BERLIN, Germany — A new vaccine given during pregnancy could prevent severe respiratory syncytial virus (RSV) illness in infants, according to the results of a recent phase 3 clinical trial. An international team finds that immunization had an association with an 81-percent rate of protection during the first six months of life.

RSV is a respiratory virus that invades the lungs and causes mild, cold-like symptoms. Since infants have tiny airways, however, the inflammation from the illness can make it hard to breathe. Lower respiratory infections due to RSV may cause shortness of breath and other fatal breathing problems. In 2019, an estimated 100,000 children under the age of five died from RSV infection, with 97 percent residing in low to middle-income countries.

“Previously, only symptomatic treatment was available for RSV. In severe cases, supplemental oxygen is vital, but in poorer countries, it is often not administered fast enough or to an adequate extent,” says Beate Kampmann, the head of the Institute of International Health at Charité and Einstein Professor for Global Health, in a media release.

One possibility to prevent infection in the first place is vaccination. Throughout history, vaccines have helped with mitigating the severity of symptoms and illness for other viruses such as flu, whooping cough, and SARS-CoV-2. The drug in the latest clinical trial would protect newborns and infants by generating antibodies in pregnant mothers. The antibodies would, in turn, pass along to the fetus through the placenta. When the child is born, it would already come with a well-prepared immune system for the first few months.

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Pregnant woman check for preeclampsia
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4 in 5 babies were RSV-free for 12 weeks

The experimental vaccine is called RSV-preF and researchers delivered it to 3,682 women in their second or third trimester of pregnancy in 18 countries between 2020 and 2022. Another group of pregnant women was randomly selected to be part of the placebo group, where they received a shot, but it contained no vaccine. The goal of the trial was to see if the vaccine actually worked in preventing RSV illness and if it produced any concerning side-effects.

Researchers tracked the health of children born from the participants and if there were any signs of respiratory illness for one to two years after birth. They were also tested for RSV, and if so, the severity of the infection.

Over 80 percent of children born from mothers given the RSV-preF vaccine were protected from severe RSV illness in the first three months of life. Additionally, over two-thirds were protected at six months of age. The vaccine played a huge difference in maintaining the health of babies living in low-income countries where healthcare is limited.

“It’s important to perform vaccine trials in the countries where the vaccines are to be used later on,” Kampmann explains. “Especially in socioeconomically disadvantaged countries, people often suffer from chronic gastrointestinal conditions due to poor hygiene conditions. That can lower the effectiveness of vaccination, as in the case of the rotavirus vaccine. And there are comorbidities such as malaria and HIV that impair the transfer of antibodies through the placenta. All these factors affect how well a vaccine works in the end.”

The study is published in The New England Journal of Medicine.

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