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Update: Influenza Activity --- United States, October 1, 2006--February 3, 2007

This report summarizes U.S. influenza activity* since the beginning of the 2006--07 influenza season (October 1, 2006) and updates the previous summary (1). Low levels of influenza activity were reported from October through early December. Activity increased from mid-December through the end of the year, declined slightly in early January, and then increased again in mid-January.

Viral Surveillance

During October 1, 2006--February 3, 2007,^† World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories in the United States reported testing 83,332 specimens for influenza viruses, and 6,244 (7.5%) tested positive (Figure 1). Of these, 5,161 (82.7%) were influenza A viruses, and 1,083 (17.3%) were influenza B viruses. A total of 1,696 (32.9%) of the 5,161 influenza A viruses have been subtyped: 1,507 (88.9%) were influenza A (H1) viruses, and 189 (11.1%) were influenza A (H3) viruses. From October 1, 2006 through January 6, 2007, 6.2% of the subtyped influenza A viruses were A (H3). From January 7 through February 3, the percentage of influenza A viruses subtyped as A (H3) increased to 16.9%. Although influenza A (H3) viruses have been identified in all nine surveillance regions,^§ of the 189 influenza A (H3) viruses reported to CDC this season, 117 (61.9%) were from the Mountain and Pacific regions.

Antigenic Characterization

CDC has antigenically characterized 161 influenza viruses collected since October 1, 2006, and submitted by U.S. laboratories: 99 influenza A (H1), seven influenza A (H3), and 55 influenza B viruses. Ninety-three (94%) of the influenza A (H1) viruses were characterized as A/New Caledonia/20/99-like, the influenza A (H1) component of the 2006--07 influenza vaccine; six (6%) had reduced titers with ferret antisera produced against A/New Caledonia/20/99. Four (57%) of the seven influenza A (H3) viruses were characterized as A/Wisconsin/67/2005-like, the influenza A (H3) component of the 2006--07 influenza vaccine, and three (43%) had reduced titers with ferret antisera produced against A/Wisconsin/67/2005. Influenza B viruses currently circulating can be divided into two antigenically distinct lineages represented by B/Victoria/02/87 and B/Yamagata/16/88. Thirty-seven (67%) of the 55 influenza B viruses characterized belong to the B/Victoria lineage of viruses: 18 (49%) were similar to B/Ohio/01/2005, the influenza B component of the 2006--07 influenza vaccine, and 19 (51%) had reduced titers with antisera produced against B/Ohio/01/2005. Eighteen (33%) of the 55 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.

Influenza-Like Illness (ILI) Surveillance

During the current influenza surveillance season, weekly percentages of patient visits for ILI^¶reported by approximately 1,300 U.S. sentinel providers in 50 states, Chicago, the District of Columbia, New York City, and the U.S. Virgin Islands have ranged from 1.0 to 3.2%. The national percentage of outpatient visits for ILI during 7 weeks was above the national baseline of 2.1%** (Figure 2). For the week ending February 3, 2007, eight of the nine influenza surveillance regions reported ILI at or above their region-specific baselines.

State-Specific Activity Levels

Widespread^†† influenza activity was reported by only seven southeastern states (Alabama, Florida, Georgia, Kentucky, Mississippi, South Carolina, and Tennessee) from the week ending November 25, 2006, through January 13, 2007. Since that time, an additional eight states (Arkansas, Delaware, Indiana, Iowa, Maryland, Minnesota, Oklahoma, and Texas) have reported widespread influenza activity for at least 1 week this season. Arkansas, Maryland, Minnesota, Oklahoma, and Texas each reported widespread activity for the first time this season during the week ending January 27, and Delaware reported widespread activity for the first time during the week ending February 3. For the week ending February 3, nine states reported widespread activity, 19 states reported regional activity, 14 states reported local activity, and seven states reported sporadic activity (Figure 3).

Pneumonia and Influenza-Related Mortality

Pneumonia and influenza (P&I) was listed as an underlying or contributing cause of death for 7.4% of all deaths reported through the 122 Cities Mortality Reporting System for the week ending February 3, 2007. During the current influenza season, the weekly percentage of deaths associated with P&I has ranged from 5.6% to 7.5% but has not exceeded the epidemic threshold^§§ (Figure 4).

Influenza-Associated Pediatric Hospitalizations

Pediatric hospitalizations associated with laboratory-confirmed influenza infections are monitored in two population-based surveillance networks, the Emerging Infections Program (EIP) and the New Vaccine Surveillance Network (NVSN). From October 1, 2006, through January 20, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate reported by EIP sites for children aged 0--17 years was 0.13 per 10,000 (0.34 per 10,000 children aged 0--4 years and 0.05 per 10,000 children aged 5--17 years). From November 5, 2006, through January 20, 2007, the preliminary laboratory-confirmed influenza associated hospitalization rate for children aged 0--4 years in NVSN was 0.63 per 10,000 children.^¶¶

Influenza-Related Pediatric Mortality

For the 2006--07 influenza season, nine influenza-related pediatric deaths have been reported from six states (Florida, Georgia, Louisiana, New York, Ohio, and Texas) through the CDC Influenza-Associated Pediatric Mortality Surveillance System. Children ranged in age from 3 months to 14 years (mean: 7.5 years). Five children were male, and four were female. All patients tested positive for influenza A virus; two specimens were further subtyped as influenza A (H1) virus.

Reported by: WHO Collaborating Center for Surveillance, Epidemiology, and Control and Influenza. L Brammer, MPH, S Wang, MPH, L Blanton, MPH, A Postema, MPH, R Dhara, MA, MPH, T Wallis, MS, D Shay, MD, J Bresee, MD, A Klimov, PhD, N Cox, PhD, Influenza Div, National Center for Immunization and Respiratory Diseases (proposed); A Johnson, DVM, PhD, EIS Officer, CDC.

Editorial Note:

During October 1, 2006--February 3, 2007, the United States experienced moderate levels of influenza activity. For the week ending February 3, 2007, state and territorial epidemiologists reported nine states with widespread activity and 19 states with regional activity. Influenza virus isolates have been reported in all nine surveillance regions in the United States. Patient visits for ILI have exceeded the national baseline during 7 weeks this season; however, P&I mortality has not exceeded the epidemic threshold.

Vaccination is the best method for preventing influenza and its potentially severe complications. Although the optimal months for influenza vaccination are October and November, vaccination in December and beyond is still beneficial. Influenza activity typically peaks in the United States between December and March (2).

The degree of antigenic match between current influenza vaccine strains and strains that are circulating this season will be determined as more strains become available for analysis. However, to date, influenza A (H1) viruses have been most frequently reported, and 94% of those sent to CDC for antigenic characterization were similar to A/New Caledonia/20/99, the influenza A (H1) component of the 2006--07 influenza vaccine.

On January 9, 2007, the executive committee of the Council of State and Territorial Epidemiologists adopted an interim position statement that adds human infections with novel influenza A viruses to the list of nationally notifiable diseases and conditions reportable to the National Notifiable Disease Surveillance System.*** Novel influenza A viruses are defined as those isolated from a human but subtyped as nonhuman, or those that cannot be subtyped by standard methods. Human infections with novel influenza A viruses that can be transmitted from person to person might signal the beginning of an influenza pandemic. Rapid reporting of such infections will facilitate characterization of influenza A viruses with pandemic potential and early implementation of public health responses. Influenza surveillance reports for the United States are posted online weekly during October--May at http://www.cdc.gov/flu/weekly/fluactivity.htm. Additional information regarding influenza viruses, surveillance, vaccine, and avian influenza is available at http://www.cdc.gov/flu.

Acknowledgments

This report is based, in part, on data contributed by participating state and territorial health departments and state public health laboratories, WHO collaborating laboratories, National Respiratory and Enteric Virus Surveillance System collaborating laboratories, the U.S. Influenza Sentinel Provider Surveillance Network, the New Vaccine Surveillance Network, the Emerging Infections Program, and the 122 Cities Mortality Reporting System.

References

* The CDC Influenza Surveillance System has seven components: 1) World Health Organization and National Respiratory and Enteric Virus Surveillance System collaborating laboratories, 2) U.S. Influenza Sentinel Provider Surveillance Network, 3) state and territorial epidemiologist reports, 4) 122 Cities Mortality Reporting System, 5) Emerging Infections Program, 6) New Vaccine Surveillance Network, and 7) Influenza-Associated Pediatric Mortality Surveillance System.

^† As of February 3, 2007.

^§ New England (Connecticut, Maine, Massachusetts, New Hampshire, Vermont, and Rhode Island); Mid-Atlantic (New Jersey, New York City, upstate New York, and Pennsylvania); East North Central (Illinois, Indiana, Michigan, Ohio, and Wisconsin); West North Central (Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, and South Dakota); South Atlantic (Delaware, District of Columbia, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, and West Virginia); East South Central (Alabama, Kentucky, Mississippi, and Tennessee); West South Central (Arkansas, Louisiana, Oklahoma, and Texas); Mountain (Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming); Pacific (Alaska, California, Hawaii, Oregon, and Washington).

^¶ Defined as a temperature of >100.0^ºF (>37.8^ºC), 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. A non-influenza week is a week during which <10% of specimens tested positive for influenza. National and regional percentages of patient visits for ILI are weighted on the basis of state population. Use of the national baseline for regional data is not appropriate.

^†† Levels of activity are 1) no activity; 2) sporadic: isolated laboratory-confirmed influenza cases or laboratory-confirmed outbreak in one institution, with no increase in ILI 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 fewer 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.

^§§ The expected seasonal 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 that occurred during the preceding 5 years. The epidemic threshold is 1.645 standard deviations above the seasonal baseline.

^¶¶NVSN conducts surveillance in Monroe County, New York; Hamilton County, Ohio; and Davidson County, Tennessee. 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 in 60 counties associated with 12 metropolitan areas: San Francisco, California; Denver, Colorado; New Haven, Connecticut; Atlanta, Georgia; Baltimore, Maryland; Minneapolis/St. Paul, Minnesota; Albuquerque, New Mexico; Las Cruces, New Mexico; Albany, New York; Rochester, New York; Portland, Oregon; and Nashville, Tennessee. 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.

*** Available at http://www.cste.org/PS/2007pdfs/NovelFluANNDSSJan10final23.pdf.

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Date last reviewed: 2/14/2007

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