Pfizer’s RSV vaccine, given during pregnancy, protects infants from severe illness

Pfizer announced Tuesday that its maternal RSV vaccine, given during pregnancy, protected infants from developing severe symptoms during the first six months after birth — a critical window of vulnerability.

The company plans to apply for approval of the vaccine before year’s end, with the hope that the shot could be the first vaccine to help protect infants against RSV — respiratory syncytial virus — as soon as next winter.

The data was announced in a news release and has not yet been published or peer reviewed. But it offers a glimmer of good news in the middle of a brutal and early RSV season that has contributed to a wave of respiratory illness that is overwhelming many pediatric hospitals.

“We’re very hopeful everything can be done in time to vaccinate mothers before the next RSV season,” said Annaliesa Anderson, chief scientific officer of vaccine research and development at Pfizer. “We’re about to come into a very heavy RSV season. We’re seeing hospitals filling up. Everyone appreciates the urgency that can really help to prevent this.”

RSV is a common respiratory illness, and most people experience it as common cold-like symptoms. But in young infants, whose airways are smaller, the virus can be life-threatening — and it is the leading cause of hospitalization for babies.

For decades, fear and failure in the hunt for an RSV vaccine. Now, success.

Unlike vaccines that are given directly to infants, the Pfizer shot provides protection through an indirect route.Antibodies are naturally passed down to infants during pregnancy, so maternal vaccination is a way of giving babies a temporary, but immediate, shield of immune protection. Vaccines for influenza, diphtheria, tetanus and pertussis are also given as maternal immunizations.

The Pfizer vaccine, given early in the third trimester, was 69 percent effective after birth in preventing severe cases of illness that required medical attention over six months. It was even more effective within the first three months after birth, probably reflecting antibody levels that naturally drop off over time.

Researchers also measured whether the vaccine prevented infant medical visits caused by RSV, including cases that were not severe. Those results did not reach statistical significance, but suggested that the vaccine could cut medical visits due to RSV in half during the first six months after birth.

The company said that there were no major safety concerns for babies or vaccinated individuals, but that data will be closely scrutinized by regulators. There were 7,400 pregnant people in the trial, and infants were followed for at least a year.

Barney Graham, a vaccine expert at Morehouse School of Medicine whose work with Jason McLellan at the University of Texas at Austin underlies much of the progress in the RSV field, said the news was gratifying.

“I think this is a big step for protecting babies against RSV and improving overall lung health,” Graham said. “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.”

Two promising tools to prevent RSV in infants could soon be available, if regulators agree they are safe and effective. In addition to Pfizer’s maternal vaccine, AstraZeneca and Sanofi showed that a single dose of a monoclonal antibody could protect infants from RSV for five months. That drug, nirsevimab, was recently recommended for approval in Europe. Two vaccines for older adults have also showed success, from Pfizer and GSK.

Many pediatricians have been waiting for tools to fight RSV for decades. Early efforts to create a vaccine backfired disastrously, causing babies to develop an enhanced illness if they became infected and casting a shadow over the field.

If both a preventive monoclonal antibody treatment and a vaccine soon become available, it will give physicians options — and lead to debates about how the different tools should be used.

“I’m so thrilled at where the field is right now. I think there are many, many people out there in the world who share my excitement,” said Ruth Karron, a pediatrician and professor of international health at the Johns Hopkins Bloomberg School of Public Health, who is working on a different vaccine that could be given directly to young children, to protect them after the first six months.

“The difference this year is the general public for the first time is probably eagerly awaiting an RSV vaccine, because they’re seeing firsthand just how much of a problem RSV can be.”