Respiratory Syncytial Virus (RSV), Tis the Season

Dec. 1, 2023

This article was originally published Dec. 6, 2022 and has since been updated by the authors.

Respiratory syncytial virus (RSV) in cross-section landing on the surface of a human respiratory endothelial cell.
Respiratory syncytial virus (RSV) in cross-section landing on the surface of a human respiratory endothelial cell.
Source: NIAID
As autumn transitions to winter, there's more than a simple chill in the air—a stark reminder that respiratory viruses like influenza (flu), SARS-CoV-2 and respiratory syncytial virus (RSV) will be following people as they migrate indoors. The U.S. Centers for Disease Control and Prevention (CDC) reports that COVID-19 activity remains high this season, and flu activity is on the upswing in most areas of the country. Furthermore, as of Nov. 17, 2023, the CDC stated that emergency department visits and hospitalizations resulting from RSV remain elevated among young children, and are increasing among older adults across the country. Overall, reports state that the entire U.S. is experiencing elevated RSV activity.

In other words, RSV has arrived again this year and is impacting hospitals, urgent care centers and clinics in numerous locations across the U.S. The good news, compared to the 2022-23 season, is that we have some new tools to prevent and treat this dangerous, and sometimes deadly, foe. Vaccines are available and can help protect people from the most serious health effects of fall and winter viruses. However, only 14.3% of adults 60 and older report having received an RSV vaccine. Knowing a bit about the disease, how to protect oneself—and one's children—and when to seek treatment are key to enduring this unwelcome seasonal guest.

What Is RSV?

RSV is an enveloped single-stranded RNA virus of the genus  Orthopneumovirus, family Pneumoviridae. It was first identified in 1956 when a colony of chimpanzees used for polio research at the Walter Reed Army Institute for Research in Washington, D.C., developed runny noses caused by inflammation (i.e., coryza). At the time, the etiologic agent was named chimpanzee coryza agent (CCA). One year later, in 1957, a "new" virus that caused severe respiratory illness in infants was found in Baltimore and discovered to be identical to CCA. The pathogen was renamed Respiratory Syncytial Virus, since the virus causes cells to fuse with neighboring cells, creating multinucleated syncytia (single cells containing multiple nuclei).

RSV Symptoms

Symptoms typically include fever, runny or stuffy nose, cough, dyspnea (shortness of breath) and wheezing. Severe infections can lead to bronchiolitis (inflamed airways clogged with mucus) and pneumonia. RSV mortality is associated with approximately 100 underlying respiratory deaths each year in children younger than 1 year, posing a greater risk to infants than influenza.

RSV Transmission and Incubation

RSV is primarily spread from human-to-human, though respiratory droplets when a person coughs or sneezes, and through direct contact to the eyes or nose after touching a contaminated surface or object. RSV can survive for up to 12 hours on hard, non-porous surfaces, but it typically persists for shorter amounts of time on soft surfaces, such as tissues and hands.

​The incubation period for RSV is 3–5 days, and people can be contagious for 3-8 days after infection. It is even possible to be contagious 1-2 days before symptoms appear. Virus shedding can last up to 4 weeks in infants and young children, elderly people and people with weakened immune systems.

When to Seek Medical Attention

With numerous respiratory viruses circulating, it can be difficult to tell the difference between influenza, COVID-19, RSV and other respiratory viruses in children and infants. Individuals, parents and caregivers should stay alert for emergency warning signs. Seek medical attention right away if any of the following symptoms occur: difficulty breathing, blue lips or face, irritability, decreased activity, decreased appetite and apnea.

Infant with nebulizer mask.
Infant with nebulizer mask.

How Common Is RSV?

Although RSV is not a new virus, many have not heard of but have likely been infected with it at some point in their lifetimes. Almost all children get an RSV infection by the time they are 2 years old, but RSV isn’t just a virus impacting young children—older adults can still be at risk for severe infection. Age, chronic health conditions and weakened immune systems are all risk factors for the disease.

Historically, in the U.S., RSV infection results in the following annual statistics:

  • 2.1 million outpatient (non-hospitalization) visits among children younger than 5 years old.
  • 58,000-80,000 hospitalizations among children younger than 5 years old.
  • Roughly 60,000-120,000 hospitalizations among adults 65 years and older.

Restrictive and unprecedented public health measures—mask use, social distancing, increased attention to hand hygiene—adopted throughout 2020 and 2021 blunted the transmission of respiratory viruses, including RSV. In fact, flu and other cold viruses virtually disappeared. However, as restrictions were lifted, RSV returned with full force in June 2021, eventually decreased as January 2022 approached and slightly increased again in summer 2022. Since winter 2022, respiratory illness from COVID-19, RSV and influenza have returned to a more usual seasonality.

How Concerning Are Recent Outbreaks?

Participating laboratories report to the CDC the total number of RSV tests performed weekly, and the number of those tests that were positive. If one looks at these CDC data, the increase in RSV cases for the 2023 season began as early as late August-early September 2023, and, similar to trends observed in 2022, RSV cases began surging in November 2023.

Plot of number of positive RSV antigen and PCR tests from Jan. 2021 through Oct. 2022.
Number of positive RSV antigen and PCR tests from November 2021 through November 2023.
Source: CDC

The CDC reported a 6-fold increase in RSV positive tests (from 2% to 12%) between Sept. 2-Nov. 16, 2023. Much like the early jump of RSV activity in 2022, it appears that the 2023-24 season is off to the races in the U.S. The country might still be experiencing what scientists are referring to as an “immunity gap” or “immunity debt,” a term proposed to describe the issue of protective immunity from global COVID-19 pandemic mitigation measures over extended periods of low exposure to a given pathogen, leaving a greater proportion of the population susceptible to the disease.

At this time, high RSV activity is being observed in the southern states of New Mexico, Mississippi, Alabama, Georgia and Florida. A “very high” activity level is currently found in Louisiana. While RSV infections are creating some alarms, due to early surges in cases that have nearly filled hospital emergency departments in Georgia, Texas and some other states, many experts are not expecting to see the types of widespread patient traffic and hospital loads as last year.

Map of public health regions. RSV activity is increasing in all public health regions except regions 4 and 6.
Map of public health regions. As of Nov. 4, 2022, RSV activity was increasing in all public health regions except regions 4 and 6.
Source: CDC

RSV Infection Prevention

Monoclonal Antibody Therapy

Nirsevimab is a long-acting monoclonal antibody approved by the Food and Drug Administration (FDA) to protect infants and some young children at increased risk for severe RSV disease. Nirsevimab is safe and efficacious. In clinical trials, 1 dose of nirsevimab, administered as an intramuscular injection, protected infants for at least 5 months (the length of an average RSV season) and reduced the risk of severe RSV disease by about 80%. The incidence of serious adverse events was not increased among nirsevimab recipients compared with placebo recipients in the clinical trials. Importantly, the medication is not a vaccine but works as a dose of protective antibodies during RSV season.

Another monoclonal antibody that is FDA-approved to help protect high-risk babies from severe RSV disease is Palivizumab (Synagis®). This medication has been approved for preemies (babies born at or before 35 weeks) and infants who are 6 months of age or less at the beginning of RSV season. The medication is also not a vaccine but works as a monthly dose of protective antibodies during RSV season.

RSV Vaccine Candidate

Highly-magnified, 1981 transmission electron microscopic (TEM) image of RSV.
Highly-magnified, 1981 transmission electron microscopic (TEM) image of RSV.
Source: CDC Public Health Library
RSVPreF3 and RSVpreF are recombinant protein vaccines that are both approved by FDA for use in adults ages 60 years and older to prevent RSV-associated lower respiratory tract disease. During the first RSV season after introduction of these vaccines, each was more than 80% efficacious in preventing RSV-associated lower respiratory tract disease. A small number of participants in clinical trials (6 out of 38,177 total participants aged ≥60 years who received either vaccine) developed inflammatory neurologic events within 6 weeks after RSV vaccination, but it was unclear whether these events were related to RSV vaccination.

For complete guidance on the use of RSV vaccines, see the RSV Advisory Committee on Immunization Practices (ACIP). ACIP recommendations should be discussed with your primary care physician or other appropriate health care professional.

Public Health Measures

Still, it is important to remember that vaccination is only 1 (very important) part of a multimodal public and personal health strategy for risk reduction when it comes to the ongoing threats posed by infectious microbial agents. It’s extremely rare for vaccines to prevent infection with 100% efficacy. Clinically speaking, in most cases, vaccination is used to greatly reduce morbidity (severe illness) and mortality (death) of patients, which means that one can still be at risk of infection. This knowledge makes public health measures, such as those that have helped keep people safe from COVID-19 over the past 4 years, another critical part of limiting the spread of respiratory viruses, like RSV, flu and SARS-CoV-2. A few of these easily implemented practices are listed below:

  • If you or someone you care for is experiencing typical respiratory illness, please visit a doctor and get tested for RSV or other appropriate testing.
  • Cover your mouth when coughing or sneezing.
  • Keep hands clean by washing them with soap and water or using alcohol-based hand sanitizer frequently.
  • Clean and disinfect frequently touched surfaces or items (such as countertops, faucets, doorknobs and toys) routinely.
  • Distance yourself from individuals who are ill.
  • Stay away from others if you feel ill.
  • Wear a well-fitting face mask.

As temperatures drop and people move indoors (often spending more time in close quarters with one another), RSV (and other respiratory viruses) are expected to increase. Practicing and promoting those simple everyday preventive measures plays an important role in keeping us safe.

Looking for more content? Check out this article about the impact of COVID-19 on other infectious diseases of public health significance.

Author: Priya Dhagat, M.S., MLS(ASCP)CM, CIC

Priya Dhagat, M.S., MLS(ASCP)CM, CIC
Priya Dhagat, M.S., MLS(ASCP)CM, CIC, is an infection preventionist and the associate director of the System-wide Special Pathogens Program at New York City Health + Hospitals.

Author: Rodney Rohde, Ph.D., SM(ASCP), SVCM, MBCM, FACSc

Rodney Rohde, Ph.D., SM(ASCP), SVCM, MBCM, FACSc
Rodney Rohde, Ph.D., is the Associate Director of the Translational Health Research Initiative at Texas State University.