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Update: Influenza Activity -- United States and Worldwide, 1993

From October 1992 through February 1993, influenza activity was reported at moderate levels worldwide. Epidemic or outbreak levels of influenza activity were associated with either influenza B or influenza A(H3N2) in many parts of the world. Isolation of influenza A(H1N1) occurred less frequently. This report summarizes worldwide influenza activity reported from March through mid-September 1993 and makes recommendations for vaccination schedules in the United States.

North America and Europe. In most countries, influenza activity peaked in late February or early March and was associated with isolation of influenza B viruses. In March, an increase in the isolation of influenza A(H3N2) that began in mid-January continued throughout the rest of the season. Canada reported influenza A(H3N2) or influenza B outbreaks in nursing homes and other institutions from March through April and detection of sporadic infections caused by influenza A continuing through July. In the United Kingdom, sporadic cases of influenza A(H3N2) were reported during July and August.

In the United States, influenza A(H3N2) was isolated during outbreaks in nursing homes and other institutions during March-May; sporadic isolation of influenza A(H3N2) continued through June. During August, laboratory-confirmed influenza A(H3N2) outbreaks were reported in two nursing homes and among workers on a dredging barge in southern Louisiana (1). Influenza A(H3N2) viruses from the Louisiana outbreaks are antigenically similar to the A/Beijing/32/92 strain (2).

Asia. During March, epidemic level activity associated with the isolation of influenza B viruses was reported in Beijing. Japan reported widespread outbreaks and epidemic levels of influenza activity caused by influenza A(H3N2) viruses continuing into March. Since March, moderate levels of influenza activity caused by influenza A(H3N2) and influenza B have been reported in Hong Kong. From March through August, sporadic isolations of influenza A (untyped), A(H3N2), and influenza B were reported from China, Hong Kong, Indonesia, Korea, Malaysia, Singapore, and Thailand.

Central and South America. Epidemics caused by influenza A(H3N2) were reported in Brazil in March and in Uruguay in May. During May and June, institutional outbreaks caused by influenza A(H3N2) were reported in Argentina. Concomitantly, influenza-like illness (ILI) among all age groups was reported to be widespread in Cordoba, Argentina. From May through July, Chile reported outbreaks due to influenza A(H3N2) and influenza B.

Oceania. Epidemic level activity caused by influenza A (untyped) occurred in Fiji during March and declined in May. During April and May, sporadic isolation of influenza B was reported in Papua New Guinea. In New Zealand, outbreaks caused by influenza B occurred in May followed by widespread outbreaks due to influenza A(H3N2) and sporadic isolation of influenza B from May through July. From March through August, Australia reported mild influenza activity with sporadic isolations of influenza A(H3N2), influenza B, and one case of influenza A(H1N1). During August 15- August 22, a continuing increase in ILI was reported in Victoria and Queensland.

Africa. During February and March, epidemic levels of influenza A(H3N2) occurred in Tunisia. Isolation of influenza A(H3N2) was reported in Madagascar from April through June. Outbreaks due to influenza A(H3N2) occurred in South Africa during May and June.

Characterization of influenza virus isolates. During the 1992- 93 influenza season, 873 influenza isolates collected worldwide were characterized antigenically by the World Health Organization Collaborating Center for Surveillance, Epidemiology, and Control of Influenza at CDC; of these, 551 (63%) were from the United States. Of the 457 influenza A(H3N2) isolates characterized, 384 (84%) were closely related to the 1993-94 vaccine strain, A/Beijing/32/92, and 73 (16%) resembled A/Beijing/353/89, the 1992-93 vaccine strain (2). Of the 343 influenza B isolates, 339 (99%) resembled B/Panama/45/90, the 1993-94 vaccine strain, and four (1%) resembled the strain B/Victoria/02/87 (3). All 73 influenza A(H1N1) viruses analyzed were similar to A/Taiwan/01/86 or to the closely related 1993-94 vaccine strain, A/Texas/36/91 (4). Reported by: World Health Organization National Influenza Centers, Communicable Diseases Div, World Health Organization, Geneva. World Health Organization Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, Influenza Br, and Epidemiology Activity, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Circulation of influenza A/Beijing/32/92(H3N2)-like viruses late in the 1992-93 season and the association of this virus strain with outbreaks in August suggest that influenza A(H3N2) viruses may be the predominant circulating viruses in the United States during the 1993-94 influenza season. Since the emergence of influenza type A(H3N2) in 1968, influenza seasons during which this strain has predominated have been accompanied by a concomitant increase in the proportion of influenza-associated deaths, particularly among persons aged greater than or equal to 65 years.

Although sporadic cases of influenza can occur at any time, outbreaks rarely occur during the summer in the United States. Sporadic cases of influenza are often first detected during October or November, but outbreaks usually do not begin until December. Although it is unknown whether the outbreaks investigated in Louisiana indicate an early influenza season this year, in the past, similar outbreaks have been followed by early influenza activity in other parts of the United States (5-7). Therefore, CDC recommends that, if possible, vaccination providers complete vaccination programs by the end of October 1993 rather than conducting routine vaccination programs through mid-November, as is usually recommended (8).

The Advisory Committee on Immunization Practices recommends vaccination against influenza for 1) persons aged greater than or equal to 65 years; 2) persons who reside in nursing homes or other chronic-care facilities; 3) persons with chronic cardiovascular or pulmonary disorders, including children with asthma; 4) persons who required medical follow-up or hospitalization during the past year because of chronic metabolic disease, renal dysfunction, hemoglobinopathies, or immunosuppression; and 5) children and teenagers who are receiving long-term aspirin therapy and, therefore, may be at risk for developing Reye syndrome after influenza (8). In addition, vaccination is recommended for health-care workers and other persons who are in close contact with persons in high-risk groups, including household members.

The 1993-94 trivalent influenza vaccine contains virus strains of the three distinct groups of influenza viruses in worldwide circulation: A/Texas/36/91-like (H1N1), A/Beijing/32/92-like (H3N2), and B/Panama/45/90-like. Most influenza viruses isolated since March 1993 are closely related to the 1993-94 influenza vaccine.

Even though the vaccine and circulating virus strains appear to be closely matched, antiviral agents can still be a useful adjunct to vaccination (9). Rimantadine hydrochloride, approved for marketing in September by the Food and Drug Administration, and amantadine hydrochloride are specifically active against influenza type A viruses and can be used for prophylaxis or for treatment of influenza A infections in certain situations, including 1) as a control measure when influenza outbreaks occur in institutions -- both for treatment of ill persons and as prophylaxis for others; 2) as short-term prophylaxis for high-risk persons vaccinated after influenza activity has begun and who need protection for the 2-week period during which immunity is developing; 3) as prophylaxis during peak influenza activity for persons for whom vaccine is contraindicated or for immunocompromised persons who may not produce protective levels of antibody in response to vaccination; and 4) as prophylaxis for unvaccinated health-care workers and household contacts of high-risk persons either during peak influenza activity or until immunity develops after vaccination. Because amantadine and rimantadine are effective only against influenza type A, use of a rapid diagnostic test for influenza A may assist in determining influenza-control measures (10).

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


  1. CDC. Influenza A outbreaks -- Louisiana, August 1993. MMWR 1993;42:689-92.

  2. CDC. Update: influenza activity -- United States and worldwide, and composition of the 1993-94 influenza vaccine. MMWR 1993;42:177-

  3. World Health Organization. Recommended composition of influenza virus vaccines for use in the 1993-94 season. Wkly Epidemiol Rec 1993;68:57-60.

  4. CDC. Update: influenza activity -- United States, 1991-92 season. MMWR 1992;41:63-5.

  5. CDC. Influenza -- Arizona, worldwide. MMWR 1980;29:354-5.

  6. CDC. Influenza -- United States, worldwide. MMWR 1980;29:503-4.

  7. CDC. Influenza -- United States, worldwide. MMWR 1980;29:530-2.

  8. ACIP. Prevention and control of influenza: part 1, vaccines -- recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1993;42(no. RR-6):1-13.

  9. ACIP. Prevention and control of influenza: recommendations of the Immunization Practices Advisory Committee (ACIP). MMWR 1992;41(no. RR-9).

  10. Waner JI, Todd SJ, Shalaby H, Murphy P, Wall LV. Comparison of directigen FLU-A with viral isolation and direct immunofluorescence for the rapid detection and identification of influenza A virus. J Clin Microbiol 1990;29:479-82.

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