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Update: Influenza Activity --- United States and Worldwide, May--October 2004

During May--October 2004, influenza A (H3N2) viruses circulated worldwide and were associated with mild-to-moderate levels of disease activity. Influenza A (H1N1)* and B viruses were reported less frequently. In North America, isolates of influenza A (H3N2), A (H1N1), and B were identified sporadically. This report summarizes influenza activity in the United States and worldwide during May--October 2004. Influenza activity in North America typically peaks during December--March (1).

United States

Until recently, in the United States, national influenza surveillance was conducted by four systems that operated during October--May. One of these systems consists of approximately 1,000 sentinel health-care providers, who regularly report data to CDC on patient visits for influenza-like illness (ILI). In addition, during 2004, approximately 350 sentinel providers continued to submit weekly reports during May--September. A second system consists of approximately 120 U.S.-based World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories; these laboratories report the number of respiratory specimens tested and the number and types of influenza viruses identified throughout the year.

For the 2004--05 influenza season, CDC has added two new surveillance systems: one that tracks naturally reported pediatric deaths associated with laboratory-confirmed influenza infections and another that tracks hospitalizations associated with laboratory-confirmed influenza infections in children aged <18 years. The latter system, which will continue at a minimum of nine sites through CDC's Emerging Infections Program, augments CDC's ongoing surveillance at the three National Vaccine Surveillance Network sites of children aged <5 years hospitalized with fever or respiratory illness.

During May 23--October 2, the weekly percentage of patient visits to sentinel providers for ILI ranged from 0.4% to 0.8%. WHO and NREVSS collaborating laboratories tested 11,916 respiratory specimens; 54 (0.5%) were positive for influenza. Of the positive results, 29 (54%) were influenza B viruses, 14 (26%) were influenza A (H3N2) viruses, and 11 (20%) were influenza A viruses that were not subtyped. Both influenza A and B viruses were reported during late May--September 2004.

During October 3--16, influenza activity occurred at low levels in the United States. Since October 3, WHO and NREVSS collaborating laboratories in the United States have tested 1,414 respiratory specimens; eight (0.6%) were positive. Of these, six were influenza A viruses, and two were influenza B viruses. The proportion of patient visits to sentinel providers for ILI and the proportion of deaths attributed to pneumonia and influenza were below baseline levels. During the week ending October 16, nine states and New York City reported sporadic influenza activity, and 40 states and the District of Columbia reported no influenza activity.


During May--July, influenza A (H3N2) viruses predominated in Africa (Madagascar, Senegal, and South Africa). In Asia, influenza A (H3N2) viruses predominated in China, Hong Kong, and Thailand and also were reported in Japan. Influenza A (H3N2) viruses were responsible for regional outbreaks in Taiwan in August and September (2).

In Oceania (Australia, New Caledonia, and New Zealand), influenza A (H3N2) viruses predominated and were associated with multiple nursing home outbreaks in Australia and New Zealand in August and September. In South America, influenza A (H3N2 and non-subtyped) viruses predominated in Argentina, Brazil, Chile, Peru, and Uruguay. Influenza A (H3N2) viruses were associated with widespread outbreaks in Argentina, Chile, and Paraguay during May--June.

During May--July, influenza A (H1N1) viruses predominated in the Philippines and also were reported in China, Japan, New Caledonia, Peru, and Thailand. Influenza B viruses were reported in South America (Argentina, Brazil, Chile, Colombia, and Peru), Asia (China, Japan, and Korea), Africa (South Africa), and North America (United States). Influenza B viruses were associated with widespread outbreaks in Brazil during May--June.

Characterization of Influenza Virus Isolates

WHO's Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, located at CDC, analyzes influenza virus isolates received from laboratories worldwide. During May--October, 236 influenza A (H3N2) viruses (110 from Latin America, 100 from Asia, 24 from North America [including 10 from the United States], one from Africa, and one from Oceania) were collected and characterized antigenically. A total of 208 (88.1%) were A/Fujian/411/02-like and similar to A/Wyoming/03/2003, the A (H3N2) component of the 2004--05 influenza vaccine; 28 (11.9 %) had reduced titers to A/Wyoming/03/2003. The eight influenza A (H1N1) viruses (one from Canada, three from Hong Kong, two from Singapore, and two from the United Kingdom) collected during May--September and characterized antigenically at CDC were similar to A/New Caledonia/20/99, the A (H1N1) component of the 2003--04 influenza vaccine.

Influenza B viruses circulating worldwide can be divided into two antigenically distinct lineages: B/Yamagata/16/88 and B/Victoria/2/87. Before 1991, B/Victoria lineage viruses circulated worldwide; from late 1991 to early 2001, no viruses of the B/Victoria lineage were identified outside Asia. However, since March 2001, B/Victoria-lineage viruses have been identified in many countries outside Asia, including the United States. Viruses of the B/Yamagata lineage began circulating worldwide in 1990 and continue to be identified (3). The type-B component of the 2004--05 influenza vaccine (B/Shanghai/361/2002-like) belongs to the B/Yamagata lineage. Of the 73 influenza B isolates collected during May--September and characterized antigenically at CDC, 54 belonged to the B/Yamagata lineage, and 19 belonged to the B/Victoria lineage.

Of the B/Yamagata lineage viruses, 50 (92.6%) were B/Shanghai/361/2002-like, and four (7.4%) had reduced titers to B/Shanghai/361/2002. Twenty-one of the B/Yamagata lineage viruses were from North America (including 16 from the United States), 25 were from South America, five were from Asia, two were from Oceania, and one was from Europe.

Human Infections with Avian Influenza A (H5N1) Viruses

Since December 2003, nine countries (Cambodia, China, Indonesia, Japan, Laos, Malaysia, South Korea, Thailand, and Vietnam) have reported outbreaks of avian influenza A (H5N1) infection affecting poultry and, in some countries, other animals. As of October 25, a total of 44 laboratory-confirmed cases of avian influenza A (H5N1) virus infection in humans had been reported in Vietnam and Thailand in 2004 (4). Of these 44 patients, 32 died. The cases occurred in association with recurring H5N1 outbreaks among poultry in those countries.

Four human H5N1 cases occurred in Vietnam (three in children and one in a young adult) during July--September. In Thailand, four cases occurred in September and one case in October. The cases were associated with severe respiratory illness, with persons requiring hospitalization; all but one patient died. The cumulative case-fatality proportion for confirmed H5N1 cases since January 2004 is 73% (Vietnam: 27 cases, 20 deaths; Thailand: 17 cases, 12 deaths).

Reported by: WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza; K Teates, MPH, L Brammer, MPH, A Balish, T Wallis, H Hall, A Klimov, PhD, K Fukuda, MD, N Cox, PhD, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; M Katz, MD, EIS Officer, CDC.

Editorial Note:

During May--October 2004, influenza A (H3N2) viruses were the most frequently reported virus subtype worldwide; however, influenza A (H1N1) and influenza B viruses also circulated. At this time, neither the influenza virus subtype that will predominate in the United States nor the severity and timing of the 2004--05 season can be predicted.

The ongoing widespread epizootic of highly pathogenic H5N1 viruses in Asia remains a major concern. Since December 2003, nine Asian countries have reported H5N1 poultry outbreaks, with human cases reported from two of these countries. No evidence of sustained person-to-person transmission has been identified to date, although a probable instance of limited person-to-person transmission in a family cluster was identified recently in Thailand. CDC continues to recommend enhanced surveillance for suspected H5N1 cases among travelers with severe unexplained respiratory illness returning from H5N1-affected countries. Additional information about avian influenza is available at

Influenza surveillance reports for the United States are published weekly during October--May and are available through CDC's voice (telephone, 888-232-3228) and fax (telephone, 888-232-3299, document number 361100) information systems and at Additional information about influenza viruses, influenza surveillance, and the influenza vaccine is available at


This report is based on data contributed by WHO collaborating laboratories; National Respiratory and Enteric Virus Surveillance System laboratories; Sentinel Providers Influenza Surveillance System; WHO National Influenza Centers, Communicable Diseases, Surveillance and Response, WHO, Geneva, Switzerland. A Hay, PhD, WHO Collaborating Centre for Reference and Research on Influenza, National Institute for Medical Research, London, England. I Gust, MD, A Hampson, WHO Collaborating Center for Reference and Research on Influenza, Parkville, Australia. M Tashiro, MD, WHO Collaborating Center for Reference and Research on Influenza, National Institute of Infectious Diseases, Tokyo, Japan.


  1. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2004;53(No. RR-6).
  2. World Health Organization. Influenza in the world. Wkly Epidemiol Rec 2004;79:385--92.
  3. CDC. Update: influenza activity---United States and worldwide, May--September 2003. MMWR 2003;52:911--3.
  4. World Health Organization. Communicable disease surveillance and response: avian influenza. Geneva, Switzerland: World Health Organization; 2004. Available at

* Includes both the A (H1N1) and A (H1N2) influenza virus types. Although H1N2 viruses have not been identified since February 2004, not all isolated H1 viruses have been tested for the subtype of their neuraminidase. Thus, this subtype might continue to circulate in some parts of the world. Influenza A (H1N2) viruses appear to have resulted from reassortment of the genes of the circulating influenza A (H1N1) and A (H3N2) subtypes. Because the hemagglutinin proteins of the A (H1N2) viruses are similar to those of the circulating A (H1N1) viruses, and the neuraminidase proteins are similar to the circulating A (H3N2) viruses, the 2004--05 influenza vaccine should provide protection against A (H1N2) viruses.

As of October 16, 2004.

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