Update: Influenza Activity --- United States and Worldwide, May--September 2003

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During May--September 2003, influenza A(H3N2) viruses circulated worldwide and were associated with mild to moderate levels of disease activity. Influenza A(H1)* and B viruses were reported less frequently. In North America, isolates of influenza A(H3N2), A(H1), and B were identified sporadically. This report summarizes influenza activity in the United States and worldwide during May--September 2003. 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

In the United States, influenza surveillance is conducted by a network comprising four components, including approximately 900 sentinel health-care providers who regularly report data on patient visits for influenza-like illness (ILI) and approximately 120 U.S.-based World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories that report the number of respiratory specimens tested and the number and type of influenza viruses identified during October--mid-May (2). In 2003, approximately 300 sentinel providers and 65 WHO and NREVSS collaborating laboratories continued to submit weekly reports after mid-May. During May 18--September 13, the weekly percentage of patient visits to sentinel providers for ILI ranged from 0.5% to 0.9%, and WHO and NREVSS collaborating laboratories tested 9,145 respiratory specimens, of which 68 (0.7%) were positive. Of the positive results, 31 (45.6%) were influenza A(H3N2) viruses, 25 (36.8%) were influenza type-B viruses, seven (10.3%) were influenza A(H1) viruses, and five (7.0%) were influenza A viruses that were not subtyped. Influenza A viruses were reported each week during mid-May--mid-August. Influenza B viruses were reported for 5 consecutive weeks during mid-May--mid-June and during the week ending August 2. As of September 19, no influenza viruses have been reported for September.

Worldwide

During May--July, influenza A(H3N2) viruses predominated in Africa (Madagascar and South Africa). In Asia, influenza A(H3N2) viruses predominated in Hong Kong and Thailand and were reported in Bangladesh, China, Guam, Indonesia, Japan, and Singapore. In Oceania (Australia, New Caledonia, and New Zealand), A(H3N2) viruses predominated and were associated with widespread activity in Australia and New Zealand. In Latin America, influenza A(H3N2) viruses predominated in Brazil, Chile, and Uruguay. Influenza A(H3N2) viruses also circulated widely in Argentina and were isolated in El Salvador, French Guiana, Paraguay, and Peru. During May--August, sporadic cases of influenza A(H3N2) infection were reported in North America (Canada and Mexico) and Europe (Latvia, Norway, and the United Kingdom). Influenza A(H1) viruses predominated in Argentina and also were reported from Brazil, Chile, French Guiana, Iceland, New Zealand, Peru, South Africa, Trinidad and Tobago, the United Kingdom, and Uruguay. In Africa, influenza B viruses were reported in May (Morocco) and July (South Africa). A small number of influenza B viruses were identified in Asia (Bangladesh, Hong Kong, Japan, and Thailand), South America (Argentina, Brazil, Peru, and Uruguay), and Australia. During May, influenza B viruses were reported in Canada, Latvia, Mexico, and the United Kingdom.

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. Of 91 influenza A(H1) viruses (84 from Latin America, five from the United States, one from Africa, and one from Oceania) collected during May--September and characterized antigenically at CDC, all were similar to A/New Caledonia/20/99, the H1N1 component of the 2003--04 influenza vaccine. Of the 254 influenza A(H3N2) viruses (172 from Latin America, 49 from Asia, 28 from North America [including 22 from the United States], four from Africa, and one from Oceania) that were characterized antigenically, 178 (70.1%) were similar to A/ Panama/2007/99, the H3N2 component of the 2003--04 influenza vaccine, and 76 (29.9%) had reduced titers to A/Panama/2007/99.

Influenza B viruses circulating worldwide can be divided into two antigenically distinct lineages represented by B/Yamagata/16/88 and B/Victoria/2/87. Before 1991, B/Victoria lineage viruses circulated worldwide, but 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 B component of the 2003--04 influenza vaccine belongs to the B/Victoria lineage. Of the four influenza B isolates collected during May--September and characterized antigenically at CDC, two belonged to the B/Victoria lineage and two to the B/Yamagata lineage. Both B/Victoria-lineage viruses were similar to B/Hong Kong/330/01, the B component of the 2003--04 influenza vaccine, and both were from North America (including one from the United States). Of the two B/Yamagata-lineage viruses, one was from the United States, and one was from Asia.

Reported by: WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza; L Brammer, MPH, E Murray, MSPH, A Postema, MPH, 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 May--September 2003, influenza A(H3N2) viruses were the most frequently reported influenza virus type/subtype worldwide, but influenza A(H1) and B viruses also circulated. The influenza virus type/subtype that will predominate and the severity of influenza-related disease activity for the 2003--04 influenza season cannot be predicted.

Influenza vaccine is recommended for persons at high risk for experiencing influenza-related complications (e.g., persons aged >65 years and persons aged 6 months--64 years with certain medical conditions), health-care workers, and household contacts of persons at high risk (1). Influenza vaccine also is recommended for persons aged 50--64 years because they have an elevated prevalence of certain chronic medical conditions. Because young, healthy children are at increased risk for influenza-related hospitalization, vaccination of children aged 6--23 months and household contacts and caregivers of children aged <23 months is encouraged when feasible. In addition to the groups for whom influenza vaccination is recommended, influenza vaccine can be administered to anyone who wants to reduce the likelihood of becoming ill with influenza.

Because vaccine supplies for 2003 are expected to be plentiful, no staggering of vaccination is recommended. The optimal time for influenza vaccination is during October--November. Influenza vaccine manufacturers have indicated that production and distribution of influenza vaccine for the 2003--04 season is proceeding on schedule, allowing for sufficient supply of influenza vaccine during October--November (3). Therefore, influenza vaccination can proceed for all persons, whether healthy or at high risk, either individually or through mass campaigns, as soon as vaccine is available.

In June 2003, the Food and Drug Administration approved live, attenuated influenza vaccine for use among healthy persons aged 5--49 years. This vaccine is administered intranasally rather than by intramuscular injection and offers another option for the prevention of influenza among the approved groups, including health-care workers and close contacts of persons at high risk. The Advisory Committee on Immunization Practices has published supplementary recommendations for the use of this vaccine (4).

This report is based on data contributed by WHO collaborating laboratories; National Respiratory and Enteric Virus Surveillance System laboratories; Sentinel Providers Influenza Surveillance System; World Health Organization National Influenza Centers, Communicable Diseases, Surveillance and Response, World Health Organization, Geneva, Switzerland. A Hay, PhD, WHO Collaborating Center for Reference and Research on Influenza, National Institute 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

* Includes both the A(H1N1) and A(H1N2) influenza virus types. The influenza A(H1N2) strain appears to have resulted from the 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 2003--04 influenza vaccine should provide protection against A(H1N2) viruses.

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