In collaboration with the World Health Organization (WHO), the WHO international network of approximately 110 collaborating laboratories in 83 countries, and U.S. state and local health departments, CDC conducts surveillance to monitor influenza activity and to detect antigenic changes in the circulating strains of influenza viruses. During October 1997-April 1998. influenza activity was moderate to severe in the Northern Hemisphere (1). Influenza A(H3N2) viruses were predominant, but influenza A(H1N1) viruses were associated with outbreaks, and influenza B viruses were identified sporadically in all regions. Since April 1998, increased influenza outbreak and epidemic level activity, primarily associated with influenza A(H3N2), has been reported in the Southern Hemisphere. This report summarizes worldwide influenza activity during April-September 1998 and the antigenic characteristics of influenza isolates collected during April-September.

Africa. Influenza activity in South Africa primarily was associated with influenza A(H3N2) viruses. Activity began earlier, in March, and was more extensive than usual. Influenza A(H3N2) activity peaked from mid-May through the first week of June, but viruses continued to be isolated through July. Influenza A(H1N1) and influenza B viruses were detected sporadically during June-July. In Mauritius, outbreaks of influenza A(H3N2) were detected from the end of April through May; viruses were isolated sporadically through June. Influenza A(H3N2) viruses were isolated in Senegal during May-July, and in Reunion during July.

Asia. Influenza A(H3N2) viruses predominated in Asia, but influenza A(H1N1) and influenza B viruses also were isolated. In China, outbreaks of influenza A(H3N2) viruses were reported each month from April through June. Influenza B viruses were isolated less frequently, but were detected each month from April through June with outbreaks reported in April; influenza A(H1N1) viruses were isolated during April. In Hong Kong, influenza A(H3N2) viruses were predominant. The number of virus isolations peaked during March with a second smaller peak during July. Influenza B viruses were isolated every month from April through August with the numbers increasing during June-July. Influenza A(H1N1) virus isolations declined after the beginning of 1998, but H1N1 viruses were detected sporadically during April and June. In Japan, influenza A(H3N2) viruses predominated during the winter and continued to be isolated through April; influenza B viruses were detected each month from April to July. In Sri Lanka, outbreaks of influenza A(H3N2) began during the last week of April and continued through May. In Myanmar, an influenza A outbreak was detected in late June; four of five isolates were influenza A(H3N2) and one was influenza A(H1N1). In Thailand, influenza A viruses were detected during June-July; most subtyped viruses were A(H3N2), but a few influenza A(H1N1) viruses were isolated. In Guam, influenza A(H3N2) viruses were isolated during April-June, and in Nepal during April-May. In Malaysia, influenza A(H3N2) and A(H1N1) viruses were isolated during June. Influenza B viruses were isolated in Turkey during May.

Europe. During the 1997-98 season, influenza activity was lower in Europe than last season and began later than in North America. Activity levels peaked in many European countries during March, but isolation of influenza viruses was common through April. Influenza A(H3N2) viruses were most frequently isolated, but influenza A(H1N1) viruses predominated in Belarus, Latvia, and Portugal. In Finland, Sweden, Switzerland, and the United Kingdom, influenza A(H3N2) viruses continued to be isolated during May and during June in France. Influenza B viruses were isolated in Finland and France during May, in Spain during June, and in Finland during September.

North America. In the United States, influenza A(H3N2) viruses predominated during the winter and continued to be isolated every month through September. Influenza B viruses were isolated through June and during August. No influenza A(H1N1) viruses have been isolated in the United States since March. Summer outbreaks associated with influenza A(H3N2) viruses were reported in Montana, Florida, Tennessee (2), and Alaska and the Yukon Territory, Canada (3,4). During September, an outbreak of influenza A(H3N2) in a nursing home was reported in California. In addition to the influenza activity in the Yukon Territory, Canada reported influenza A and influenza B isolates through May and April, respectively, and influenza A again in September.

Oceania. Influenza activity peaked in southern Australia during July and in Sydney and Brisbane during August and September, respectively. Influenza A(H3N2) viruses predominated, but influenza B viruses were detected sporadically; no influenza A(H1N1) viruses were isolated. In New Zealand, rates of influenza-like illness were lower than in recent years. Influenza A viruses were isolated during June-August; influenza A(H1N1) viruses were isolated more frequently, but influenza A(H3N2) viruses also were detected. A small number of influenza B viruses were detected in New Zealand during May. In New Caledonia, outbreaks of influenza A(H3N2) viruses were reported from the beginning of April through the first week of July.

South America. Influenza A(H3N2) viruses predominated in South America during 1998. In Argentina, influenza A(H3N2) viruses were associated with outbreaks during March and each month from May through August. In Brazil, influenza A(H3N2) viruses predominated and were isolated from March through August. Influenza type B viruses were isolated less frequently but were associated with outbreaks in S o Paulo during April-June; a single influenza A(H1N1) virus was identified in June. In Chile, influenza A(H3N2) viruses first were detected during March. Influenza activity peaked during the first half of May, and lower activity levels were reported through August. All influenza viruses identified from Chile were influenza A(H3N2) with the exception of one influenza A(H1N1) virus. In Uruguay, influenza activity peaked during June-July and was associated with influenza A(H3N2) viruses. Influenza A(H3N2) viruses were identified in Peru during April, in French Guyana during May, and in Venezuela during June. Influenza A(H1N1) viruses were detected in French Guyana during May. Outbreaks of influenza A were reported from Colombia in September.

Characterization of influenza virus isolates. The WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza at CDC analyzes isolates received worldwide. This report describes isolates collected during April-September, includ-ing those from the end of the influenza season in the Northern Hemisphere and from the epidemic season in the Southern Hemisphere. Of the 40 influenza B isolates that were characterized antigenically or genetically, 35 were similar to B/Harbin/07/94, the B/Beijing/184/93-like virus contained in the 1998-99 influenza vaccine. Of the 35 B/Harbin/07/94-like viruses, 19 were collected in the United States and Europe, four were from South America, two were from New Zealand, and 10 were from Asia. The remaining five influenza B isolates were B/Victoria/02/87-like viruses collected in Asia. These viruses have not been identified outside of Asia since 1991.

Among 10 influenza A(H1N1) viruses collected during April-September, seven H1N1 viruses from Asia were similar to A/Beijing/262/95, the H1N1 component of the 1998-99 influenza vaccine. Three influenza A(H1N1) viruses from Brazil and New Zealand were antigenically related to A/Bayern/07/95.

Of 312 influenza A(H3N2) viruses tested, 305 (98%) were similar to A/Sydney/05/97, the H3N2 component of the 1998-99 influenza vaccine, and seven were similar to older H3N2 strains. Of the H3N2 isolates, 45 (14%) were from North America, four (1%) were from Europe, 88 (28%) were from Asia, 146 (47%) were from Central and South America, and 29 (9%) were from South Africa, Australia, or New Zealand.

Reported by: World Health Organization National Influenza Centers, Emerging and Other Communicable Diseases Div, World Health Organization, Geneva, Switzerland. World Health Organization 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: Worldwide surveillance for influenza viruses provides the basis for selecting influenza vaccine strains. Vaccine strains are chosen approximately 7 months before the start of the following influenza vaccination season. In the United States, all of the influenza isolates identified during the past summer have been similar to the strains contained in the 1998-99 influenza vaccine. The influenza vaccine for the 1998-99 influenza season contains A/Beijing/262/95-like (H1N1), A/Sydney/05/97-like (H3N2), and B/Beijing/184/93-like antigens. U.S. vaccine manufacturers will use the antigenically equivalent strain B/Harbin/07/94 for the B/Beijing/184/93-like antigen, because of its growth properties (1).

Annual vaccination against influenza is recommended by the Advisory Committee on Immunization Practices for persons aged greater than or equal to 65 years; persons who reside in nursing homes or chronic-care facilities; persons with chronic cardiovascular or pulmonary disorders, including children with asthma; persons who required medical follow-up or hospitalization during the previous year because of diabetes or other chronic metabolic diseases, renal dysfunction, hemoglobinopathies, or immunosuppression; children and teenagers (aged 6 months-18 years) receiving long-term aspirin therapy (who may therefore be at risk for developing Reye syndrome after influenza); and women who will be in the second or third trimester of pregnancy during the influenza season. Vaccination also is recommended for health-care workers and other persons, including household members, in frequent contact with persons at high risk for influenza-related complications. Influenza vaccine also can be administered to other persons who want to reduce their likelihood of acquiring influenza and for whom vaccination is not contraindicated (5).

In the United States, the optimal time for organized influenza vaccination campaigns is October through mid-November. After mid-November, health-care providers should continue to offer influenza vaccine to unvaccinated high-risk persons even after influenza activity has begun in the community. Influenza surveillance reports from local health departments can be useful for determining the period during which continuing influenza vaccination is beneficial.

Influenza vaccine production during the 1997-98 influenza season was approximately 80 million doses (Food and Drug Administration, unpublished data, 1998), and vaccine production for the 1998-99 influenza season is expected to match or exceed that amount. Although vaccination against influenza is the most effective means of reducing the impact of influenza, antiviral agents provide a useful adjunct (5). Amantadine and rimantadine are available for the prophylaxis or treatment of influenza type A infection, but neither is effective against influenza type B viruses.

Information about influenza surveillance is available through the toll-free CDC Voice Information System, telephone (888) 232-3238, fax (888) 232-3299 (document no. 361100), or CDC's World-Wide Web site http://www.cdc.gov/ncidod/diseases/flu/weekly.htm. From October through May, the information is updated weekly.

References

1. CDC. Update: influenza activity -- United States and worldwide, 1997-98 season, and composition of the 1998-99 influenza vaccine. MMWR 1998;47: 280-4.

2. CDC. Influenza -- Florida and Tennessee, July-August 1998, and virologic surveillance for influenza, May-August 1998. MMWR 1998;47: 756-9.

3. CDC. Update: outbreak of influenza A infection -- Alaska and the Yukon Territory, July-August 1998. MMWR 1998;47:685-8.

4. CDC. Outbreak of influenza A infection -- Alaska and the Yukon Territory, June-July 1998. MMWR 1998;47:638.

5. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on immunization Practices (ACIP). MMWR 1998;47(no. RR-6).

   
   

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