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Pfizer on Tuesday announced its maternal vaccine for RSV, administered during pregnancy, was effective at preventing infants from developing severe symptoms from birth through their first six months.

The company said that it plans to file the data on the vaccine with regulators by the end of the year, with the hope of having it available by next winter. If authorized, it would be the first vaccine on the market to help protect young infants from the common but potentially life-threatening respiratory illness.

There are currently no vaccines available for RSV, and the only treatment is monoclonal antibodies, usually reserved for extremely high-risk cases, including infants born prematurely or those with chronic diseases related to the heart and lungs. 

According to Pfizer’s news release, the vaccine reduced the rate of severe illness in infants that required medical attention by 81.8% through the first 90 days of life. That efficacy dropped to 69% over a six-month follow-up period.

Researchers also tested how well the vaccine prevents all RSV-related medical visits, including for non-severe cases. The study found the vaccine reduced medical visits in vaccinated patients by about 50% compared to mothers who got placebo, though the results were not considered statistically significant. 

The company said the vaccine was well-tolerated with no safety concerns for both the vaccinated individuals and their newborns. However, the data has not yet been submitted to a journal or been peer-reviewed.

Like the flu, RSV season usually occurs during colder weather, though it’s been hitting unusually hard and early this year, contributing to a wave of respiratory infections that is overwhelming children’s hospitals nationwide.  

In healthy adults and older children, RSV typically causes mild, cold-like symptoms that go away with moderate rest and self-care. 

Younger children, especially those less than 6 months old, have the highest risk of developing severe cases that could lead to hospitalization. RSV is the leading cause of hospitalization for infants. 

Worldwide, RSV results in the death of hundreds of thousands of children annually, with the vast majority in developing countries.

Unlike some other RSV vaccine candidates, Pfizer’s shot is administered during pregnancy, with the aim of transferring antibodies from mothers to infants. Other maternal vaccines include the flu shot, as well as one for tetanus, diphtheria and pertussis.

Pfizer’s study enrolled approximately 7,400 pregnant individuals in 18 countries. The trial began in June 2020, so it spanned multiple RSV seasons in both the northern and southern hemispheres.