Update: Influenza Activity -- Worldwide, 1996

From October 1995 through August 1996, influenza activity occurred at moderate to severe levels worldwide. Epidemic activity in Europe, Asia, and North America was associated with influenza A(H1N1) and influenza A(H3N2) viruses. Influenza A(H1N1) viruses caused an epidemic in Japan and predominated in Canada, most regions of the United States, and a few countries in Europe. Influenza A(H3N2) viruses predominated in most European countries, China, and some regions of the United States. Influenza B viruses were isolated in association with sporadic cases throughout most of the world (1,2). This report summarizes influenza activity worldwide during March-August 1996, indicating that, during these months, influenza activity occurred at peak levels in the Southern Hemisphere.

Africa. During June, the first localized outbreak of influenza-like illness (ILI) associated with influenza A(H1N1) infections occurred among adult workers in Durban, South Africa. Influenza A(H3N2) viruses were isolated in Senegal in June and in Madagascar and South Africa in June and July.

Asia. During March-August, influenza A(H3N2) viruses predominated in Asia, but influenza A(H1N1) and influenza B viruses also were identified. In Korea during March and April, influenza A(H3N2) and influenza B viruses were isolated. From March through June, influenza A(H3N2), influenza A(H1N1), and influenza B viruses circulated in China. In southern China, outbreaks associated with influenza A(H3N2) viruses occurred in Guangzhou Province during March, April, and May; Guangxi Province during April and May; Hainan Province during May; and Fujian Province during May and June. In Hong Kong, the number of influenza viruses isolated peaked during March and April, then again during July and August; influenza A(H1N1) and influenza B viruses were isolated sporadically, but influenza A(H3N2) viruses were associated with outbreaks. From May through July, influenza A(H3N2) and influenza B viruses were isolated sporadically in Taiwan and Guam.

Europe. In March, several countries reported sporadic isolation of influenza A(H1N1) (Croatia, Germany, and the United Kingdom), influenza A(H3N2) (Germany and Iceland), and influenza B viruses (Austria, Czech Republic, France, Germany, Greece, Sweden, and Switzerland). Sporadic isolation of all three influenza viruses also was reported in Europe during the summer.

North America. Influenza activity in the United States increased during November and December 1995, peaked during mid-December through early January 1996, and declined thereafter with widespread * and regional activity last reported for the weeks ending March 2 (week 9) and April 20 (week 16), respectively. CDC received influenza B isolates collected during every month from March through July for antigenic characterization. During June and July, these viruses were associated with sporadic cases in Alaska, Hawaii, Ohio, Pennsylvania, and Texas. Influenza A(H3N2) viruses were collected during every month from March through August. These included influenza A(H3N2) isolates from a nursing home outbreak in Washington in which 31 (40%) of 77 residents and 29 (19%) of 150 staff members became ill during May 31-June 24; two residents died. Influenza A(H3N2) viruses also were isolated from residents and staff in association with an outbreak of ILI in a nursing home in Hawaii; 70 (38%) of 183 residents and 36 (29%) of 125 staff became ill during July 17-30. In addition, from July 1 through August 22, a total of 14 (19%) of 74 respiratory specimens collected from military personnel and their family members at Tripler Army Medical Center in Hawaii were positive for influenza type A. All viruses subtyped were influenza A(H3N2). Sporadic influenza A(H3N2) isolates were identified in Alaska during July and in Wisconsin during August and September.

In Canada, influenza A and influenza B viruses were isolated from sporadic cases throughout May and June. Only one isolate of influenza B virus was reported in July. In August, Ontario reported isolation of influenza A(H1N1) and influenza A(H3N2) viruses.

Central and South America. Chile reported isolation of influenza A viruses in the northern, central, and southern regions during June. During May and June, outbreaks of ILI associated with influenza A(H1N1) and influenza A(H3N2) occurred in Santiago and Valparaiso. Brazil reported outbreaks of influenza A(H3N2) viruses during April and June and influenza A(H1N1) viruses during May and June.

Oceania. Epidemic level activity was associated with influenza A(H3N2) viruses while influenza A(H1N1) and influenza B viruses circulated at low levels. In Australia, influenza A(H3N2) virus activity increased sharply in June and peaked in July at a level substantially higher than reported in 1995. In August, influenza activity declined in most regions of Australia, but Queensland reported increased activity and the Northern Territory reported severe outbreaks. In New Zealand, seasonal activity began in May with school outbreaks of influenza type A(H3N2). The number of influenza isolates and consultation rates for ILI increased rapidly and peaked in June and early July. The 1996 influenza type A(H3N2) epidemic reported from New Zealand was the largest since the revision of their surveillance program in 1990 (3).

Characterization of influenza virus isolates. Influenza A(H1N1) viruses predominated in most parts of the United States during the 1995-96 influenza season, but influenza A(H3N2) and influenza B viruses accounted for 35% and 15%, respectively, of isolates reported by the World Health Organization Collaborating Laboratories from October 1, 1995, through May 18, 1996. From October 1, 1995, through September 6, 1996, a total of 1016 influenza isolates collected worldwide were antigenically characterized by the World Health Organization Collaborating Center for Surveillance, Epidemiology, and Control of Influenza at CDC. Of these, 566 (56%) were from North America; 130 (13%), from Europe; 263 (26%), from Asia; and 57 (6%), from South America and Oceania. Of the viruses subtyped, 457 (45%) were influenza A(H3N2), 348 (34%) were influenza A(H1N1), and 211 (21%) were influenza B. Of the 457 influenza A(H3N2) isolates characterized, 306 (67%) were antigenically related to A/Johannesburg/33/94, the 1995-96 vaccine strain, and 151 (33%) were more closely related to A/Wuhan/359/95, the A(H3N2) component of the 1996-97 influenza vaccine. The proportion of A/Wuhan/359/95(H3N2)-like viruses have increased since January 1996. Of the 211 influenza B viruses, 207 (98%) were similar to B/Beijing/184/93, the current vaccine strain. Four (2%) of the influenza B viruses were antigenically related to B/Victoria/02/87. B/Victoria/02/87-like viruses circulated in 1988-89 and since then have been isolated sporadically only in China and Hong Kong. Of the 348 influenza A(H1N1) viruses, 318 (91%) were A/Texas/36/91-like or related to the antigenically similar A/Taiwan/01/86-like viruses, and 30 (9%) were antigenically similar to a recently identified variant that has been isolated only in China and Hong Kong. The influenza A(H1N1) component of the 1996-97 vaccine is A/Texas/36/91.

Reported by: J Sasaki, MPH, Epidemiology Br, Hawaii State Dept of Health; P Toyama, MS, Dept of Pathology, Tripler Army Medical Center, Honolulu, Hawaii. J Russell, N Furness, C Spitters, MD, Snohomish Health District, Everett, Washington. World Health Organization National Influenza Centers, Emerging and Other Communicable Diseases Div, World Health Organization, Geneva. 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: In the United States, sporadic cases of influenza are common during the summer, but outbreaks of influenza, such as those observed in Washington and Hawaii during June and July, are not common. Although specific patterns of influenza activity and the time and extent of virus circulation cannot be predicted with certainty, the recent worldwide pattern of influenza suggests that all three influenza virus strains -- type A(H3N2), type A(H1N1), and type B -- will circulate during the 1996-97 influenza season in the United States.

The influenza vaccine is updated annually to include viruses antigenically similar to the strains of the three distinct groups of influenza viruses that are in worldwide circulation. The influenza vaccine for the 1996-97 influenza season contains A/Texas/36/91-like (H1N1), A/Wuhan/359/95-like (H3N2), and B/Beijing/184/93-like antigens (2,4). For the A/Wuhan/359/95-like and B/Beijing/184/93-like antigens, U.S. manufacturers will use the antigenically equivalent strains A/Nanchang/933/95(H3N2) and B/Harbin/07/94 viruses, respectively, because of their growth properties. Since March 1996, most influenza viruses isolated worldwide have been antigenically similar to the vaccine strains.

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; and children and teenagers (aged 6 months-18 years) receiving long-term aspirin therapy and who therefore may be at risk for developing Reye syndrome after influenza. 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. Pregnant women who will be in the third trimester during the influenza season may be at increased risk for medical complications following influenza infection and should consult with their health-care providers about receiving the vaccine. Influenza vaccine also can be administered to persons who want to reduce the likelihood of acquiring influenza (4).

The optimal time for organized influenza vaccination campaigns is October through mid-November, but beginning in September, health-care providers should offer influenza vaccine to persons at high risk who are seen for routine care or as a result of hospitalization. Health-care providers should continue to offer influenza vaccine to high-risk persons until and even after influenza activity has been documented in the community.

Although vaccination against influenza is the most effective means of reducing the impact of influenza, antiviral agents provide a useful adjunct. Antiviral agents available for the prophylaxis or treatment of influenza type A infection are amantadine hydrochloride and rimantadine hydrochloride. Neither drug is effective against influenza type B viruses. Use of antivirals may be considered 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 who are 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 rapid diagnostic testing for influenza type A and close monitoring of local influenza surveillance reports may assist health-care providers in making treatment decisions for patients with ILI.

Information about influenza surveillance is available through the CDC Voice Information System (influenza update) by telephone ({404} 332-4555) or fax ({404} 332-4565) (document no. 361100) or through the CDC Information Service on the Public Health Network electronic bulletin board. From October through May, the information is updated weekly. Periodic updates about influenza are published in MMWR and information about local influenza activity is available through county and state health departments.

References

1. CDC. Update: influenza activity -- United States and worldwide, 1995-96 season, and composition of the 1996-97 influenza vaccine. MMWR 1996;45: 326-9.

2. World Health Organization. Recommended composition of influenza virus vaccines for use in the 1996-97 flu season. Wkly Epidemiol Rec 1996;71: 57-61.

3. Anonymous. Influenza update: New Zealand. Communicable Diseases Intelligence 1996;20:341.

4. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1996;45(no. RR-5).

Levels of activity are 1) no activity; 2) sporadic sporadically occurring ILI or culture-confirmed influenza, with no outbreaks detected; 3) regional outbreaks of ILI or culture-confirmed influenza in counties with a combined population of <50% of the state's total population; and 4) widespread outbreaks of ILI or culture-confirmed influenza in counties with a combined population of greater than or equal to 50% of the state's total population.

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