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Respiratory Syncytial Virus (RSV) Season Varies by Region and Year

Knowledge of regional differences in the timing of the RSV season helps doctors decide when to protect children at high risk for serious RSV illness. Such children include premature infants, those less than 2 years of age with chronic lung or heart conditions, and those with weakened immune systems.

Chart: Respiratory Syncytial Virus (RVS) Season. United States, by Region and Florida

Respiratory syncytial virus (RSV) is the most common cause of severe lower respiratory tract disease among infants and young children. In high-risk children and older adults, the disease can lead to more serious illnesses, such as pneumonia (inflammation of the lungs) or bronchiolitis (inflammation of the small airways in the lungs). No vaccine or effective treatment for RSV is currently available. However, palivizumab, a medication that contains virus-fighting antibodies to RSV, can help prevent severe RSV disease in high-risk children. Since these antibodies are given to protect children during RSV outbreaks in their communities, the monitoring of outbreak patterns has helped physicians determine when the drug should be given. Yearly community outbreaks of RSV usually last 3 to 4 months during the fall, winter, and/or spring months.

Laboratory data from the United States National Respiratory and Enteric Virus Surveillance System (NREVSS) showed that the 2006–2007 RSV season1 varied by geographic region2. The season started and ended the earliest in Florida and the latest in the West. The 2006–2007 RSV seasons for Florida and for the South (excluding Florida), Northeast, Midwest, and West are summarized below.

  • Florida: early July to late January
  • South (excluding Florida): late October to late February
  • Northeast: mid-November to early February
  • Midwest: mid-November to mid-March
  • West: mid-December to late March

Within a region, timing of the RSV season can change from year to year. As shown in the table below, the 2006 seasons in several regions started later than those in 20073.



Start of RSV Season




Early July

Early July

South (excluding Florida)

Late October




Early November



Late November



Early December

It is not known why community RSV outbreaks occur when they do, but temperature, humidity, and other environmental factors are likely to contribute to the timing of outbreaks.

1NREVSS estimates that RSV seasons start when the median percentage of specimens testing positive for antigen is greater than or equal to 10 for 2 consecutive weeks and ends when the median percentage of specimens testing positive is less than or equal to 10 for 2 consecutive weeks.

2Regions: Northeast Region: Connecticut, Massachusetts, New Hampshire, New Jersey, New York, and Rhode Island; Midwest Region: Illinois, Indiana, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin; South Region: Alabama, Arkansas, Delaware, District of Columbia, Georgia, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, and Virginia; West Region: Alaska, Arizona, California, Colorado, Hawaii, Montana, Washington, and Wyoming; and the state of Florida.

3The 2007 season start dates were determined on the basis of preliminary reports. These dates have been updated since the publication of the RSV MMWR on 7 December 2007 (see data source section below). The 2007 season start estimates include data from Surveillance Data, Inc. (SDI), a private company that conducts RSV surveillance with support from MedImmune, LLC. In fall 2006, CDC and SDI signed a memorandum of understanding to share RSV surveillance data to make the most complete RSV dataset available. The relationship between CDC and SDI is limited to data sharing, as outlined in the memorandum. CDC does not make recommendations regarding the administration of RSV immune prophylaxis. For additional information, contact NREVSS by email at

Data Source: MMWR. Respiratory syncytial virus activity---United States, July 2006 -- November 2007. MMWR. 7 Dec 2007. 56(48);1263-1265.

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