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Update: Influenza Activity --- United States and Worldwide, June--September, 2002

During June--September 2002, influenza A (H3N2) and B viruses circulated worldwide and were associated with mild to moderate levels of disease activity. Influenza B viruses predominated in Africa, and both influenza A (H3N2) and B viruses circulated widely in Asia, Oceania, and Latin America, except in Chile and Taiwan, where A (H1)* viruses predominated. In North America, sporadic isolates of influenza A (H3N2), A (H1), and B viruses were identified. This report summarizes influenza activity in the United States and worldwide during June--September 2002. Influenza activity in North America typically peaks during December--March, which underscores the need to begin vaccinating against influenza in October and to continue vaccination into December and throughout the influenza season (1).

United States

Influenza surveillance is conducted by a network comprising four components, including approximately 700 sentinel providers and approximately 120 U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories (2). During previous influenza seasons, active surveillance was conducted only from October to mid-May. In 2002, approximately 120 sentinel providers and approximately 60 U.S. WHO and NREVSS collaborating laboratories continued to submit weekly reports after mid-May. During May 19--September 21, the weekly percentage of patient visits to sentinel providers for influenza-like illness ranged from 1.0% in mid-May to 0.5% in mid-September. During this period, WHO and NREVSS collaborating laboratories tested 10,370 respiratory specimens for influenza viruses, of which 145 (1.4%) were positive. Of the positive results, 138 (95.2%) were influenza B viruses and seven (4.8%) were influenza A viruses. Influenza viruses were reported each week from mid-May through mid-July and during the weeks ending July 27 and August 31. No influenza viruses have been reported for September.

From mid-May through early June, outbreaks of influenza B viruses were reported in schools in Hawaii, Oregon, and Texas. In mid-August, a cluster of five influenza A (H3N2) cases associated with a cruise and land tour in Alaska and the Yukon was reported by Health Canada. Ongoing surveillance conducted by the tour company detected no increase in respiratory illness (3).

Worldwide

During June--September, influenza A (H3N2) and B viruses circulated widely in Asia and Oceania; influenza A (H1) viruses were identified infrequently and were not associated with widespread activity, except in Taiwan, where they predominated. In Africa, influenza B viruses predominated. However, Madagascar reported an outbreak of influenza A (H3N2) viruses associated with elevated morbidity and mortality (4). In Latin America, influenza A (H3N2) and B viruses circulated widely and were associated with outbreaks. Influenza A (H1) viruses were identified less frequently, except in Chile, where they predominated. Influenza B viruses predominated in Argentina and were associated with an outbreak in August among school-aged children and their contacts. Influenza A (H3N2) and B viruses circulated widely in Brazil and Peru. In Canada, an outbreak in a long-term--care facility in August was associated with influenza A (H3N2) viruses; influenza A (H3N2), A (H1), and B viruses were identified sporadically throughout the summer.

Characterization of Influenza Virus Isolates

WHO's Collaborating Center for Surveillance, Epidemiology, and Control of Influenza at CDC analyzes influenza virus isolates from laboratories worldwide. Of 23 influenza A (H1) viruses that were collected worldwide during June--September and characterized antigenically at CDC, all were similar to A/New Caledonia/20/99, the H1N1 component of the 2002--03 influenza vaccine; 21 isolates were from Latin America, one was from Asia, and one was from Oceania. Of the 42 influenza A (H3N2) viruses that were characterized antigenically, all were similar to A/Panama/2007/99, the H3N2 component of the 2002--03 influenza vaccine; 21 were from Latin America, 13 were from Oceania, seven were from Asia, and one was from the United States.

Influenza B viruses circulating worldwide can be divided into two antigenically distinct lineages represented by B/Yamagata/16/88 and B/Victoria/2/87. Viruses of the B/Yamagata lineage have circulated worldwide since 1990. From late 1991 to early 2001, no viruses of the B/Victoria lineage were identified outside Asia. Since March 2001, B/Victoria-lineage viruses have been identified in many countries, including the United States. The B component of the 2002--03 influenza vaccine belongs to the B/Victoria lineage. Of the 96 influenza B isolates that were collected worldwide during June--September and characterized antigenically at CDC, 93 belonged to the B/Victoria lineage and three belonged to the B/Yamagata lineage. All 93 B/Victoria-lineage viruses were similar to B/Hong Kong/330/01, the B component of the 2002--03 influenza vaccine. Of the 93 B/Victoria-lineage viruses, 65 were from Latin America, 13 were from Asia, eight were from the United States, and seven were from Oceania. Of the three B/Yamagata-lineage viruses, two were from Latin America and one was from Asia.

Reported by: WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza; E Murray, MSPH, A Postema, MPH, L Brammer, MPH, C Bridges, MD, H Hall, A Klimov, PhD, K Fukuda, MD, N Cox, PhD, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note:

During June--September 2002, influenza A (H1), A (H3N2), and B viruses circulated worldwide. In North America, sporadic cases of influenza were identified each month. The identification of influenza isolates and sporadic influenza outbreaks in the summer and fall is not unusual. Although the number of influenza viruses reported to CDC this summer is greater than the number reported in the previous 12 summers, this increase probably reflects the institution of systematic reporting rather than a true increase in summer influenza activity.

Although the influenza virus type/subtype that will predominate and the onset, peak, and severity of influenza-related disease activity for the 2002--03 influenza season cannot be predicted, the optimal time to receive influenza vaccine is October--November. Persons at high risk for influenza-related complications (e.g., persons aged >65 years and persons aged 6 months--64 years with certain medical conditions), health-care workers, household members of persons at high risk, and children aged 6 months to <9 years receiving influenza vaccine for the first time are recommended to receive vaccine beginning in October (1). Because children aged 6--23 months are at increased risk for influenza-related hospitalizations, starting this fall, the Advisory Committee on Immunization Practices is encouraging, when feasible, the vaccination of all children aged 6--23 months and their household contacts and out-of-home caretakers beginning in October (1,5,6). Other healthy persons, including those aged 50--64 years, are recommended to begin receiving vaccine in November. Influenza vaccine should continue to be offered to all unvaccinated persons in December and throughout the influenza season, as long as vaccine supplies are available (1).

The three manufacturers distributing influenza vaccine in the United States are expected to produce approximately 94 million doses combined, the largest number of trivalent influenza vaccine doses ever projected for a single season. Vaccine manufacturers estimate that approximately 80% of the 94 million doses of influenza vaccine will be distributed by the end of October.

Each February, WHO recommends influenza virus strains for inclusion in the following season's Northern Hemisphere influenza vaccine (7). In the United States, the Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee selects vaccine strains to be used by vaccine manufacturers that distribute influenza vaccine in the United States. Substitution of an antigenically equivalent virus with better growth or processing properties for one or more of the WHO-recommended vaccine components occurs frequently. For the 2002--03 influenza season, WHO recommended A/New Caledonia/20/99--like (H1N1), A/Moscow/10/99-like (H3N2), and B/Hong Kong/330/01--like viruses for inclusion in the Northern Hemisphere influenza vaccine (7). Influenza vaccines sold in the United States will use A/New Caledonia/20/99 for the H1N1 component and the antigenically equivalent strains of A/Panama/2007/99 (H3N2) for the A/Moscow/10/99-like strain and B/Hong Kong/330/01 or B/Hong Kong/1434/02 for the B/Hong Kong/330/01--like strain.

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 http://www.cdc.gov/ncidod/diseases/flu/weekly.htm. The first surveillance report for the 2002--03 season will be available October 11, 2002. Additional information about influenza viruses and influenza surveillance is available at http://www.cdc.gov/ncidod/diseases/flu/fluvirus.htm, and additional information on influenza vaccine is available at http://www.cdc.gov/nip/flu/default.htm.

Acknowledgments

This report is based on data contributed by Health Canada, Ottawa, Canada. T Ayers, MS, Hawaii Dept of Health, Honolulu. A Markum, Klamath County Dept of Public Health, Klamath Falls, Oregon. N Pascoe, Texas Dept of Health, Austin. WHO collaborating laboratories. National Respiratory and Enteric Virus Surveillance System laboratories. U.S. Influenza Sentinel Provider Surveillance System. WHO National Influenza Centers, Communicable Diseases, Surveillance and Response, Geneva, Switzerland. A Hay, PhD, WHO Collaborating Center 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.

References

  1. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2002;51(No. RR-3).
  2. CDC. Influenza activity---United States, 1999--2000 season. MMWR 1999;48:1039--42.
  3. Health Canada. Flu watch August 11 to August 24, 2002 (week 33 and 34). Available at http://www.hc-sc.gc.ca/pphb-dgspsp/fluwatch/01-02/w34_02/index.html.
  4. World Health Organization. Disease outbreaks reported: influenza in Madagascar---update 6. Available at http://www.who.int/disease-outbreak-news/n2002/august/30august2002.html.
  5. Izurieta HS, Thompson WW, Kramarz P, et al. Influenza and the rates of hospitalization for respiratory disease among infants and young children. New Engl J Med 2000;342:232--9.
  6. Neuzil KM, Mellen BG, Wright PF, Mitchel EF, Griffin MR. Effect of influenza on hospitalizations, outpatient visits, and courses of antibiotics in children. New Engl J Med 2000;342:225--31.
  7. World Health Organization. Recommended composition of influenza virus vaccines for use in the 2002--03 season. Wkly Epidimiol Rec 2002;77:57--68.

* Includes both the A (H1N1) and A (H1N2) influenza virus subtypes. The influenza A (H1N2) strain appears to have resulted from the reassortment of the genes of currently circulating influenza A (H1N1) and A (H3N2) subtypes. Because the hemagglutinin proteins of the A (H1N2) viruses are similar to those of currently circulating A (H1N1) viruses and the neuraminidase proteins are similar to currently circulating A (H3N2) viruses, the 2002--03 influenza vaccine should provide protection against A (H1N2) viruses.

Data reported as of September 27, 2002.

Use of trade names and commercial sources is for identification only and does not imply endorsement by the U.S. Department of Health and Human Services.


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