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Accurate data on the RSV vaccine is raising hopes after years of futility

New research shows that vaccinating pregnant women has helped protect their newborns from a common but scary respiratory virus called RSV that is flooding hospitals with wheezing in children every autumn.

Preliminary results support hope that after decades of failures and disappointments, RSV vaccines may finally be on the way.

Pfizer announced Tuesday that a large international study found that vaccinating expectant mothers was nearly 82% effective in preventing severe cases of RSV in the most vulnerable babies in the first 90 days of life. At 6 months of age, the vaccine was still 69% effective against serious disease – and there were no signs of safety issues in mothers or babies.

“Moms always give their antibodies to their baby,” said virologist Kena Swanson, Pfizer’s vice president of viral vaccines. “The vaccine just puts them in a much better position” to make and transmit RSV-fighting antibodies.

Finding a vaccine isn’t just about protecting babies. RSV is dangerous for the elderly, too, and both Pfizer and rival GSK recently announced that their competing shots have also been shown to be protective in the elderly.

No results will help this year, when an early RSV outbreak is already overwhelming children’s hospitals. But they raise the prospect that one or more vaccines may be available before the next RSV season in the fall.

“I’m crossing my fingers,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University. “We’re making our way through.”

The data for Tuesday was presented in a press release and has not been verified by independent experts.

Here’s a look at the long quest for RSV vaccines.


For most healthy people, RSV, or respiratory syncytial virus, is a nuisance similar to the common cold. But for the very young, the elderly, and people with certain health problems, it can be serious, even life-threatening. The virus can travel deep into the lungs, causing pneumonia, and in babies it can make breathing difficult by inflaming the tiny airways.

In the US, about 58,000 children under the age of 5 are hospitalized each year because of RSV, and several hundred die. Among adults age 65 and older, about 177,000 are hospitalized with RSV and 14,000 die each year.

Worldwide, RSV kills about 100,000 children a year, mostly in poor countries.


The tragedy of the 1960s set the whole field back. Using the approach that led to the first polio vaccine, scientists made an experimental RSV vaccine by growing the virus in the lab and killing it. But tests on children showed that not only did the vaccine not protect, but children who caught RSV after vaccination did worse. Two died.

“For 20 years, even though the science was advancing, no one wanted to come close to developing an RSV vaccine,” Schaffner said.

He noted that even today’s RSV vaccine candidates were first tested on the elderly, not children.


Current vaccines tend to target the outer surface of the virus, which is seen by the immune system when the microbe invades. For RSV, this target is the so-called F protein, which helps the virus attach to human cells. Again, there’s an obstacle: this protein changes shape, rearranging its shape before and after “fusion” with cells.

It turns out that the immune system makes effective RSV antibodies only when it detects a so-called pre-fusion version of this protein, explained structural biologist Jason McLellan of the University of Texas at Austin.

In 2013, McLellan and virologist Barney Graham were working at the National Institutes of Health when they studied the correct shape and figured out how to freeze it in that shape. This discovery paved the way for the current development of many experimental RSV vaccine candidates.

(This same discovery was the key to highly successful vaccines against COVID-19, since the coronavirus also hides in a shape-changing surface protein.)


Several companies are developing RSV vaccines, but Pfizer and rival GSK are leading the way. Both companies recently reported the latest phase of testing in the elderly. Competing vaccines are made somewhat differently, but each has proven highly effective, especially against serious diseases. Both companies plan to receive regulatory approval in the US by the end of the year, as well as in other countries.

The data for the elderly “looks fantastic,” said McLellan, who has followed vaccine development closely. “I think we’re on the right track.”

And if vaccinating pregnant women succeeds, it could be “a win for two people, not one,” offering protection for both mother-to-be and baby, said Dr. Wilbur Chen of the University of Maryland School of Medicine.

Pfizer’s maternal vaccine is the same formulation it has successfully tested in older adults — and it also plans to get food and drug approval for those shots by the end of the year.

The new study included 7,400 pregnant women in 18 countries, including the United States, and covered multiple RSV seasons. Preliminary results reported Tuesday show the vaccine was most effective against severe disease. For milder disease, the effectiveness ranged from 51% to 57% — not meeting the study’s statistical requirements, but a result that Pfizer still called clinically meaningful because it could mean fewer visits to the doctor.


The Associated Press Department of Health and Science receives support from the Department of Science Education at the Howard Hughes Medical Institute. AP is solely responsible for all content.


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