Update: Influenza Activity --- United States, October 2--December 3, 2005

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During October 2--December 3, 2005, low level influenza activity was reported in the United States. This report summarizes U.S. influenza activity* since the beginning of the 2005--06 influenza surveillance season and updates the previous summary (1).

Influenza Viral Surveillance and Characterization

During the current influenza surveillance season, U.S. World Health Organization (WHO) collaborating laboratories and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories in the United States tested 20,336 respiratory specimens for influenza viruses; 173 (0.9%) were positive. The weekly percentages of specimens testing positive for influenza virus ranged from 0.4% to 1.4%. Since October 2, influenza viruses have been reported from 30 states. Of the 173 influenza viruses identified, a total of 151 (87.3%) were influenza A viruses, and 22 (12.7%) were influenza B viruses. Of the 151 influenza A viruses, 78 (51.7%) have been subtyped, with 76 (97.4%) determined to be influenza A (H3N2) viruses and two (2.6%) determined to be influenza A (H1N1) viruses.

CDC has characterized antigenically 16 influenza viruses collected by U.S. laboratories since October 1, 2005. These include 14 influenza A (H3N2) viruses that are similar to A/California/07/2004, the influenza A (H3N2) component included in the 2005--06 influenza vaccines, and two influenza B viruses, one that belongs to the B/Victoria lineage and one that belongs to the B/Yamagata lineage and was characterized as B/Florida/07/2004-like. Recently circulating influenza B viruses have belonged to two antigenically and genetically distinct lineages represented by B/Victoria/2/87 viruses and B/Yamagata/16/88 viruses. The influenza B/Florida/07/2004-like virus isolated is a minor antigenic variant of B/Shanghai/361/2002, the recommended influenza B component for the 2005--06 influenza vaccine.

Influenza-Related Pediatric Mortality

During the current influenza surveillance season, California reported two influenza-related pediatric deaths. One occurred during the 2004--05 influenza surveillance season, and one occurred during the 2005--06 season, the only influenza-related pediatric death reported during the current surveillance season.

Pneumonia and Influenza (P&I) Mortality Surveillance

During the current influenza surveillance season, 5.7%--6.7% of all deaths reported to the 122 Cities Mortality Reporting System were attributable to P&I. Each week, the percentage of P&I deaths was below the epidemic threshold^§ (Figure 1).

Patient Visits for Influenza-Like Illness (ILI)

During the current influenza surveillance season, weekly percentages of patient visits for ILI^¶ reported by approximately 1,000 U.S. sentinel providers in 50 states, New York City, Chicago, and the District of Columbia have ranged from 1.2% to 1.7%. During the week ending December 3, the percentage of patient visits for ILI was 1.6%, which is below the national baseline of 2.2%.**

Influenza Activity Levels Reported by State and Territorial Epidemiologists

No state has reported widespread or regional influenza activity during the current influenza surveillance season. During the week ending December 3, Nebraska was the only state to report local influenza activity; 29 states, New York City, and Puerto Rico reported sporadic influenza activity; 20 states and the District of Columbia reported no influenza activity (Figure 2).

Pediatric Hospitalizations Associated with Laboratory-Confirmed Influenza Infection

CDC monitors laboratory-confirmed influenza-associated pediatric hospitalizations by using two population-based surveillance networks: the Emerging Infections Program (EIP),^§§ which began surveillance for the 2005--06 season on October 1, 2005, and the New Vaccine Surveillance Network (NVSN), which began surveillance for the 2005--06 season on October 30, 2005. Surveillance methods and case definitions differ slightly between the two systems.^¶¶ During October 1--November 26, 2005, the preliminary influenza-associated hospitalization rate for children aged 0--4 years reported by EIP was 0.06 per 10,000. EIP also monitors hospitalizations in children aged 5--17 years; no influenza-associated hospitalizations for this older group were reported during the same period. During October 30--November 26, 2005, NVSN reported no laboratory-confirmed influenza-associated hospitalizations among children aged 0--4 years. EIP and NVSN hospitalization rate estimates are preliminary and might change as data continue to be collected.

Human Cases of Avian Influenza A (H5N1)

No human case of avian influenza A (H5N1) virus infection has been identified in the United States. From January 2004 through December 9, 2005, a total of 137 laboratory-confirmed human cases of avian influenza A (H5N1) infections were reported to the World Health Organization (2). Of these, 70 (51%) were fatal (Table). All cases were reported from five countries in Asia (Cambodia, China, Indonesia, Thailand, and Viet Nam).

Reported by: WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza; S Wang, MPH, R Dhara, MPH, L Brammer, MPH, A Postema, MPH, M Katz, MD, T Uyeki, MD, J Bresee, MD, A Balish, T Wallis, H Hall, A Klimov, PhD, N Cox, PhD, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; J Ortiz, MD, EIS Officer, CDC.

Editorial Note:

During October 2--December 3, the United States experienced a low level of influenza activity. During the week ending December 3, state and territorial epidemiologists reported only one state (Nebraska) with local influenza activity and 29 states, New York City, and Puerto Rico with sporadic activity; 20 states and the District of Columbia reported no activity. In addition, P&I mortality and patient visits for ILI have remained below national baseline levels.

Vaccination is the best way to prevent influenza (3). Although influenza vaccinations begin in October, vaccination in December and beyond is still beneficial; influenza activity usually does not peak in the United States until December--March (3). The degree of antigenic match between the current vaccine strains and strains that will circulate this season will be determined as more strains become available for analysis.

Influenza surveillance reports for the United States are posted online weekly during October--May and are available at http://www.cdc.gov/flu/weekly/fluactivity.htm. Additional information about influenza viruses, influenza surveillance, and the influenza vaccine is available at http://www.cdc.gov/flu.

Sporadic cases of avian influenza A (H5N1) in humans continue to be reported in Asia; in November, for the first time during the current outbreak (December 26, 2003 through December 9, 2005), China reported laboratory-confirmed cases (4). The majority of cases appear to have been acquired from direct contact with infected poultry. No evidence of sustained human-to-human transmission of H5N1 has been detected, although rare cases of human-to-human transmission likely have occurred (5).

Recently, influenza A (H5N1) was reported for the first time in avian species in Europe (6), although the likely Asian origin of the outbreaks has been confirmed by virus sequencing analysis and virus isolation (7). This westward spread of disease might be attributed to transport of virus by wild migratory birds from Asia (8); further research is needed to better understand the role of migratory birds in the current H5N1 epizootic.

CDC continues to recommend enhanced surveillance for suspected H5N1 cases among travelers with unexplained severe respiratory illness returning from H5N1-affected countries (1) as a defense against further spread of the disease from H5N1-affected countries. Additional information regarding avian influenza is available at http://www.cdc.gov/flu/avian/index.htm.

The findings in this report are 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 System, 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 Providers Surveillance Network, 3) 122 Cities Mortality Reporting System, 4) state and territorial epidemiologist reports, 5) influenza-associated pediatric mortality reports, 6) Emerging Infections Program, and 7) New Vaccine Surveillance Network.

^§ 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.

^¶ Temperature of >100.0^°F (>37.8^°C) and cough and/or sore throat in the absence of a known cause other than influenza.

** The national baseline was calculated as the mean percentage of visits for ILI during noninfluenza weeks for the preceding three seasons, plus two standard deviations. Noninfluenza weeks are those in which <10% of laboratory specimens are positive for influenza. Wide variability in regional data precludes calculating region-specific baselines; therefore, applying the national baseline to regional data is inappropriate.

Levels of activity are 1) no activity, 2) sporadic: small numbers of laboratory-confirmed influenza cases or a single influenza outbreak reported but no increase in cases of ILI, 3) local: outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in a single region of a state, 4) regional: outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in at least two but less than half the regions of a state, and 5) widespread: outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in at least half the regions of a state.

^§§ The EIP Influenza Project 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. 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 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 positive influenza test results from testing (i.e., culture, direct or indirect fluorescent antibody assays, PCR, or a rapid test) conducted as part of their routine care.

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