Nov. 1, 2022 – New trial data from drugmaker Pfizer shows promising results of a vaccine given to mothers during pregnancy that later protects infants in their first months from the worst effects of respiratory syncytial virus, or RSV. 

Pfizer will apply for FDA approval by the end of the year, the company said in a statement on Tuesday. 

Trial results are so promising that – after talking with government regulators – the company will stop enrolling new people in the study. 

Specifically, the company reported the vaccine prevented severe illness particularly well during the first 90 days of life, with measurable protection against severe illness continuing through 6 months of age. (That period is when infants are the most fragile if they get sick with RSV.) 

RSV is a respiratory illness than can affect anyone, usually resulting in no symptoms or those similar to the common cold. But it can be particularly dangerous – and even deadly – for babies and for people over the age of 65. Pfizer and another drug company, GSK, are developing promising vaccines for older adults, The Washington Post reported.

RSV is the leading cause of hospitalization for infants, the Post noted. 

The Pfizer study, called MATISSE, enrolled 7,400 pregnant women in 18 countries worldwide. Those who received the vaccine were given it during the late second to third trimester of pregnancy. Women in the study were monitored for safety through the rest of their pregnancy and 6 months after their children were born. Infants were monitored for at least 1 year for safety and effectiveness; more than half of them were monitored for 2 years. 

The Pfizer vaccine works by passing maternal antibodies to the infant during pregnancy, the Post reported, noting that other vaccines transmitted via maternal immunization include those for influenza, diphtheria, tetanus, and pertussis.

Annually,  RSV has a devastating impact on young children, hospitalizing tens of thousands and causing up to 300 deaths, data show.

For every 100 children who get RSV under 6 months of age, one or two of them may need to be hospitalized, according to the CDC. Those hospitalized infants may need oxygen, intubation, or even mechanical ventilation to help with breathing. 

“Most improve with this type of supportive care and are discharged in a few days,” the CDC said.

“I think this is a big step for protecting babies against RSV and improving overall lung health,” vaccine researcher Barney Graham, PhD, told the Post. “Overall, it’s an exciting time for RSV. It’s also a troubling time, because you see how the patterns of infection have been changed by COVID, and we’re having an earlier, bigger season this year than we have for a couple of years – and it’s causing a lot of hospitalization and misery for people.”

As many as four RSV vaccines may have applications submitted to the FDA  this year, according to CNN. Also in development is an antibody shot given to infants just after they are born, the news outlet reported. 

Pfizer’s data, announced Tuesday, has not yet been published or peer-reviewed, but the company said it is seeking peer-reviewed publication.

“We are thrilled by these data as this is the first-ever investigational vaccine shown to help protect newborns against severe RSV-related respiratory illness immediately at birth,” Annaliesa Anderson, PhD, Pfizer chief scientific officer for vaccine research & development, said in a statement. “We look forward to working with the FDA and other regulatory agencies to bring this vaccine candidate to expectant mothers to help protect their infants against severe RSV during their most vulnerable first six months of life, which has the highest burden of RSV illness in infants.”

Show Sources


Pfizer: “Pfizer Announces Positive Top-Line Data of Phase 3 Global Maternal Immunization Trial for its Bivalent Respiratory Syncytial Virus (RSV) Vaccine Candidate.”

The Washington Post: “Pfizer’s RSV vaccine, given during pregnancy, protects infants from severe illness.”

CDC: “RSV in Infants and Young Children,” “RSV Research & Surveillance.”

CNN: “By the next RSV season, the US may have its first vaccine.”

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