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Update: Influenza Activity --- United States and Worldwide, May 20--September 15, 2007

During May 20--September 15, 2007, influenza A (H1), influenza A (H3), and influenza B viruses cocirculated worldwide and were identified sporadically in the United States. This report summarizes influenza activity in the United States and worldwide since the last MMWR update (1).

United States

In the United States, CDC uses nine systems for national influenza surveillance (2), six of which operate year-round: 1) World Health Organization (WHO) collaborating laboratories; 2) the National Respiratory and Enteric Virus Surveillance System (NREVSS); 3) the U.S. Influenza Sentinel Provider Surveillance System; 4) the 122 Cities Mortality Reporting System; 5) the Influenza-Associated Pediatric Mortality System, part of the National Notifiable Diseases Surveillance System (NNDSS); and 6) novel influenza A virus case reporting through NNDSS. Data from these six systems are included in this report.

During May 20--September 15, 2007,* WHO and NREVSS collaborating laboratories in the United States tested 21,029 respiratory specimens for influenza viruses; 398 (1.9%) were positive (Figure). Of these, 330 (83%) were influenza A viruses, and 68 (17%) were influenza B viruses. Of the influenza A viruses, 152 (46%) were subtyped: 67 (44%) were influenza A (H1) viruses, and 85 (56%) were influenza A (H3) viruses. Influenza viruses were reported from 22 states in eight of the nine public health surveillance regions. However, 200 (50%) of all the influenza viruses, including 63 (94%) of the 67 influenza A (H1) viruses, were reported from Hawaii, and 100 (25%) were reported from Florida. Of the 398 influenza viruses reported during the summer months, only 124 (31%) were reported during August and the first half of September. Among this subset of viruses, 105 (85%) were influenza A, and 19 (15%) are influenza B.

During May 20--September 15, data from the U.S. Influenza Sentinel Provider Surveillance System indicated that the weekly percentage of patient visits to U.S. sentinel providers for influenza-like illness (ILI) remained below the national baseline§ of 2.1% and ranged from 0.6% to 1.0%. The percentage of deaths attributed to pneumonia and influenza (P&I) as reported by the 122 Cities Mortality Reporting System was below the epidemic threshold. One influenza-associated pediatric death occurred during June and was reported to the Influenza-Associated Pediatric Mortality Reporting System.

Two human cases of novel influenza A were reported to NNDSS. Both persons were infected with swine influenza virus and were infected by handling ill pigs at a county fair in Ohio. Both recovered from their illness.


During May 20--September 15, influenza A (H1), influenza A (H3), and influenza B viruses cocirculated worldwide. Influenza A (H3) viruses predominated in Asia; however, influenza A (H1) and B viruses also were reported. In Africa, influenza A viruses predominated, with approximately equal numbers of influenza A (H1) and A (H3) viruses reported and a smaller number of influenza B viruses identified. In Europe and North America, small numbers of influenza A and influenza B viruses were reported. In Oceania, influenza A viruses predominated. Influenza A (H3) viruses were reported more frequently than influenza A (H1) viruses in Australia and New Caledonia; however, in New Zealand, influenza A (H1) viruses predominated. In South America, influenza A (H3) viruses were most commonly reported, although influenza B viruses also were identified.

Antigenic Characterization of Influenza Virus Isolates

The WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, located at CDC, analyzes influenza virus isolates received from laboratories worldwide. Of four influenza A (H1) viruses that were collected during May 20--September 8 (three from Asia and one from Europe) and analyzed at CDC, all four (100%) were antigenically similar to A/Solomon Islands/3/2006, the H1N1 component of the 2007--08 influenza vaccine. Of the 94 influenza A (H3) viruses that were characterized (four from Europe, 78 from Latin America, four from Asia, two from Africa, and six from the United States), 17 (18%) were antigenically similar to A/Wisconsin/67/2005, the H3N2 component of the 2007--08 influenza vaccine, whereas 77 (82%) had reduced titers to A/Wisconsin/67/2005.

Circulating influenza B viruses can be divided into two antigenically distinct lineages that have cocirculated worldwide since March 2001, represented by B/Yamagata/16/88 and B/Victoria/02/87 viruses. The B component of the 2007--08 influenza vaccine belongs to the B/Victoria lineage. Of the eight influenza B isolates collected during May 20--September 8 and characterized at CDC, one belonged to the B/Victoria lineage (from Asia). This B/Victoria-lineage virus was similar to B/Ohio/01/2005; B/Ohio/01/2005 is antigenically equivalent to B/Malaysia/2506/2004, the recommended influenza B component for the 2007--08 influenza vaccine. The remaining seven influenza B viruses (three from South America, three from Asia, and one from the United States) belonged to the B/Yamagata lineage.

Human Infections with Avian Influenza A (H5N1) Viruses

During May 20--September 10, 2007, a total of 21 human cases of avian influenza A (H5N1) infection were reported to WHO from four countries (China, Egypt, Indonesia, and Vietnam). Fourteen (67%) of the cases were fatal. Since December 1, 2003, a total of 328 human avian influenza A (H5N1) infection have been reported to WHO (3). Of these, 200 (61%) were fatal (Table). All cases were reported from Asia (Azerbaijan, Cambodia, China, Indonesia, Iraq, Laos, Thailand, Turkey, and Vietnam) and Africa (Djibouti, Egypt, and Nigeria). In addition, no human case of avian influenza A (H5N1) virus infection has been identified in the United States.

Reported by: WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza. L Blanton, MPH, L Brammer, MPH, A Budd, 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, CDC.

Editorial Note:

During May 20--September 15, 2007, influenza A (H1), influenza A (H3), and influenza B viruses cocirculated worldwide. The influenza virus strain that will predominate and the severity of influenza-related disease activity for the 2007--08 influenza season are difficult to predict.

Vaccination is the best method for preventing influenza and its potentially severe complications. In the United States, the influenza vaccine can be administered to any person aged >6 months who wants to reduce the likelihood of becoming ill with influenza or transmitting the virus to others. Annual influenza vaccination is targeted toward persons at increased risk for influenza-related complications and severe disease (e.g., children aged 6--59 months, pregnant women, persons aged >50 years, and persons aged 5--49 years with certain chronic medical conditions) and their close contacts (e.g., health-care workers and household contacts) (4). In addition, all children aged 6 months to <9 years who have never received influenza vaccination should receive 2 doses of influenza vaccine (4). For the 2007--08 influenza season, vaccine supplies are projected to be plentiful in the United States; therefore, influenza vaccination can proceed for all persons, whether healthy or at high risk, either individually or through mass campaigns, as soon as vaccine is available.

Although many of the recently examined influenza A (H3) viruses show reduced reactivity with sera produced against the A/Wisconsin/67/2005 (H3N2) vaccine strain (the H3N2 component of the 2007--08 influenza vaccine), vaccination is still the best means of protection against influenza and influenza-related complications. Even in years in which the match between the vaccine strains and circulating strains is not exact and protection against illness is reduced, the vaccine can still mitigate the severity of illness and reduce the likelihood of severe outcomes such as hospitalization and death.

Although vaccination is the best method for preventing and reducing the impact of influenza, antiviral medications are a valuable adjunct. For patients who consult a health-care provider within 48 hours of illness onset, antiviral medications can reduce the duration of illness and might reduce the likelihood of complications. Antivirals also can be used to prevent influenza in persons who have not received vaccine and to control outbreaks in institutions or group residential settings such as nursing homes.

On September 19, 2007, the Food and Drug Administration (FDA) approved the live, attenuated influenza vaccine (LAIV), FluMist, for use in healthy children aged 2--4 years (i.e., 24--59 months). Vaccination providers should ask the parents or guardians of these children about wheezing and should not use LAIV in children who have recurrent wheezing. LAIV, which is administered as a nasal spray, had already been approved for healthy children aged >5 years and healthy adults aged <50 years. Other FDA-approved changes in the use of FluMist for persons of all approved ages include 1) a reduction in the volume of vaccine used to 0.1 mL per nostril, 2) a reduction in the minimum dose spacing to 4 weeks for children who require 2 doses, and 3) a change in the temperature requirements for shipping and storage of the vaccine (now 2--8°C [35--46°F]). Trivalent inactivated influenza vaccine, which is administered as an intramuscular injection, may be used for any person aged >6 months, including those with high-risk conditions (4).

Two cases of human infection with swine influenza virus were reported in the United States during August. Although human infection with swine influenza is uncommon, sporadic cases occur in most years, usually among persons in direct contact with ill pigs or who have been in places where pigs might have been present (e.g., agricultural fairs, farms, or petting zoos). The sporadic cases detected in recent years have not resulted in sustained human-to-human transmission or community outbreaks; however, human infections with swine influenza viruses or any other nonhuman or novel influenza virus should be identified quickly and investigated. Clinicians should consider swine influenza A in the differential diagnosis among patients with ILI who have had recent contact with pigs. Testing of respiratory specimens from these patients for influenza virus should be coordinated with the state health department laboratory. In January 2007, the executive committee of the Council of State and Territorial Epidemiologists (CSTE) voted to make human infection with a novel influenza A virus, including swine influenza viruses, a nationally notifiable condition, and the proposal was approved by CSTE in June (5,6).

In collaboration with local and state health departments, CDC continues to recommend enhanced surveillance for possible influenza A (H5N1) infection among travelers with severe, unexplained respiratory illness returning from countries affected by influenza A (H5N1) (7). Updates on worldwide avian influenza are available from WHO at

Influenza surveillance reports for the United States are posted online weekly during October--May at Additional information on influenza viruses, influenza surveillance, the influenza vaccine, and avian influenza is available at


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 System, and the 122 Cities Mortality Reporting System; WHO National Influenza Centers, WHO Global Influenza Programme, Geneva, Switzerland; A Kelso, PhD, I Barr, PhD, WHO Collaborating Center for Reference and Research on Influenza, Parkville, Australia; A Hay, PhD, WHO Collaborating Center for Reference and Research on Influenza, National Institute of Medical Research, London, England; and M Tashiro, MD, WHO Collaborating Center for Reference and Research on Influenza, National Institute of Infectious Diseases, Tokyo, Japan.


  1. CDC. Update: influenza activity---United States and worldwide, 2006--07 season, and the composition of the 2007--08 influenza vaccine. MMWR 2007;56:789--94.
  2. CDC. Overview of influenza surveillance in the United States. Atlanta, GA: US Department of Health and Human Services, CDC; 2006. Available at
  3. World Health Organization. Confirmed human cases of avian influenza A (H5N1). Geneva, Switzerland: World Health Organization; 2007. Available at
  4. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2007; 56(No. RR-6).
  5. CDC. Addition of novel influenza A virus infections to the National Notifiable Diseases Surveillance System, 2007. MMWR 2007;56:307.
  6. Council of State and Territorial Epidemiologists. National reporting for initial detections for novel influenza A viruses. Atlanta, GA: Council of State and Territorial Epidemiologists; 2007. Available at
  7. CDC. Updated interim guidance for laboratory testing of persons with suspected infection with avian influenza A (H5N1) virus in the United States. Atlanta, GA: CDC; 2006. Available at

* Data as of September 21, 2007.

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 baseline is the mean percentage of visits for ILI during noninfluenza weeks for the previous three seasons plus two standard deviations. Noninfluenza weeks are those in which <10% of laboratory specimens are positive for influenza.

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.


Table 1
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Figure 1
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Date last reviewed: 9/26/2007


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