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Update: Influenza Activity -- Worldwide, May-September 1999

In collaboration with the World Health Organization (WHO), the WHO international network of collaborating laboratories, and state and local health departments, CDC conducts surveillance to monitor influenza activity and to detect antigenic changes in the circulating strains of influenza viruses. From October 1998 through April 1999, influenza activity was moderate to severe in the Northern Hemisphere. Influenza A(H3N2) viruses predominated but influenza type B viruses were isolated more frequently than influenza A in some countries. Influenza A(H1N1) viruses were isolated from sporadic cases in Asia, Europe, and North America, and from outbreaks in South America (1). Since May 1999, influenza activity associated primarily with influenza A(H3N2) viruses has peaked and is declining in the Southern Hemisphere. This report summarizes worldwide influenza activity during May-September 1999 and the antigenic characteristics of influenza isolates collected during May-August 1999.

Africa. During May-September, influenza A(H3N2) and influenza B viruses were reported in Mauritius, Senegal, and South Africa. Influenza B viruses were isolated in Madagascar. Influenza A viruses predominated in Mauritius and accounted for approximately half the influenza viruses isolated in South Africa. Influenza B viruses predominated in Senegal.

Asia. During May-August, influenza A(H3N2) viruses predominated in Asia and were reported from Hong Kong, Israel, Japan, Malaysia, Nepal, the Philippines, Singapore, Taiwan, and Thailand. Influenza A(H1N1) viruses were isolated in the Philippines, Taiwan, and Thailand. Influenza type B viruses were isolated from sporadic cases in China, Hong Kong, Israel, Taiwan, and Thailand.

Europe. From late May to early June, an outbreak of influenza A(H3H2) virus infections occurred aboard a British cruise ship sailing in the Mediterranean. During June and July, influenza A(H3N2) viruses were isolated in the United Kingdom and an influenza type B virus was isolated during July. Influenza A virus isolates were reported from Finland during July and August.

North America. During May-September 1999, an outbreak of influenza A(H3N2) virus infections occurred among tourists to Alaska and the Yukon Territory (2). A summer outbreak of influenza A(H3N2) occurred among travelers to the same region in 1998 (3,4). During June-September 1999, six additional outbreaks of influenza in the United States were reported to CDC. From late June to early July, a serologically confirmed influenza A outbreak occurred at a day care center for the elderly in Louisiana. During July, outbreaks of influenza A(H3N2) virus infection occurred in an Oklahoma nursing home, a Texas military base, and a Florida long-term-care facility for the mentally disabled. Influenza A(H3N2) outbreaks also occurred at several Florida correctional facilities during June through August. In September, an outbreak of influenza A(H3N2) infection occurred among passengers and crew aboard a ship sailing along the northeastern seaboard. During June through August, sporadic cases of influenza A were reported from Florida, Hawaii, Illinois, Washington, and Wisconsin. Influenza A(H3N2) viruses were isolated from sporadic cases in New York during June, Hawaii during July, and Texas during August. Influenza B viruses were reported from Hawaii during June. Sporadic cases of influenza A were reported from Canada throughout the summer.

Oceania. Influenza A viruses predominated in Australia, New Zealand, and New Caledonia. Influenza B viruses also were isolated and increased in number later in the influenza season. Most influenza A viruses were subtype A(H3N2). Influenza A(H1N1) viruses were isolated from sporadic cases in Australia and New Zealand and were associated with outbreaks in New Caledonia during May and June.

South and Central America and the Carribean. Influenza A(H3N2) viruses predominated in Argentina, Brazil, Chile, Panama, and Uruguay, and were reported from the Bahamas, Costa Rica, the Dominican Republic, Jamaica, and Puerto Rico. Influenza B isolates were reported from Argentina, Brazil, Chile, Colombia, Costa Rica, Paraguay, and Uruguay. Influenza A(H1N1) viruses were reported from Argentina, Brazil, Costa Rica, and Paraguay.

Characterization of influenza virus isolates. The WHO Collaborating Center for Reference and Research on Influenza at CDC analyzes isolates from laboratories worldwide. Isolates were collected during May-August, including those from the end of the 1998-99 influenza season and from summer 1999 in the Northern Hemisphere, and from the 1999 epidemic season in the Southern Hemisphere. Of the 41 antigenically characterized influenza B isolates, all 41 were similar to B/Yamanashi/166/98, the B/Beijing/184/93-like virus contained in the 1999-2000 influenza vaccine; 17 were collected from Central and South America, 12 were from Asia, 10 were from South Africa, Australia, and New Zealand, and two were from the United States.

Among 209 influenza A(H3N2) viruses tested, 180 (86%) were antigenically similar to A/Sydney/05/97, the H3N2 component of the 1999-2000 influenza vaccine; 29 (14%) H3N2 viruses, although related to A/Sydney/05/97, showed reduced titers against A/Sydney/05/97 antiserum in hemagglutination-inhibition tests. Of the 209 influenza H3N2 viruses tested, 110 were from Central and South America and the Carribean; 51 were from North America; 25 were from South Africa, Australia, or New Zealand; and 23 were from Asia.

Among 25 influenza A(H1N1) viruses collected during May-August, six (24%) were similar to A/Beijing/262/95, the H1N1 component of the 1999-2000 influenza vaccine, and 19 (76%) were antigenically related to A/Bayern/07/95. Of the A/Beijing/262/95-like viruses, five were from Australia, New Caledonia, and New Zealand, and one was from Asia. All the A/Bayern/07/95-like viruses were from Central and South America. Although A/Beijing/262/95 and A/Bayern/07/95-like viruses are antigenically distinguishable, persons vaccinated with A/Beijing/262/95 develop equivalent antibody levels against A/Bayern/05/97 and A/Beijing/262/95 (5).

Reported by: 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 for Medical Research, London, England. I Gust, MD, A Hampson, WHO Collaborating Center for Reference and Research on Influenza, Parkville, Australia. K Nerome, WHO Collaborating Center for Reference and Research on Influenza, National Institute of Infectious Diseases, Tokyo, Japan. WHO Collaborating Center for Reference and Research on Influenza, Influenza Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note:

Influenza A(H3N2) viruses continued to predominate worldwide during May-September 1999. In the United States, summer influenza activity included an outbreak of influenza A(H3N2) virus infections among tourists to Alaska and the Yukon Territory and scattered outbreaks and sporadic cases in the 48 contiguous states and Hawaii similar to those seen during the summer of 1998 (6). During the past 2 years, 12 summer influenza outbreaks were reported to CDC. Whether these outbreaks represent increased levels of summer influenza activity or improved detection and reporting is unknown.

Annual influenza vaccination is recommended for persons aged greater than or equal to 65 years, persons residing in nursing homes or long-term-care facilities, anyone aged 6 months-64 years with certain chronic medical conditions such as heart or lung disease (including asthma); diabetes; renal insufficiency; hemoglobinopathies; immunocompromising illnesses or conditions requiring the use of immunosuppressive medications; and children and adolescents aged 6 months-18 years receiving long-term aspirin therapy who may be at risk for developing Reye syndrome after influenza. Health-care providers, family members, and others in close contact with high-risk persons should be vaccinated to diminish virus transmission. Serious complications from influenza include pneumonia and worsening of underlying medical conditions and have resulted in an average of approximately 110,000 hospitalizations and 20,000 deaths annually in the United States (7).

Pregnant women with high-risk medical conditions should be vaccinated before the start of the influenza season regardless of their stage of pregnancy. Pregnant women without high-risk medical conditions, but who will be in their second or third trimester during the influenza season, are at elevated risk of complications and should be vaccinated. Some experts prefer to vaccinate these women during the second trimester to avoid a coincidental association with spontaneous abortion, which is common in the first trimester, and because exposures to vaccines traditionally have been avoided during the first trimester (7).

In the United States, the optimal time for organized influenza vaccination campaigns is October through mid-November; however, after mid-November, health-care providers should continue to offer influenza vaccine to high-risk unvaccinated persons throughout the influenza season even after influenza activity has begun in the community. The timing of influenza activity varies from year to year, and local influenza surveillance reports can be useful for determining when influenza viruses are in local circulation.

Although vaccination against influenza is the most effective method of reducing the impact of influenza, antiviral agents provide a useful adjunct. Amantadine and rimantadine are approved for the prophylaxis or treatment of influenza type A but neither is effective against influenza type B viruses (7). Zanamivir, an orally inhaled neuraminidase inhibitor drug, was approved by the Food and Drug Administration in July 1999 to treat uncomplicated influenza A and B infections.

Information about influenza surveillance and vaccination is available through the toll-free CDC Voice Information System, telephone (888) 232-3228, fax (888) 232-3299 (document no. 361100), or through CDC's World-Wide Web site, From October through May, information is updated weekly.


  1. CDC. Update: influenza activity--United States and worldwide, 1998-99 season, and composition of the 1999-2000 influenza vaccine. MMWR 1999;48:374-8.
  2. CDC. Outbreak of influenza A infection among travelers--Alaska and the Yukon Territory, May-June 1999. MMWR 1999;48:545-6,555.
  3. CDC. Outbreak of influenza A infection--Alaska and the Yukon Territory, June-July 1998. MMWR 1998;47:638.
  4. CDC. Update: outbreak of influenza A infection--Alaska and the Yukon Territory, July-August 1998. MMWR 1998;47:685-8.
  5. World Health Organization. Recommended composition of influenza virus vaccines for use in the 1998-1999 season. Wkly Epidemiol Rec 1998;73:56-61.
  6. CDC. Update: influenza activity--worldwide, April-September 1998. MMWR 1998;47:830-3.
  7. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1999;48(no. RR-4).

* When presented separately, numbers for other racial/ethnic groups were too small for meaningful analysis.

** References to sites of non-CDC organizations on the World-Wide Web are provided as a service to MMWR readers and do not constitute or imply endorsement of these organizations or their programs by CDC or the U.S. Department of Health and Human Services. CDC is not responsible for the content of pages found at these sites.

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