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In 2023, the US Food and Drug Administration (FDA) approved the first vaccine to protect babies and older adults against illness caused by the respiratory syncytial virus (RSV).

The US vaccine protects babies and young children up to 2 years of age, and adults aged 60 years and older.

At-risk groups

RSV can cause an acute respiratory infection in people of all age groups. Most babies and young children experience mild, cold-like symptoms, but some babies, especially if it is their first infection, develop lower respiratory tract illnesses like pneumonia and bronchiolitis (inflammation of the small airways in the lungs), which can become serious and even result in a medical emergency.

Premature babies and those with chronic lung disease due to prematurity or significant congenital heart disease are at greater risk of severe RSV-related illness. According to the American Academy of Pediatrics, approximately 1 to 3% of children under 12 months in the US are hospitalized each year due to this virus.

Older adults are vulnerable due to the biology of aging and deteriorating immune systems. In this age group, more complications may arise due to the virus, requiring hospitalization.

Vaccines and clinical trials

To protect infants, the FDA approved Abrysvo (Pfizer), which is recommended during weeks 32 to 36 of pregnancy.

Currently, there are three vaccines approved in the United States for older adults: a formulation from Abrysvo, Arexvy (GSK) and mRESVIA (Moderna)

Arexvy is an adjuvanted vaccine, meaning it contains a substance that helps enhance the effects of the vaccine by stimulating the immune system response. Abrysvo contains no adjuvant, but is bivalent, meaning it protects against both RSV A and RSV B. Moderna's vaccines was developed with mRNA technology.

Typically, these vaccines should be given from September to January.

In clinical trials, in adults aged 60 years and older with healthy immune systems, one dose of Arexvy RSV vaccine was 83% effective in preventing lung infections (such as pneumonia) due to RSV during the first RSV season after vaccination. During the second RSV season after vaccination, one dose of Arexvy was still 56% effective against lung infections.

In a study of 12,500 participants, adults aged 60 years and older with healthy immune systems, one dose of the Abrysvo RSV vaccine was 89% effective in preventing lung infections (such as pneumonia) due to RSV during the first season after vaccination. According to the first results of the second season of RSV, Abrysvo continues to provide protection, but the second season is ongoing and the final results have not yet been published.

In complementary studies, the side effects of Arexvy continue to be studied, especially when given together with the flu vaccine. The FDA ordered the Arexvy manufacturer to perform a postmarketing study to assess the signals of serious risks for Guillain-Barré syndrome and encephalomyelitis.

A CDC external advisors' committee just recommended these vaccines for people 75 years of age and older and for people 60 years and older with medical conditions that put them at greater risk.

The situation in the Americas

Respiratory syncytial virus is the most common cause of pneumonia and bronchiolitis in infants worldwide. What’s more, it is the leading cause of hospitalizations and deaths in the first six months of life.

In the Americas, most RSV-associated hospitalizations are reported in children under 5 years, especially in infants under 6 months.

In its 2024 report, the Sociedad Latinoamericana de Infectología Pediátrica (SLIPE, Latin American Society of Pediatric Infectious Diseases) indicated that every year at the start of winter, many countries report an uptick in pediatric cases of lower respiratory tract infections (LRTIs), with respiratory syncytial virus being the main etiological microorganism in infants and children under 5 years.

Chile, Mexico, and Argentina are some of the countries that have approved the use of a preventive RSV vaccine.

In Costa Rica, vaccines are given to high-risk babies. The Sociedad Panameña de Pediatría (Panamanian Society of Pediatrics) recommends vaccination in children under 1 year.

In most of the United States, RSV circulation is seasonal, typically starting in fall and peaking in winter. The virus is transmitted from person to person through close contact with an infected individual.

Although RSV has a seasonal pattern, areas of Latin America and the Caribbean have higher average temperatures, higher humidity, and less pronounced changes between seasons.

These climate conditions result in greater exposure to droplets in the environment and on surfaces, causing the virus to survive longer on surfaces. As the virus primarily spreads through direct or indirect contact, survival on surfaces plays a major role in transmission and could be one reason behind less-defined seasonality patterns in the region.

This story was produced using content from original studies or reports, and other medical research and health and public health sources, highlighted in related links throughout the article.