RSV vaccine research stretches back nearly 60 years, but the vaccines approved for use today are built on a scientific breakthrough from 2013 and went through roughly 6 to 7 years of human clinical trials before reaching the market in 2023 and 2024. The long gap between the first attempts and modern success is itself part of the story, because a failed trial in the 1960s set the entire field back by decades.
The 1960s Trial That Halted Progress
The first RSV vaccine was tested in infants and children across four U.S. studies in 1966. It used a killed (formalin-inactivated) version of the virus, a standard approach for vaccines at the time. The results were devastating. In 1967, when vaccinated infants encountered RSV naturally, they developed a more severe form of the disease than unvaccinated children, with high fever, pneumonia, and wheezing. Two children died.
Scientists spent decades trying to understand what went wrong. The vaccine triggered antibodies that couldn’t actually neutralize the virus, and it primed the immune system to overreact when real RSV arrived, flooding the lungs with inflammatory cells. This phenomenon, called enhanced respiratory disease, made regulators and researchers deeply cautious. For years, the consensus was that only a weakened live virus vaccine could ever be safe enough for infants, which severely narrowed the research pipeline.
The Breakthrough That Made Modern Vaccines Possible
The turning point came from structural biology work at the National Institutes of Health. Researchers mapped the atomic structure of the RSV fusion protein, the molecule the virus uses to break into human cells. That protein exists in two shapes: a “prefusion” form before it attacks a cell, and a “postfusion” form after. The prefusion shape turned out to be far better at generating protective antibodies.
In 2013, NIH scientists stabilized the prefusion protein and tested it as a vaccine in mice and primates, with strong results. By 2017, the first human trial began at the NIH Clinical Center, enrolling 40 healthy adults. Early data showed the stabilized protein triggered a surge of neutralizing antibodies, confirming the concept worked in people. This prefusion protein technology became the foundation for all three RSV vaccines now on the market.
Clinical Trial Timelines for Approved Vaccines
Three RSV vaccines have received FDA approval, each following its own testing path through multiple trial phases.
The first two approvals came in mid-2023, both for adults 60 and older. One (GSK’s Arexvy) was approved on May 3, 2023, and the other (Pfizer’s Abrysvo) followed on May 31, 2023. Pfizer’s vaccine also received approval for use in pregnant women to protect newborns. A third vaccine, Moderna’s mRESVIA, which uses mRNA technology similar to COVID vaccines, was approved on May 31, 2024.
Moderna’s phase 2/3 trial enrolled 36,685 adults aged 60 and older between November 2021 and December 2022, with an additional 106 participants in an earlier phase 1 safety study. The initial efficacy analysis, based on a median follow-up of 3.7 months per participant, showed the vaccine prevented about 79% of symptomatic RSV lower respiratory disease. With longer follow-up of about 19 months, efficacy settled to around 47%, suggesting protection wanes over time. Across all participants, the rate of serious adverse events was identical between the vaccinated and placebo groups.
Maternal Vaccine Testing and a Safety Signal
Testing RSV vaccines in pregnant women added another layer of complexity. The goal is to vaccinate the mother during pregnancy so she passes protective antibodies to her baby before birth. GSK ran a phase 2 trial of its maternal RSV candidate, followed by a phase 3 trial that enrolled participants across nine countries starting in August 2021.
In February 2022, an independent safety board flagged a potential increase in preterm births in a separate phase 3 maternal study of the same vaccine. All maternal studies of that candidate were immediately halted and unblinded on February 25, 2022. GSK ultimately did not pursue approval for its maternal vaccine. Pfizer’s maternal vaccine (Abrysvo) did receive approval, but the preterm birth signal across the field led to ongoing scrutiny and specific labeling about the recommended timing of vaccination during pregnancy.
Post-Approval Monitoring So Far
Since the vaccines reached the public, the U.S. Vaccine Adverse Event Reporting System (VAERS) has been collecting real-world safety data. As of March 2025, that means roughly two years of post-marketing surveillance for the first two approved vaccines and about ten months for the mRNA vaccine. This monitoring supplements the clinical trial data and is specifically watching for rare events like Guillain-Barré syndrome and inflammatory heart conditions, neither of which appeared in the clinical trials within 42 days of vaccination.
In 2024, the CDC’s Advisory Committee on Immunization Practices updated its recommendations for RSV vaccines in older adults based on accumulated trial and real-world evidence. The current guidance reflects a shared clinical decision-making approach, where adults 60 and older discuss with their provider whether vaccination makes sense for their individual risk level, rather than a blanket recommendation for everyone in that age group.
From First Attempt to Approval: The Full Timeline
Depending on how you count, RSV vaccine development spans either about 60 years from the first human trials in 1966, or about 10 years from the prefusion protein breakthrough in 2013 to the first approvals in 2023. The modern vaccines that people actually receive today were in structured human clinical trials for roughly 6 to 7 years before approval, progressing from small safety studies in 2017 through large phase 3 efficacy trials enrolling tens of thousands of participants. That timeline is consistent with the typical development path for major vaccines, though it built on decades of prior immunology research that was essential to getting the vaccine design right.