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*  This summary includes data reported as of May 19, 2007.  Reporting is incomplete and numbers may changes as more reports are received.
¥  The number and/or proportion of an influenza virus type/subtype may differ between the virus detections reported and the viruses antigenically characterized by CDC because 1) some isolates initially reported as influenza A (unsubtyped) through weekly surveillance may be submitted to CDC for further characterization that will include subtyping of those viruses, and 2) influenza virus type/subtype less frequently seen during a given influenza season may be actively solicited for antigenic characterization. The number and relative proportion of virus type/subtype reported through weekly surveillance should be considered as representative of the currently circulating viruses, whereas, antigenic characterization information should be used to compare the relative proportion of a virus strain within a type or subtype.
¶ The expected baseline proportion of P&I deaths reported by the 122 Cities Mortality Reporting System is projected using a robust regression procedure in which a periodic regression model is applied to the observed percentage of deaths from P&I over the previous 5 years. The epidemic threshold is 1.654 standard deviations above the seasonal baseline. 
‡ The Emerging Infections Program (EIP) Influenza Project conducts surveillance in 60 counties associated with 12 metropolitan areas: San Francisco, CA; Denver, CO; New Haven, CT; Atlanta, GA; Baltimore, MD; Minneapolis/St. Paul, MN; Albuquerque, NM; Las Cruces, NM; Albany, NY; Rochester, NY; Portland, OR; and Nashville, TN.  The New Vaccine Surveillance Network (NVSN) conducts surveillance in Monroe County NY, Hamilton County OH, and Davidson County TN. 
†  NVSN provides population-based estimates of laboratory-confirmed influenza hospitalization rates in children aged <5 years admitted to NVSN hospitals with fever or respiratory symptoms.  Children are prospectively enrolled, and respiratory samples are collected and tested by viral culture and reverse transcription-polymerase chain reaction (RT-PCR).  EIP conducts surveillance for laboratory-confirmed, influenza-related hospitalizations in persons aged <18 years.  Hospital laboratory and admission databases and infection-control logs are reviewed to identify children with a positive influenza test (i.e., viral culture, direct fluorescent antibody assays, RT-PCR, or a commercial rapid antigen test) from testing conducted as a part of their routine care.
§  Defined as a temperature of ≥ 100.0°F (≥ 37.8°C), oral or equivalent, and cough and/or sore throat, in the absence of a known cause other than influenza
** The national and regional baselines are the mean percentage of visits for ILI during non-influenza weeks for the previous three seasons plus two standard deviations.  Non-influenza weeks are defined as weeks in which the percentage of specimens tested for influenza that is influenza positive is less than 10%.  It is not appropriate to use the national baseline for regional data.
¥¥  Surveillance Regions: New England (Connecticut, Maine, Massachusetts, New Hampshire, Vermont, Rhode Island); Mid-Atlantic (New Jersey, New York City, Pennsylvania, Upstate New York); East North Central (Illinois, Indiana, Michigan, Ohio, Wisconsin); West North Central (Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota); South Atlantic (Delaware, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, Washington, D.C., West Virginia); East South Central (Alabama, Kentucky, Mississippi, Tennessee); West South Central (Arkansas, Louisiana, Oklahoma, Texas); Mountain (Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, Wyoming); Pacific (Alaska, California, Hawaii, Oregon, Washington).
¶¶  Levels of activity are 1) no activity; 2) sporadic: isolated laboratory-confirmed influenza cases or a laboratory-confirmed outbreak in one institution, with no increase in activity; 3) local: increased ILI , or at least two institutional outbreaks (ILI or laboratory-confirmed influenza) in one region with recent laboratory evidence of influenza in that region; virus activity no greater than sporadic in other regions; 4) regional: increased ILI activity or institutional outbreaks (ILI or laboratory-confirmed influenza) in at least two but less than half of the regions in the state with recent laboratory evidence of influenza in those regions; and 5) widespread: increased ILI activity or institutional outbreaks (ILI or laboratory-confirmed influenza) in at least half the regions in the state with recent laboratory evidence of influenza in the state