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Update: Influenza Activity --- United States and Worldwide, May 22--September 3, 2005, and 2005--06 Season Vaccination Recommendations

Please note: An erratum has been published for this article. To view the erratum, please click here.

Influenza A (H3N2) viruses circulated worldwide, and Influenza A (H1)* and B viruses were reported less frequently during May 22--September 3, 2005. In North America, isolates of influenza A (H3N2), A (H1), and influenza B were identified sporadically. This report summarizes influenza activity in the United States and worldwide since the last MMWR update.

United States

In the United States, CDC uses seven systems for national influenza surveillance, including the following four that operate year-round: 1) collaborating laboratories of the World Health Organization (WHO) and the National Respiratory and Enteric Virus Surveillance System (NREVSS) report the number, types, and subtypes of influenza viruses detected; 2) approximately 2,250 sentinel health-care providers report patient visits for influenza-like illness (ILI), and approximately 500 of these providers continue regular reporting throughout the summer; 3) 122 U.S. cities report mortality attributed to influenza and pneumonia on a weekly basis; and 4) a national surveillance system records pediatric deaths associated with laboratory-confirmed influenza (1).

During May 22--September 3,§ WHO and NREVSS collaborating laboratories tested 14,016 respiratory specimens; 120 (0.9%) were positive for influenza. Of the positive results, 66 (55%) were influenza B viruses, 33 (28%) were influenza A (H3N2) viruses, one (0.8%) was an influenza A (H1) virus, and 20 (17%) were influenza A viruses that were not subtyped. The majority (78%) of these isolates were tested from mid-May through late June, during which time 1.3% of specimens tested were positive for influenza. Since July, 0.4% of specimens tested were positive for influenza.

During May 22--September 3, the weekly percentage of patient visits to sentinel providers for ILI remained below the national baseline of 2.5% and ranged from 0.7% to 1.3%. The percentage of deaths attributable to pneumonia and influenza (P&I) as reported by the 122 Cities Mortality Reporting System remained below the epidemic threshold,** and no influenza-related pediatric deaths were reported as occurring during this period.


During May 22--September 3, influenza A (H3N2) viruses predominated in Asia (China, Hong Kong, Japan, Korea, and Thailand). Influenza A (H3N2) viruses were also identified in Oman and Singapore. Influenza A (H1) viruses were reported in China, Hong Kong, India, Indonesia, Japan, Korea, and Malaysia. Influenza B viruses were reported in China, Hong Kong, Indonesia, Korea, Nepal, Philippines, and Thailand.

In Oceania, during the same period, influenza A (H3N2 and non-subtyped) viruses predominated in Australia; influenza B viruses were responsible for outbreaks in New Zealand. Influenza B viruses were also reported in Australia and New Caledonia. In Africa, both influenza A virus subtypes (H3N2 and H1) and influenza B viruses were reported in South Africa, and influenza A (H3N2) and influenza B viruses were reported in Madagascar. Influenza B viruses also were reported in Kenya.

In South America, influenza A (H3N2 and non-subtyped) viruses were associated with regional outbreaks in Argentina and Chile during May 22--September 3 and were reported in Brazil, Colombia, Peru, and Uruguay. Influenza B viruses were associated with an outbreak in Colombia in July and also were reported in Argentina, Brazil, Chile, and Uruguay. Influenza A (H1) viruses were reported in Peru. In North America, influenza A viruses (H3N2 and non-subtyped) and influenza B viruses were reported in Canada, Mexico, and the United States. The United States reported one influenza A (H1) virus. Influenza A (H3N2) viruses also were reported in El Salvador and Panama (2--4).

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. During May 22--September 3, a total of 77 influenza A (H3N2) viruses (47 from Latin America, 21 from Asia, eight from the United States, and one from Oceania) were collected and characterized antigenically. All 77 influenza A (H3N2) viruses were antigenically related to the A/California/07/2004 reference virus. However, four South American viruses and nine Asian viruses had reduced titers to A/California/07/2004. An A/ California/07/2004-like virus was recommended as the H3 component for the 2005--06 Northern Hemisphere vaccine. No influenza A (H1) viruses collected during this period were received and characterized by CDC.

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. The type-B component of the 2005--06 influenza vaccine (B/Shanghai/361/2002-like) belongs to the B/Yamagata lineage. Of the 46 influenza B isolates collected during May 22--September 3 and characterized antigenically at CDC, three belonged to the B/Yamagata lineage, and 43 belonged to the B/Victoria lineage. All three of the B/Yamagata-lineage viruses had reduced titers to B/Shanghai/361/2002. Two of the B/Yamagata-lineage viruses were from Asia, and one was from the United States. Of the 43 B/Victoria-lineage viruses, 18 came from Asia, 18 from South America, and seven from the United States.

Avian Influenza A (H5N1)

Since December 2003, a total of 11 countries (Cambodia, China, Indonesia, Japan, Kazakhstan, Laos, Malaysia, Russia, South Korea, Thailand, and Vietnam) have reported outbreaks of highly pathogenic avian influenza A (H5N1) virus affecting poultry. Russia and Kazakhstan reported outbreaks of H5N1 virus among poultry for the first time in late July 2005 (5). Mongolia reported detection of H5N1 virus in migratory birds in August (6). In Southeast Asia, where H5N1 continues to be detected among poultry, approximately 150 million birds have died or been culled since 2003 (5).

Since December 2003, a total of 112 H5N1 cases in humans have been reported to WHO in four countries (Cambodia, Indonesia, Thailand, and Vietnam); 57 (51%) persons died. In August 2005, three cases (including two deaths) were reported in Vietnam. In July, one fatal case was reported in Indonesia (5).

Influenza Vaccine Supply and Recommendations

Vaccination is the primary method for preventing influenza (1). For the 2005--06 influenza vaccine, four manufacturers expect to provide influenza vaccine to the U.S. population. Sanofi Pasteur, Inc., projects production of up to 60 million doses of trivalent inactivated influenza vaccine (TIV). Chiron Corporation projects production of 18--26 million doses of TIV. GlaxoSmithKline, Inc. projects production of 8 million doses of TIV. MedImmune Vaccines, Inc., producer of the nasal-spray, live attenuated influenza vaccine (LAIV), projects production of approximately 3 million doses (7).

Because of the uncertainties regarding production of influenza vaccine, the exact number of available doses and timing of vaccine distribution for the 2005--06 influenza season remain unknown. As a result, CDC recommends that only the following priority groups receive TIV before October 24, 2005:

  • persons aged >65 years with comorbid conditions
  • residents of long-term--care facilities
  • persons aged 2--64 years with comorbid conditions
  • persons aged >65 years without comorbid conditions
  • children aged 6--23 months
  • pregnant women
  • health-care personnel who provide direct patient care
  • household contacts and out-of-home caregivers of children aged <6 months

These groups correspond to tiers 1A--1C in the previously published table of TIV priority groups in the event of vaccination supply disruption (8). Beginning October 24, 2005, influenza vaccine should be made available to all persons. Healthy persons aged 5--49 years who are not pregnant, including health-care workers who are not caring for severely immunocompromised patients in special-care units, can receive LAIV at any time (1).

Vaccination Recommendations for Persons Displaced by Hurricane Katrina

On September 6, 2005, CDC issued interim vaccination recommendations for persons displaced by Hurricane Katrina (9). Any displaced persons aged >6 months living in crowded group settings should be administered influenza vaccine; children aged <8 years should be administered 2 doses, at least 1 month apart.

Reported by: WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza; L Brammer, MPH, A Postema, MPH, R Dhara, MPH, A Balish, T Wallis, H Hall, A Klimov, PhD, T Uyeki, 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 22--September 3, 2005, influenza A (H3N2) viruses were the most frequently reported virus worldwide; however, influenza A (H1) and influenza B viruses also circulated. In North America, sporadic cases of influenza were identified each month. The identification of influenza isolates and even sporadic outbreaks in the summer in North America is not unusual. Neither the influenza virus that will predominate in the United States nor the severity and timing of the 2005--06 season can be predicted.

The ongoing widespread epizootic of highly pathogenic avian influenza A (H5N1) viruses in Asia remains a major public health concern. Since December 2003, a total of 12 countries have reported H5N1 outbreaks in poultry or migratory birds, with human cases reported from four of these countries. Since July 2005, H5N1 infections in poultry and migratory birds have spread beyond their initial focus in Southeast Asia to Kazakhstan, Mongolia, and Russia; a human case was reported in Indonesia for the first time. No evidence of sustained person-to-person transmission has been identified to date, although probable limited person-to-person transmission has been reported (10). To date, no evidence has indicated genetic reassortment among avian influenza A (H5N1) and human influenza A viruses. CDC recommends enhanced surveillance for suspected H5N1 cases among travelers with unexplained severe respiratory illness returning from H5N1-affected countries. Additional information about avian influenza is available at

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


This report is based, in part, on data contributed by WHO collaborating laboratories; National Respiratory and Enteric Virus Surveillance System laboratories; Sentinel Providers Influenza Surveillance System; 122 Cities Mortality Reporting 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 2005;54(No. RR-8).
  2. World Health Organization. Influenza. Wkly Epidemiol Rec 2005;80:239--40.
  3. World Health Organization. Global influenza surveillance. Geneva, Switzerland: World Health Organization; 2005. Available at
  4. Public Health Agency of Canada. FluWatch. Ottawa, Canada: Public Health Agency of Canada; 2005. Available at
  5. World Health Organization. Communicable disease surveillance and response: avian influenza. Geographical spread of H5N1 avian influenza in birds---Update 28. Geneva, Switzerland: World Health Organization; 2005. Available at
  6. World Organisation for Animal Health (OIE). Highly pathogenic avian influenza in Mongolia in migratory birds. Paris, France: World Organisation for Animal Health (OIE); 2005. Available at
  7. CDC. Update: influenza vaccine supply and recommendations for prioritization during the 2005--06 influenza season. MMWR 2005;54:850.
  8. CDC. Tiered use of inactivated influenza vaccine in the event of a vaccine shortage. MMWR 2005;54:749--50.
  9. CDC. Interim immunization recommendations for individuals displaced by Hurricane Katrina. Atlanta, GA: US Department of Health and Human Services, CDC; 2005. Available at
  10. Ungchusak K, Auewarakul P, Dowell SF, et al. Probable person-to-person transmission of avian influenza A (H5N1). N Engl J Med 2005;352:333--40.

* 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, H1N2 viruses 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 2005--06 influenza vaccine should provide protection against A (H1N2) viruses.

CDC. Update: influenza activity---United States and worldwide, 2004--05 season. MMWR 2005;54:631--4.

§ As of September 9, 2005; reporting is incomplete.

The national baseline was calculated as the mean percentage of patient visits for ILI during noninfluenza weeks plus two standard deviations. Wide variability in regional data precludes calculating region-specific baselines and makes applying the national baseline to regional data inappropriate. National and regional percentages of patient visits for ILI are weighted on the basis of state population.

** The expected seasonal baseline proportion of P&I deaths reported by the 122 Cities Mortality Reporting System is projected by using a robust regression procedure in which a periodic regression model is applied to the observed percentage of deaths from P&I during the previous 5 years. The epidemic threshold is 1.654 standard deviations above the seasonal baseline.

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Date last reviewed: 9/14/2005


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