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Update: Respiratory Syncytial Virus Activity -- United States, 1998-1999 Season

Respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract disease in infants and young children worldwide (1). In temperate climates, RSV infections occur primarily during annual outbreaks, which peak during winter months (2). In the United States, RSV activity is monitored by the National Respiratory and Enteric Virus Surveillance System (NREVSS), a voluntary, laboratory-based system. This report summarizes trends in RSV activity reported to NREVSS during July 1998-June 1999 and presents preliminary surveillance data during July 1-November 12, 1999, which show that RSV community outbreaks are becoming widespread.

Clinical and public health laboratories report weekly to CDC the number of specimens tested for RSV by antigen-detection and/or virus-isolation methods and the number of positive results. RSV activity is considered widespread by NREVSS when at least half of participating laboratories report any RSV detections for at least 2 consecutive weeks and when greater than 10% of all specimens tested by antigen detection for RSV are positive. RSV community outbreaks are defined similarly (greater than 2 consecutive weeks with greater than 10% positive tests, by city).

From July 1998 through June 1999, 72 laboratories in 45 states reported 128,579 tests for RSV, of which 18,418 were positive for RSV (Figure 1). In the United States, widespread RSV activity began in early November 1998 and continued for 27 weeks, until late April. Timing of RSV community outbreaks varied from onset (range: September 11 to April 2) to conclusion (range: January 8 to June 18). Overall, RSV outbreaks were observed earlier in laboratories in the South (19 sites; median weeks of onset and conclusion: November 20 and April 2, respectively), later in Northeast laboratories (seven sites; November 27 and April 23), and latest in the Midwest (11 sites; December 18 and May 14) and West (12 sites; January 1 and April 30).

Although most positive tests (91%) were reported from the week ending November 27 through the week ending April 30, RSV was detected throughout the year. For example, during July-August 1999, one or two sporadic RSV isolates were reported from single laboratories in Colorado, Nebraska, Oklahoma, South Dakota, Tennessee, Texas, and Washington. In addition, during July-August, an outbreak of RSV-related lower respiratory tract infections, including 18 cases of pneumonia and 15 hospitalizations, was detected among residents and staff in a long-term-care facility in Maryland. As of the week ending November 12, 1999, widespread RSV activity has been reported in communities in the South (eight of 20 sites), West (three of 15 sites), Northeast (one of 8 sites), and Midwest (one of 18 sites).

Reported by: National Respiratory and Enteric Virus Surveillance System collaborating laboratories. B Mitchell, MD, C Groves, MS, JC Roche, MD, Acting State Epidemiologist, Maryland Dept of Health and Mental Hygiene. Respiratory and Enteric Viruses Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note:

For the July 1998-June 1999 surveillance period, the total number of specimens positive for RSV, average months of peak activity, and regional trends were similar to trends observed during previous years. The duration of the 1998-1999 season was longer than previous years, with later-than-usual RSV outbreaks reported by several western and midwestern laboratories. Although RSV community outbreaks occurred largely during winter months, sporadic RSV detections were found throughout the year, including the summer.

NREVSS consists of 72 widely distributed laboratories and is a useful system for characterizing the geographic and temporal trends of RSV infections in the United States. NREVSS data can alert public health officials and physicians to the timing of seasonal RSV activity.

When reviewing NREVSS data, at least three limitations should be considered. First, laboratory results are not confirmed by CDC. Second, laboratory data serve as an indicator of when RSV is circulating in a community; however, the correlation of these data to disease burden in the population is uncertain. Finally, some regions have few laboratories; recruitment of additional laboratories is needed. To alert the public to RSV trends, regional summary data are frequently updated on the CDC World-Wide Web site ( As in the 1998-1999 season, timing of community RSV outbreaks may vary considerably within and among regions.

Severe manifestations of RSV infection (e.g., pneumonia and bronchiolitis) most commonly occur in infants aged 2-6 months, and hospitalization rates for these diagnoses have been used as an indicator for severe RSV disease among young children. In the United States, bronchiolitis hospitalization rates among children aged less than 1 year increased substantially from 12.9 per 1000 in 1980 to 31.2 per 1000 in 1996; the reasons for this increase are unclear (3). Considerably higher hospitalization rates (61.8 per 1000 children aged less than 1 year) have been identified among American Indian/ Alaska Native children receiving care through the Indian Health Service (4).

Symptomatic RSV disease can recur throughout life because of limited protective immunity induced by natural infection. As a result, health-care providers should consider RSV as a cause of acute respiratory disease in children and adults during community outbreaks. Persons with underlying cardiac or pulmonary disease or compromised immune systems and the elderly are at increased risk for serious complications of RSV infection, such as pneumonia and death (5,6). RSV infection among recipients of bone marrow transplants has resulted in high mortality rates (83%) (7).

The risk for nosocomial transmission of RSV increases during community outbreaks; nosocomial outbreaks of RSV can be controlled by adhering to contact-isolation procedures (8). No RSV vaccines are available, although both live attenuated and subunit vaccines have entered clinical trials. RSV immune globulin intravenous and a humanized murine anti-RSV monoclonal antibody are recommended as prophylaxis for some high-risk infants and young children (e.g., those born prematurely or with chronic lung disease) to prevent serious RSV disease (9).


  1. Institute of Medicine. [Appendix N]: Prospects for immunizing against respiratory syncytial virus. In: Institute of Medicine. New vaccine development: establishing priorities. Vol II. Disease importance in developing countries. Washington, DC: National Academy Press, 1986:299-307.
  2. Gilchrist S, Török TJ, Gary HE Jr, Alexander JP, Anderson LJ. National surveillance for respiratory syncytial virus, United States, 1985-1990. J Infect Dis 1994;170:986-90.
  3. Shay DK, Holman RC, Newman RD, Liu LL, Stout JW, Anderson LJ. Bronchiolitis-associated hospitalizations among US children, 1980-1996. JAMA 1999;282;15:1440-6.
  4. Lowther SA, Shay DK, Holman RC, Clarke MJ, Kaufman SF, Anderson LJ. Bronchiolitis-associated hospitalizations among American Indian and Alaska Native Children. Pediatr Infect Dis J 2000 (in press).
  5. Dowell SF, Anderson LJ, Gary HE Jr, et al. Respiratory syncytial virus is an important cause of community-acquired lower respiratory infection among hospitalized adults. J Pediatr 1996;174:456-62.
  6. Wang EEL, Law BJ, Stephens D, et al. Pediatric Investigators' Collaborative Network on Infections in Canada (PICNIC): prospective study of risk factors and outcomes in patients hospitalized with respiratory syncytial viral lower respiratory tract infections. J Pediatr 1995;126:212-9.
  7. Whimbey E, Couch RB, Englund JA, et al. Respiratory syncytial virus pneumonia in hospitalized adult patients with leukemia. Clin Infect Dis 1995;21:376-9.
  8. CDC. Guideline for infection control in health care personnel, 1998. Am J Infect Control 1998;26:289-354.
  9. Committee on Infectious Diseases, Committee on Fetus and Newborn, American Academy of Pediatrics. Prevention of respiratory syncytial virus infections: indications for the use of palivizumab and update on the use of RSV-IGIV. Pediatrics 1998;102:1211-6.

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