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Experimental RSV vaccine for pregnant women protects infants against severe illness, Pfizer says

Children through 12 months old are among those with the highest risk for severe RSV infection. Pfizer plans to submit its results to the FDA by the end of the year.

An experimental RSV vaccine for pregnant women from Pfizer is effective at protecting newborns against severe illness for at least six months, the company said in a press release Tuesday.

In a phase 3 study, the Pfizer vaccine was about 82% effective against severe illness from respiratory syncytial virus, or RSV, in the first 90 days of life in babies born to vaccinated women, according to the company. The vaccine was also about 70% effective against severe infections through the first six months of life.

Protection against any RSV infection was lower: about 57% in the first 90 days and about 51% in the first six months.

The results were announced in a news release and have not been published in a medical journal or reviewed by outside scientists.

Pfizer said it plans to submit the data to the Food and Drug Administration by the end of the year. The company's CEO, Albert Bourla, said on a call with investors Tuesday that he expects the vaccine could be available by late 2023 or early 2024. An independent group of outside experts monitoring the trial recommended that the company end trial enrollment early because of the vaccine’s potential benefits.

The results "sound very promising," said Dr. Ofer Levy, the director of the Precision Vaccines Program at Boston Children’s Hospital.

He noted he couldn't draw any firm conclusions because the results were announced in a news release and he hasn't seen the full data.

"Obviously, we'd love it to be 100%," he said, referring to the vaccine's effectiveness. "But many vaccines don't achieve that, so this is a very respectable level of efficacy."

The trial included about 7,400 pregnant women who received either a single dose of the vaccine or a placebo during the late second to third trimester of their pregnancy. Babies were followed for at least one year after birth. 

Pfizer's release didn't say whether there was a difference in protection in infants based on when the mother got vaccinated. The company said it expects that data will be available when the final results are published in a peer-reviewed journal.

In April, the company published an interim analysis that suggested pregnant women who received its vaccine passed their protective antibodies to their babies in the womb.

Levy noted that because the babies are acquiring RSV antibodies from their mothers, rather than getting them directly — also known as passive immunization — the protection may not be as long-lasting and the babies could eventually need an additional vaccine dose.

Pfizer said in a statement that it is currently not conducting an RSV vaccine trial in infants. However, it said, it will continue to evaluate babies born to vaccinated mothers for up to 24 months as part of its ongoing analysis.

The trial results come amid an early surge in pediatric cases of RSV that is contributing to a shortage of beds in children’s hospitals across the United States. Hospitals are also facing rising flu cases, as well as Covid.

There is currently no vaccine available for RSV,  the most common cause of bronchiolitis — an inflammation of the airways — and pneumonia in children through 12 months old, according to the Centers for Disease Control and Prevention. The virus is responsible for thousands of pediatric hospitalizations and hundreds of deaths each year. 

Like many other respiratory infections, RSV can cause severe breathing problems in infants and toddlers, whose smaller airways can lead to a rapid buildup of mucus in the throat and lungs. 

Given that RSV puts many babies in the hospital, "a vaccine that prevents severe disease 70% of the time until the age of six months is very good," said Dr. Céline Gounder, a senior fellow at KFF, formerly known as Kaiser Family Foundation, and an infectious diseases specialist.

The vaccine's effectiveness appears to wane over time, she added, but if timed correctly, "it could prevent a lot of hospitalizations and deaths in this age group."

Dr. William Schaffner, an infectious diseases expert at Vanderbilt University Medical Center, agreed, saying he can "imagine the thousands of parents that won't undergo the agony and distress of having their baby hospitalized" if there were a vaccine for RSV.

A number of other vaccines are already recommended during pregnancy to help prevent illness in infants, including vaccines against flu, whooping cough and Covid.

Pfizer's RSV vaccine could be "a major step forward so that we can attack this last really bad communicable disease of the neonatal period," Schaffner said.

Pfizer’s vaccine, called RSVpreF, uses the same technology seen in a number of other vaccines, including those for hepatitis B and shingles. It targets a protein that the virus uses to enter human cells.

Aside from pregnant women, Pfizer is testing its vaccine in older adults, who are also at risk for severe RSV infections. The company reported positive results from that trial in August. 

Pfizer has previously said it intends to test its vaccine in younger age groups. 

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