Influenza Activity in the United States

The Centers for Disease Control and Prevention (CDC) has updated its annual report on influenza activity in the United States. “Update: Influenza Activity—United States and Worldwide, 2003–04 Season, and Composition of the 2004–05 Influenza Vaccine” is available online athttp://www.cdc.gov/mmwr/preview/mmwrhtml/mm5325a1.htm.

During last year’s influenza season in the United States, influenza A accounted for 99 percent of all influenza infections. The remaining 1 percent consisted of influenza B. Among the influenza A viruses, 99.9 percent were H3N2 viruses, and 0.1 percent were H1 viruses.

As measured by the 122 Cities Mortality Reporting System, the percentage of deaths in the United States attributed to pneumonia and influenza (P&I) exceeded the epidemic threshold during nine consecutive weeks. The percentage of P&I mortality reached a peak of 10.3 percent during the weeks ending January 10 and 17, 2004. During the previous four influenza seasons, the peak percentage of P&I mortality ranged from 8.1 to 11.2 percent.

As of May 31, 2004, 152 influenza-associated deaths in U.S. residents younger than 18 years were reported to the CDC by 40 states. All patients had influenza virus infection detected by rapid antigen testing, viral culture, or other laboratory methods.

•Avian Influenza. In December 2003, one confirmed case of avian influenza A (H9N2) virus infection was reported in a child in Hong Kong. The child had fever, cough, and nasal discharge in late November, was hospitalized for two days, and fully recovered. The source of this child’s H9N2 infection is unknown.

Between January and March 2004, 34 confirmed human cases of avian influenza A (H5N1) virus infection were reported in Vietnam and Thailand. The cases were associated with severe respiratory illness requiring hospitalization and a case-fatality proportion of 68 percent (Vietnam: 22 cases, 15 deaths; Thailand: 12 cases, eight deaths). A substantial proportion of the cases were children and young adults. These cases were associated with widespread outbreaks of highly pathogenic H5N1 influenza among domestic poultry.

In March 2004, health authorities in Canada reported two confirmed cases of avian influenza A (H7N3) virus infection in poultry workers who were involved in culling of poultry during outbreaks of highly pathogenic H7N3 on farms in the Fraser River Valley, British Columbia. One patient had unilateral conjunctivitis and nasal discharge, and the other had unilateral conjunctivitis and headache. Both illnesses resolved without hospitalization.

During the 2003–2004 influenza season, a case of avian influenza A (H7N2) virus infection was detected in an adult male from New York, who was hospitalized for upper and lower respiratory tract illness in November 2003. Influenza A (H7N2) virus was isolated from a respiratory specimen from the patient, whose acute symptoms resolved. The source of this person’s infection is unknown.

•Composition of the Influenza Vaccine for the 2004–2005 Season. The Food and Drug Administration’s Vaccines and Related Biological Products Advisory Committee (VRB-PAC) recommended that the 2004–2005 trivalent influenza vaccine for the United States contain A/New Caledonia/20/99-like (H1N1), A/Fujian/411/2002-like (H3N2), and B/Shanghai/361/2002-like viruses.

Because of the growth properties of the A/Wyoming/3/2003 and B/Jiangsu/10/2003 viruses, U.S. vaccine manufacturers are using these antigenically equivalent strains in the vaccine as the H3N2 and B components, respectively. The A/New Caledonia/20/99 virus will be retained as the H1N1 component of the vaccine.

•Influenza Vaccination Recommendations. Beginning with the 2004–2005 inf luenza season, the Advisory Committee on Immunization Practices (ACIP) recommends that all children six to 23 months of age and close contacts of children aged zero to 23 months receive annual influenza vaccination. The ACIP continues to recommend that all persons aged older than six months with certain chronic underlying medical conditions, their household contacts, and health care workers receive annual influenza vaccination.