high-risk infants, like babies who were born prematurely or have a low birth weight.

Older adults who become severely ill from the virus are often treated with antibiotics in case there are bacteria in the lungs, Levy said, and may need to be placed in intensive care with supplemental oxygen.

“It’s so important to develop a vaccine,” he said. 

Pfizer’s vaccine is a so-called bivalent vaccine, targeting two strains of the virus called RSV A and B.

The company said it plans to submit a vaccine application to the Food and Drug Administration for full approval in adults ages 60 and older later this fall. 

Dr. William Gruber, Pfizer’s senior vice president of vaccine clinical research and development, noted that infections from RSV do not confer lifelong immunity, meaning people may eventually have to get another dose of the vaccine.

The company is still gathering data to see how long protection from the initial RSV vaccine lasts, he said, but it is possible it could become an annual immunization like the flu shot.

“We don’t know yet,” he said.

He said the company is also looking to expand testing the vaccine in other age groups, pending discussion and clearance from federal regulators.

The company announced in Thursday’s release that a separate phase 3 trial testing the vaccine in pregnant women remains ongoing. 

Gruber said he expects data on effectiveness of the vaccine for that cohort before the end of the year. “We are hopeful and optimistic that we’ll have positive results,” he said.

A study published in the New England Journal of Medicine in April found that pregnant women who received Pfizer’s vaccine passed their protective antibodies on to their newborns. 

Schaffner, of Vanderbilt University Medical Center, noted the importance of an RSV vaccine for children, especially those under age 5. The virus hospitalizes 58,000 kids in the age group each year, according to the CDC.

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