Updated on September 22, 2023.
Each year in the U.S., tens of thousands of people are hospitalized and up to 10,000 people die due to respiratory syncytial virus (RSV) and related complications. Following decades of development, new treatments that target this virus promise to protect those most vulnerable to infection.
On August 3, the U.S. Centers for Disease Control and Prevention (CDC) recommended the use of nirsevimab—a monoclonal antibody that has been shown to reduce the risk of both hospitalizations and healthcare visits for RSV in infants by about 80 percent. Monoclonal antibodies are lab-made proteins that mimic the antibodies that our bodies naturally produce.The single-dose shot made by AstraZeneca was approved by the U.S. Food and Drug Administration (FDA) on July 17. It is the first treatment available in the U.S. that could protect all infants from RSV and provide long-term protection against the infection.
The CDC recommends one dose of nirsevimab for all infants younger than 8 months, born during—or entering—their first RSV season (usually fall through spring). For some children between 8 and 19-months old who are at greater risk of severe RSV disease, such as those who are severely immunocompromised, one dose is recommended in their second RSV season.
“This new RSV immunization provides parents with a powerful tool to protect their children against the threat of RSV,” said CDC director Mandy Cohen, MD, MPH in an August 3 news release. “RSV is the leading cause of hospitalizations for infants and older babies at higher risk and today we have taken an important step to make this life saving product available.”
RSV vaccines to protect older people and newborns
A few months earlier in May 2023, the FDA approved the world’s first RSV vaccine. The vaccine called Arexvy, which is made by GSK, was approved for the prevention of lower respiratory tract disease caused by RSV in adults ages 60 and older.
In a late-stage clinical trial involving 12,500 people, the single-dose shot reduced the risk of developing lower respiratory tract disease by 82.6 percent and lowered the risk for more severe disease by 94.1 percent.
That same month the FDA approved another similar vaccine for older adults developed by Pfizer called Abrysvo. On August 21, the FDA also approved Pfizer's vaccine for pregnant people to protect newborns during the first six months of life. The shot, which may be given between 32 and 36 weeks of pregnancy, is the first vaccine to protect infants from RSV.
Each of these drugs help protect against RSV by targeting a protein called RSV F glycoprotein that lives on the surface of the virus and enables it to invade cells. By binding to this protein, the drugs prevent the virus from infecting cells.
The CDC approved the use of the GSK and Pfizer RSV vaccines for people ages 60 years and older, using shared clinical decision-making. This means these people may receive one dose of the vaccine based on discussions with their healthcare provider (HCP) about whether RSV vaccination is right for them. For the 2023-24 RSV season, the CDC advises those age 60 or older who've been advised by their HCP to receive an RSV vaccine to not wait—and get their shot as soon as it becomes available in their area. This will help ensure they are protected before the virus begins circulating in their community, health officials explain.
On September 22, the CDC also approved one dose of the Pfizer RSV vaccine for people between 32 and 36 weeks pregnant. “This is another new tool we can use this fall and winter to help protect lives,” said CDC Director Dr. Mandy Cohen in a news release. “I encourage parents to talk to their doctors about how to protect their little ones against serious RSV illness, using either a vaccine given during pregnancy, or an RSV immunization given to your baby after birth.”
Why the timing matters
RSV is a common cause of lower respiratory tract disease. Anyone can get it. In fact, most kids are infected by the age of 2. For otherwise healthy people, RSV is like a cold. But some infants, young children, older adults, and those with weakened immune systems can get very sick. RSV can lead to life-threatening pneumonia and bronchiolitis (swelling of the small airway passages in the lungs).
“Older adults, in particular those with underlying health conditions, such as heart or lung disease or weakened immune systems, are at high risk for severe disease caused by RSV,” said Peter Marks, M.D., Ph.D., director of the FDA’s Center for Biologics Evaluation and Research in a May 3 news release.
Like the flu, RSV is a seasonal virus that tends to spread in the fall and winter months. But RSV surged earlier than usual in 2022 and scientists are still trying to understand why. Research suggests that COVID precautions over the past few years have led to an “immunity gap”— an increase in vulnerable new babies and reduced immunity from lack of exposure to the virus.
Tracking safety and effectiveness
Three ongoing trials are evaluating the safety and effectiveness of AstraZeneca’s monoclonal antibody. The studies suggest the treatment is well tolerated. The most commonly reported side effects are fever and rash within two weeks of receiving the shot. There have been no reports of severe allergic reactions or other more serious complications related to the injection.
Pfizer’s Abrysvo should not be given to anyone with a history of severe allergic reaction, and the vaccine may not be as effective for those with weakened immune systems. But trials investigating the vaccine show that among adult age 60 and older, side effects from the shot are typically mild, including pain at the injections site, fatigue, headache, and muscle pain.
The FDA’s approval of GSK’s Arexvy was based on data for a single dose of the vaccine from one RSV season. Participants will remain in the study through three RSV seasons to assess the duration of effectiveness and the safety and effectiveness of repeat vaccination.
The most commonly reported side effects among those who received the RSV shot were injection site pain, fatigue, muscle pain, headache, and joint pain or stiffness. Within 30 days of vaccination, 10 people who received the shot developed a type of irregular heart rhythm called atrial fibrillation, compared to only four people who received the placebo.
In a smaller study, in which some participants received Arexvy along with a flu shot, two people developed acute disseminated encephalomyelitis (ADEM), a rare type of inflammation that affects the brain and spinal cord soon after vaccination. One of the participants who developed ADEM died. In another small study, one participant developed Guillain-Barré syndrome (a rare disorder that leads to muscle weakness and sometimes paralysis) nine days after receiving Arexvy.
The FDA has required GSK to further investigate the risks for Guillain-Barré syndrome and ADEM. GSK is also continuing to assess the possible risk for atrial fibrillation.
