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Update: Influenza Activity --- United States, 2004--05 Season

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

Influenza activity was low in the United States during October through early December but has increased steadily since mid-December. Current surveillance indicators suggest that influenza activity for the season has not yet peaked. Laboratory-confirmed influenza infections have been reported from 45 states, and this season's influenza vaccine strains have been well matched antigenically to the influenza viruses isolated so far this season. In response to this season's influenza vaccine supply shortage, the Department of Health and Human Services (DHHS) has purchased 1.2 million doses of 2004--05 inactivated influenza vaccine from GlaxoSmithKline (GSK). The GSK vaccine is produced, licensed, and distributed globally but is not licensed for use in the United States; therefore, it will be administered in the United States under an Investigational New Drug (IND) protocol. This report summarizes influenza activity during October 3, 2004--January 1, 2005* and provides information on the availability of additional influenza vaccine from GSK.

Influenza Surveillance Reports

During October 3--January 1, World Health Organization (WHO) collaborating laboratories and National Respiratory and Enteric Virus Surveillance System laboratories in the United States tested 34,497 respiratory specimens for influenza viruses; 1,369 (4.0%) were positive. The percentages of specimens testing positive for influenza ranged each week from 0.7% to 12.1% and first exceeded 10% during the week ending December 25. During the 2001--02, 2002--03, and 2003--04 influenza seasons, peak percentages of specimens testing positive for influenza ranged from 24.7% to 35.2% (CDC, unpublished data, 2004). During October 3--January 1, influenza viruses were reported from 45 states. As of January 1, approximately one half of the viruses have been reported from the Mid-Atlantic (26.4%) and New England§ (23.1%) regions. Of the 1,369 influenza viruses identified since October 3, a total 1,128 (82.4%) were influenza A viruses, and 241 (17.6%) were influenza B viruses. Of the 1,128 influenza A viruses, 406 (36.0%) have been subtyped; 404 (99.5%) were influenza A (H3N2) viruses, and two (0.5%) were influenza A (H1) viruses.

CDC has characterized antigenically 107 influenza viruses collected by U.S. laboratories since October 3. All 85 of the influenza A (H3N2) isolates were A/Fujian/411/2002-like (H3N2), the influenza A (H3N2) strain recommended for the 2004--05 influenza vaccine**. Nineteen influenza B isolates were from the B/Yamagata lineage and were characterized as B/Shanghai/361/2002-like, the influenza B strain in the 2004--05 influenza vaccine. Three B isolates belonged to the B/Victoria lineage and were characterized as B/Hong Kong/330/2001-like. Influenza B viruses fall into one of two antigenically and genetically distinct lineages represented by B/Yamagata/16/88 and B/Victoria/2/87 viruses. During 1990--2001, B/Yamagata lineage viruses circulated worldwide, whereas B/Victoria-like viruses were identified only in Asia. However, during March 2001--October 2003, B/Victoria-like viruses were the predominant B viruses in several countries, including the United States. Victoria-lineage and Yamagata-lineage viruses continue to be reported worldwide. However, Yamagata-lineage viruses have been reported more frequently and are represented in the current vaccine.

During October 3--January 1, weekly percentages of patient visits for influenza-like illness (ILI)†† reported by approximately 1,500 U.S. sentinel providers in 50 states, New York City (NYC), Chicago, and the District of Columbia have ranged from 1.0% to 3.0%. During the week ending January 1, the percentage of patient visits for ILI was 3.0%, exceeding the national baseline of 2.5% for the first time this season§§. During the 2001--02, 2002--03, and 2003--04 influenza seasons, national weekly peak percentages of patient visits for ILI ranged from 3.2% to 7.6% (CDC, unpublished data, 2004).

Since the week ending October 9, a total of 16 states and NYC have reported widespread or regional influenza activity. During the week ending January 1, two states and NYC reported widespread activity, 12 states reported regional activity, and 13 states and the District of Columbia reported local activity. During the same week, 6.7% of recorded deaths in the 122 Cities Mortality Reporting System were attributed to pneumonia and influenza (P&I), which is below the epidemic threshold of 7.9%¶¶ for that week. The percentage of P&I deaths exceeded the epidemic threshold for 1 week during October 3--January 1 but otherwise has remained below.

The New Vaccine Surveillance Network (NVSN) consists of three sites (Cincinnati, Ohio; Nashville, Tennessee; and Rochester, New York) that conduct population-based surveillance for laboratory-confirmed influenza among children aged <4 years who are admitted to the hospital with fever or acute respiratory illnesses. During October 3--December 25, 2004, two such hospitalizations occurred (preliminary rate: 0.42 per 10,000 children). During 2000--2003, the end-of-season hospitalization rates in the NVSN sites ranged from 3.7 to 12.0 per 10,000 children.

In June 2004, the Council of State and Territorial Epidemiologists changed nationally notifiable conditions to include deaths in children aged <18 years associated with laboratory test-confirmed influenza a nationally notifiable condition. Data collection began in October 2004, and as of January 8, 2005, one pediatric death has been reported to CDC by the Bureau of Health in Maine.

Purchase of Additional Inactivated Influenza Vaccine

DHHS has purchased 1.2 million doses of the GSK influenza vaccine, Fluarix®, for use in areas with continued vaccine shortages. The Fluarix vaccine obtained by DHHS is similar to other injectable U.S.-licensed influenza vaccines and is licensed and used in more than 78 countries, including the 25 countries of the European Union, Australia, and New Zealand. Because both Fluarix and U.S.-licensed influenza vaccines adhere to the WHO vaccine strain recommendations, components of the two vaccines are similar. However, Fluarix must be used under an IND protocol in the United States because it is not currently licensed by the Food and Drug Administration (FDA) and the time available is not sufficient to obtain FDA licensure for this vaccine to be administered as a licensed product for this season. This vaccine will be available for:

  • Adults aged >50 years,
  • Persons aged >3 years with underlying chronic medical conditions***,
  • Pregnant women in the 2nd or 3rd trimester or women in the 1st trimester with other high-risk conditions for influenza complications***,
  • Residents of nursing homes and long-term--care facilities,
  • Children aged 3--18 years on chronic aspirin therapy***,
  • Health-care workers involved in direct patient care,
  • Out-of-home caregivers and household contacts of persons with high-risk conditions†††.

During the next several weeks, the IND vaccine will be available in identified clinics in selected areas of the United States where need for more influenza vaccine persists. CDC is working with state and local public health officials to finalize the clinic locations. When decisions have been finalized, information on the clinic locations will be available through local and state public health authorities, at telephone 800-232-4636, and on the CDC website. Persons who wish to receive this vaccine must call the clinic for appointments during regular clinic hours, sign a consent form, and provide limited additional information for monitoring the IND vaccine program. The cost of vaccination will be paid by Medicare for persons with Medicare part B coverage. These persons should be prepared to provide their Medicare number and other billing information at the time of vaccination. Persons without Medicare will be responsible for the cost of the vaccine and its administration at the time of their appointment.

Reported by: L Brammer, MPH, E Murray, MSPH, K Teates, MPH, S Harper, MD, K Fukuda, MD, A Klimov, PhD, N Cox, PhD, WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, Div of Viral and Rickettsial Diseases; L Rotz, MD, National Center for Infectious Diseases; R Seither, MPH, M Iwane, PhD; Epidemiology and Surveillance Div; J Copeland, MS, National Immunization Program, CDC.

Editorial Note:

Influenza activity has been low but is increasing in the United States and does not appear to have reached a peak. Influenza viruses might continue to circulate for several more months, and persons for whom influenza vaccine is recommended are strongly encouraged to seek vaccination. Influenza vaccine coverage estimates from December suggest that many persons in vaccine priority groups had not yet been vaccinated and that vaccination rates lagged substantially behind vaccination coverage estimates for the previous year (1).

The influenza vaccine strains are well-matched antigenically to the circulating influenza virus strains. The match between vaccine demand and vaccine availability varies depending on the area. Overall, supplies of both inactivated vaccine and live, attenuated influenza vaccine licensed for use in the United States are available. Beginning January 3, 2005, the priority groups for influenza vaccine have been expanded to include persons aged 50--64 years and household contacts of any person at increased risk for influenza-related complications. Efforts should continue to utilize existing licensed influenza vaccine to vaccinate persons in priority groups. Additional information is available at

In addition, IND inactivated influenza vaccine will become available this month to further increase supply. Thus, persons who were not successful in obtaining vaccination earlier in the season are encouraged to contact their personal physicians or their local health departments to determine where vaccine is available in their areas. Influenza surveillance reports for the United States are published weekly during October--May and are available at or through CDC's voice (888-232-3228) and fax (888-232-3299, document number 361100) information systems.


This report is based on data contributed by participating state and territorial epidemiologists and state public health laboratory directors, WHO collaborating laboratories, National Respiratory and Enteric Virus Surveillance System collaborating laboratories, U.S. Influenza Sentinel Provider Surveillance System, New Vaccine Surveillance Network.


  1. CDC. Estimated influenza vaccination coverage among adults and children---United States, September 1--November 30, 2004. MMWR 2004;53:1147--53.

New Jersey, New York, and Pennsylvania.

§ Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont.

Includes both the A (H1N1) and A (H1N2) influenza virus subtypes.

** The A/Fujian/411/2002-like virus used by U.S. vaccine manufacturers was A/Wyoming/03/2003, an antigenically equivalent virus appropriate for vaccine production.

†† Temperature of >100.0ºF (>37.8ºC) and either cough or sore throat in the absence of a known cause.

§§ The national baseline was calculated as the mean weighted percentage of visits for ILI during noninfluenza weeks, plus two standard deviations. Wide variability in regional data precludes calculating region-specific baselines; applying the national baseline to regional data is inappropriate.

¶¶ The expected seasonal baseline proportion of P&I deaths reported by 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 1.645 standard deviations above the seasonal baseline.

*** Groups differ from the Advisory Committee on Immunization Practices recommendations to accommodate for differences in Fluarix licensed indications or lack of pediatric doses of the IND vaccine

††† Persons at high risk include adults aged >65 years, children aged 6--23 months, persons aged 2--64 years with underlying chronic medical conditions, women who will be pregnant during the influenza season, residents of nursing homes and long-term--care facilities, and children aged 2--18 years on chronic aspirin therapy.

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