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Current Trends Update: Influenza Activity -- Worldwide, 1990-91, and Influenza Vaccination -- United States

During the 1990-91 influenza season, influenza occurred at relatively low levels throughout much of the world. All reporting countries (except Brazil and Papua New Guinea, which reported epidemic levels) indicated either sporadic cases, small local outbreaks, or regional outbreaks. This report summarizes worldwide influenza activity reported from April through September 1991.

Asia. From April through August, sporadic isolations of influenza B were reported from Hong Kong, Japan, Singapore, and Thailand; influenza A(H3N2), from southern China, India, and Singapore; and influenza A(H1N1), from China, Hong Kong, Japan, Singapore, and Taiwan.

Europe. Influenza activity decreased in Europe during April and May, with only sporadic isolations of influenza B, A(H3N2), and A(H1N1) reported in 10 countries.

Canada and the United States. Canada reported continued sporadic isolation of influenza A and influenza B from April through July; most influenza A isolates were subtype A(H3N2). In the United States, influenza A(H3N2) activity increased during late February, was associated with culture-confirmed outbreaks in a military facility and nursing homes from March through May, and continued to be reported sporadically through June.

Central and South America. From June through August, sporadic isolations of influenza B were reported in Argentina, Brazil, Chile, and Uruguay. During May, an outbreak of influenza A(H3N2) was reported in Guayaquil, Ecuador; during July, epidemic levels of influenza A(H3N2) were reported in Brazil. During June, influenza A(H1N1) virus was reported from Brazil.

Oceania. Influenza B outbreaks occurred in Papua New Guinea from May through August and throughout the South Island of New Zealand in May and June. Australia reported sporadic influenza B activity from April through July. From May through June, Papua New Guinea reported epidemic levels of influenza A(H3N2) in one highland region community (estimated attack rate: 45%). During July, influenza A(H3N2) was isolated sporadically in Australia, and influenza A(H1N1) was isolated sporadically in New Zealand.

South Africa. Influenza B (32 isolates) and influenza A(H3N2) (42 isolates) were identified from May through August.

Characterization of influenza virus isolates. During the 1990-91 worldwide influenza season, 812 isolates were antigenically characterized by the World Health Organization (WHO) Collaborating Center for Surveillance, Epidemiology, and Control of Influenza at CDC; of these, 584 (72%) were from the United States. Of 450 influenza B viruses characterized, 4% were B/Victoria/02/87-like and 96%, B/Yamagata/16/88-like. Most (66%) of the B/Yamagata-like viruses were most closely related to B/Panama/45/90, a minor variant of B/Yamagata/16/88. A total of 188 influenza A(H3N2) viruses were characterized antigenically, and 168 (89%) of these were A/Beijing/353/89-like; all 145 influenza A(H1N1) viruses analyzed were A/Taiwan/1/86-like.

Reported by: National Influenza Centers. Communicable Diseases Div, World Health Organization, Geneva, Switzerland. Epidemiology Activity, and WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, Influenza Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note:

Both influenza A and influenza B circulated at low levels worldwide from October 1990 through September 1991. An increase in the proportion of U.S. isolates subtyped as influenza A(H3N2) after March 1991 and the identification of sporadic isolates of influenza A(H1N1) and influenza A (not subtyped) in mid-September 1991 suggest that influenza A may predominate in the United States during the 1991-92 influenza season.

Morbidity and mortality associated with influenza can be reduced by annual vaccination of persons at increased risk for influenza-related complications and their contacts (1). However, in the United States, only 30% of persons belonging to groups at high risk for influenza-related complications are vaccinated each year (2). One of the year 2000 national health objectives is to achieve influenza vaccination levels of at least 80% in institutionalized older or chronically ill persons, and at least 60% in noninstitutionalized high-risk persons (3). Persons at increased risk for complications from influenza include those greater than or equal to 65 years of age; all residents of nursing homes or chronic-care facilities; persons with chronic pulmonary or cardiovascular disorders (including children with asthma); persons requiring medical follow-up during the past year for chronic metabolic diseases, renal dysfunction, hemoglobinopathies, or immunosuppression; and children and teenagers on long-term aspirin therapy, who are at increased risk for Reye syndrome if infected with influenza. In addition, vaccination is recommended for all persons who provide care for or live with high-risk persons, including health-care providers and household members.

Antibody titers that are protective against influenza infection are achieved approximately 2 weeks following vaccination and begin to decline after approximately 4-6 months. The 1991-92 trivalent influenza vaccine contains hemagglutinin antigens from A/Beijing/353/89-like(H3N2), A/Taiwan/1/86-like(H1N1), and B/Panama/45/90-like viruses, which closely resemble recently identified strains. Because substantial influenza activity in the United States rarely occurs before December, November is the optimal time for vaccination campaigns. When influenza surveillance indicates the occurrence of regional influenza activity before December, vaccination programs should be initiated as soon as the currently recommended vaccine is available. Age-specific vaccination recommendations for the 1991-92 U.S. influenza season have been published (Table 1) (1).

Amantadine is an adjunct to vaccination for prevention and control of influenza A, particularly in institutional settings. Advanced contingency planning (e.g., individualized standing orders for amantadine that can be implemented at the start of an influenza A outbreak) can facilitate rapid implementation of chemoprophylaxis. Amantadine can provide effective prophylaxis for persons who are unvaccinated and, because a full protective response from vaccination requires 2 weeks to develop, for those vaccinated after influenza A is already circulating in the community. Because amantadine is ineffective against influenza B, culturing pharyngeal or nasal secretions of persons with an influenza-like illness can be helpful in guiding influenza control measures (i.e., by detecting and identifying specific influenza types/subtypes in the community).

From October through May, surveillance information is updated weekly at CDC and is available by telephone (CDC Voice Information System (influenza update) (404) 332-4555) or through the CDC Information Service on the Public Health Network electronic bulletin board. In addition, periodic updates about influenza are published in MMWR. Additional information on local influenza activity is available from state and local health departments.


  1. ACIP. Prevention and control of influenza: recommendations of the Immunization Practices Advisory Committee (ACIP). MMWR 1991;40(no. RR-6).

2. Rodgers DV, Strikas RA, Hardy AM, Park C, Zell ER, Williams WW. Influenza and pneumococcal vaccination in the elderly: results of the 1989 National Health Interview Survey (Abstract). In: Program and abstracts of the 119th annual meeting of the American Public Health Association. Washington, DC: American Public Health Association, 1991 (in press). 3. Public Health Service. Healthy people 2000: national health promotion and disease pre vention objectives. Washington, DC: US Department of Health and Human Services, Public Health Service, 1991:122; DHHS publication no. (PHS)91-50213.

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