Brief Report: Respiratory Syncytial Virus Activity --- United States, July 2007--December 2008

Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis and pneumonia in children aged <1 year and is a major cause of respiratory illness in older adults (1,2). RSV is transmitted person-to-person via close contact, droplets, and fomites. Each year in the United States, an estimated 75,000--125,000 children aged <1 year are hospitalized with RSV (1). Those at increased risk for hospitalization include premature infants meeting certain criteria and persons of any age with compromised respiratory, cardiac, and immune systems (3,4). RSV incidence follows a seasonal pattern. In temperate climates, the RSV season generally occurs during the fall, winter, and spring months. However, the timing of RSV circulation can vary by location and year (5). CDC analyzed laboratory data from the National Respiratory and Enteric Virus Surveillance System (NREVSS) to summarize RSV temporal and geographic trends in the United States during the weeks ending July 7, 2007--June 28, 2008, and for the first 5 months of the current reporting season (the weeks ending July 5--December 6, 2008). This report describes the results of that analysis, which indicated that the 2007--08 RSV season onset* for the 10 U.S. Department of Health and Human Services (HHS) regions^† and Florida ranged from early July to mid-December, and the season offset ranged from late January to mid-April; the current 2008--09 season onset occurred in eight of the 10 HHS regions by December 6, 2008. These findings support previous observations that the RSV season not only varies by location, but can vary by year.

NREVSS is a passive surveillance system that relies on a voluntary network of laboratories that report weekly the number of specimens submitted to that laboratory and the number of positive results for various pathogens, including RSV. During July 2007--June 2008, a total of 636 laboratories reported at least 1 week of RSV testing data using antigen detection methods, virus culture, or polymerase chain reaction.^§ For this analysis, CDC included 217 laboratories (34.0%) from 44 states that met the following criteria: reported >30 weeks and averaged >10 antigen detection tests per week. The analysis was restricted to antigen detection methods to provide consistency because this method is used by 98.0% of NREVSS laboratories.

Data are presented for each of the 10 HHS regions, allowing greater characterization of geographic variability in RSV detections than the four U.S. Census regions used in previous MMWR reports (6); the findings can be used to determine the optimal timing of RSV prophylaxis for infants and children at high risk in each region. The HHS regions (listed by region number and headquarter city) include Region 1 (Boston), Region 2 (New York), Region 3 (Philadelphia), Region 4 (Atlanta), Region 5 (Chicago), Region 6 (Dallas), Region 7 (Kansas City), Region 8 (Denver), Region 9 (San Francisco), and Region 10 (Seattle). Florida is summarized separately because, historically, the RSV season in Florida has been distinct from the remainder of Region 4 (Atlanta) (6) (Table and Figure).

During the 2007--08 season, the 217 laboratories reported a total of 369,944 tests, of which 58,957 (15.9%) were positive. The national RSV season onset occurred in the week ending October 20, 2007, and continued for 22 weeks until the season offset in the week ending March 15, 2008. When data from Florida were excluded, the national RSV season onset began 2 weeks later (week ending November 3, 2007); the season offset was not affected.

The season onset date for all 10 HHS regions ranged from mid-October (week ending October 13, 2007) to mid-December (week ending December 15, 2007); however, in Florida, the season onset occurred in early July (week ending July 7, 2007). After Florida, the RSV season began the earliest in Region 6 (Dallas) and Region 2 (New York) (mid-October), followed by Region 4 (Atlanta) (late October). The RSV season started in Region 3 (Philadelphia) in early November, followed by Region 5 (Chicago) in mid-November, and Region 7 (Kansas City) and Region 9 (San Francisco) in late November. The RSV season began in Region 1 (Boston) and Region 10 (Seattle) in early December and started the latest in Region 8 (Denver) (mid December).

The season offset for all 10 HHS regions and Florida ranged from late January (week ending January 26, 2008) to mid-April (week ending April 12, 2008). The season offset occurred the earliest in Florida (late January), followed by Region 2 (New York) and Region 6 (Dallas) (early February), Region 1 (Boston) and Region 3 (Philadelphia) (mid-February), and Region 4 (Atlanta) (late February). The RSV season ended in Region 7 (Kansas City) in early March, followed by Region 8 (Denver) and Region 9 (San Francisco) in late March. The RSV season ended the latest in Region 5 (Chicago) (early April) and Region 10 (Seattle) (mid-April).

The median RSV season duration among the 10 HHS regions was 17 weeks (range: 12--21 weeks). The regions with the shortest seasons were Region 1 (Boston) (12 weeks), followed by Region 3 (Philadelphia) and Region 7 (Kansas City) (15 weeks). The regions with the longest seasons were Region 5 (Chicago) (21 weeks), followed by Region 9 (San Francisco) (19 weeks) and Region 10 (Seattle) (19 weeks). The season in Florida lasted 30 weeks.

Preliminary data for the current 2008--09 RSV season are available from the week ending July 5, 2008, through the week ending December 6, 2008. A total of 548 laboratories from all 50 states and the District of Columbia reported 94,180 RSV antigen detection tests and 10,410 (11.1%) positive results to NREVSS. Reports received through December 6, 2008, indicated that the RSV season onset had begun in mid-October in Region 4 (Atlanta) (excluding Florida [week ending October 11, 2008]) and in late October in Region 6 (Dallas) (week ending October 25, 2008). The season had begun in Region 1 (Boston) and Region 2 (New York) in mid-November (week ending November 15, 2008), followed by Region 3 (Philadelphia) (week ending November 22, 2008), and Region 9 (San Francisco) and Region 10 (Seattle) (week ending November 29, 2008). The Region 5 (Chicago) season onset occurred in early December (week ending December 6, 2008). As of December 6, 2008, the RSV season onset had not started in Region 7 (Kansas City) and Region 8 (Denver). In Florida, reports indicate that the season onset occurred in mid-July (week ending July 12, 2008), 1 week later than in 2007. Nationally, the 2008--09 RSV season onset occurred the week ending November 1, 2008; however, when data from Florida are excluded, the national season onset occurred 2 weeks later (week ending November 15, 2008). Weekly updates showing RSV national, regional, and state trends are available from the NREVSS website at http://www.cdc.gov/surveillance/nrevss. Additional information about Florida RSV trends is available from the Florida Department of Health website at http://www.doh.state.fl.us/disease_ctrl/epi/rsv/rsv.htm.

Although no RSV vaccine exists, infants and children at risk for severe RSV infection can receive monthly doses of palivizumab, a humanized murine anti-RSV monoclonal antibody, during the RSV season. The most recent edition of the American Academy of Pediatrics Red Book should be consulted for specific recommendations (3).

Reported by: National Respiratory and Enteric Virus Surveillance System laboratories. CA Panozzo, MPH, AL Fowlkes, MPH, GE Fischer, MD, EE Schneider, MD, LJ Anderson, MD, Div of Viral Diseases, National Center for Immunization and Respiratory Diseases, CDC.

References

1. Shay DK, Holman RC, Newman RD, Liu LL, Stout JW, Anderson LJ. Bronchiolitis-associated hospitalizations among U.S. children, 1980--1996. JAMA 1999;282:1440--6.
2. Falsey AR, Hennessey PA, Formica MA, Cox C, Walsh EE. Respiratory syncytial virus infection in elderly and high-risk adults. N Engl J Med 2005;352:1749--59.
3. American Academy of Pediatrics. Respiratory syncytial virus. In: Pickering LK, Baker CJ, Long SS, McMillan JA, eds. Red book: 2006 report of the Committee on Infectious Diseases. 27th ed. Elk Grove Village, IL: American Academy of Pediatrics; 2006:560--6.
4. Welliver RC. Review of epidemiology and clinical risk factors for severe respiratory syncytial virus (RSV) infection. J Pediatr 2003;143(5 Suppl):S112--7.
5. Mullins JA, LaMonte AC, Bresee JS, Anderson LJ. Substantial variability in community RSV season timing. Pediatr Infect Dis J 2003;22:857--62.
6. CDC. Respiratory syncytial virus activity---United States, July 2006--November 2007. MMWR 2007;56:1263--5.

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Date last reviewed: 12/17/2008