Update: Influenza Activity -- Worldwide, March-August 1997

During the October 1996-March 1997 influenza epidemic season, influenza activity was moderate to severe in the Northern Hemisphere. In Europe, Japan, and North America, influenza A(H3N2) viruses predominated, but influenza B viruses were more commonly isolated by the end of the season (1). In contrast, in some countries in the World Health Organization (WHO) region of Asia (e.g., China, Iran, and Israel), influenza B viruses were isolated more frequently than influenza A(H3N2) viruses. Influenza A(H1N1) viruses were isolated infrequently worldwide, except in Europe, where 14 countries reported sporadic * isolations, and a late-season outbreak in April affected children in the Czech Republic. Since March 1997, influenza activity has increased in the Southern Hemisphere, and outbreaks and epidemic level activity have been associated with both influenza A(H3N2) and influenza B viruses. This report summarizes worldwide influenza activity during March-August 1997, as reported through WHO, the WHO international network of collaborating laboratories, and U.S. state and local health departments, and characterizes influenza isolates collected during March-July.

Africa. In Madagascar, increased activity during May was associated with influenza B viruses. Senegal reported isolation of several influenza A(H3N2) viruses during March and April. Most isolations reported from sporadic cases in South Africa during June-August were influenza A(H3N2), but influenza A(H1N1) and influenza B viruses also were detected.

Asia. In Hong Kong, during April-July, influenza A(H3N2) viruses were more frequently isolated than influenza B viruses, which had predominated during January-March. Several outbreaks associated with influenza A(H3N2) occurred in nursing homes for the elderly during May-July. In addition, Hong Kong reported sporadic isolation of influenza A(H1N1) virus during June-August. Influenza A(H5N1), a strain of influenza virus that usually infects only birds, was isolated from a 3-year-old child in Hong Kong who died in May of multiple complications including Reye syndrome during an acute respiratory illness. In Japan, at the end of a season predominated by influenza A(H3N2) viruses, the number of influenza B isolates increased and peaked during March. Influenza B viruses continue to predominate in China, although isolation of influenza A(H3N2) was reported during April, and influenza A(H1N1) was reported during April and May. Since March, influenza B viruses have been isolated from sporadic cases in Guam, Israel, Korea, Nepal, Taiwan, and Thailand, and influenza A(H3N2) viruses in Israel, Nepal, and Thailand.

Europe. From the end of March through May, most postseason sporadic isolates in Europe were influenza B. Influenza B viruses were isolated in Croatia, Czech Republic, France, Germany, Netherlands, Norway, Poland, and Switzerland. During April, the Czech Republic reported outbreaks of influenza A(H1N1) virus among schoolchildren. Other countries reporting isolation of influenza A(H1N1) from sporadic cases include Germany, Norway, and the United Kingdom. Germany also reported postseason isolation of influenza A(H3N2) viruses.

North America. Influenza A(H3N2) viruses were isolated from sporadic cases in the United States during March and April. In addition, two nursing home outbreaks associated with influenza A(H3N2) were reported: one in Delaware during March and one in California during June. As in Europe, influenza B viruses were isolated more frequently than influenza A(H3N2) viruses after mid-February. CDC received isolates of influenza B virus from sporadic cases each month from March to June. In Canada, influenza type A continued to be isolated through April and again in August; influenza B virus was isolated through March. Mexico reported isolates of influenza B virus in March.

Oceania. Overall, influenza activity during the 1996-97 season in Oceania was predominantly associated with influenza A(H3N2). In Australia, influenza-like illness as reported by sentinel medical practices increased during late June with notable activity in Melbourne and Sydney. Initially, preseason isolates and outbreaks in June among school-aged children were associated with influenza B viruses. By July, influenza A(H3N2) viruses had become predominant. In New Zealand and Niue, local outbreaks occurring during May-July were caused by influenza B. Since July, isolation of influenza A(H3N2) viruses has increased in New Zealand. In Oceania, only New Zealand has reported influenza A(H1N1) (one isolate).

South America. Since March, influenza activity has increased in South America. During May-July, Brazil reported outbreaks associated with both influenza A(H3N2) and influenza B viruses. In Chile, influenza B was the predominant virus type isolated, and isolation peaked in June; however, influenza A viruses also were isolated during June. Argentina reported isolation of influenza B viruses from sporadic cases, and French Guiana reported isolation of influenza B and influenza A(H3N2) viruses from sporadic cases.

Characterization of influenza virus isolates. The World Health Organization Collaborating Center for Surveillance, Epidemiology, and Control of Influenza at CDC analyzes isolates received worldwide. Isolates collected during March-July are described here and include those from the end of the influenza season in the Northern Hemisphere and during the epidemic season in the Southern Hemisphere. All 48 influenza A(H3N2) isolates were antigenically similar to A/Wuhan/359/95, the A(H3N2) component of the 1997-98 influenza vaccine. Of the 48 influenza A(H3N2) isolates, 14 (29%) were collected from the United States and Canada as season-end isolates. The number of characterized influenza A(H3N2) isolates from recent epidemic activity in the Southern Hemisphere included five (10%) from South America and 15 (31%) from New Zealand and Australia. Fourteen (29%) isolates were from current activity in Asia.

During March-August, a total of 128 influenza B isolates were collected and analyzed. Of these, 45 (33%) were from North America, and 18 (14%) were from Australia, New Zealand, and parts of South America, where influenza B viruses were isolated early during the Southern Hemisphere epidemic season. All of the isolates from North America, Australia, New Zealand, and South America are antigenically related to B/Beijing/184/93, the influenza B component of the 1997-98 influenza vaccine. In Asia, of the 65 influenza B viruses associated with sporadic and outbreak activity since March, 21 (32%) were characterized as A/Beijing/184/93-like, and 44 (68%) were B/Victoria/02/87-like. Although the proportion of B/Victoria/02/87-like viruses has increased, these viruses have not been identified outside of Asia since 1991.

No recent influenza A(H1N1) viruses related to either A/Texas/36/91 or A/Bayern/07/95, the H1N1 component of the 1997-98 influenza vaccine, have been analyzed. Only six influenza A(H1N1) viruses from China and Hong Kong were characterized and were antigenically similar to A/Wuhan/371/95, an antigenically distinct group of viruses identified in only China, Hong Kong, and Singapore (1).

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: In the United States, influenza A(H3N2) viruses were the most frequently isolated influenza viruses during the 1996-97 influenza season; however, from mid-February through May, influenza B viruses were more frequently isolated than influenza A viruses. Both influenza A(H3N2) and influenza B viruses circulated from March through August in the Southern Hemisphere during a moderate influenza season. Although the type of influenza virus and timing and extent of circulation cannot be predicted with certainty for any given influenza season, data from previous seasons suggest that influenza B viruses may circulate more widely in the United States during the 1997-98 influenza season than during the 1996-97 influenza season, and that influenza A viruses will continue to circulate.

The influenza vaccine is updated annually to include viruses antigenically similar to the strains of the three distinct groups of influenza viruses that are expected to predominate worldwide circulation. The influenza vaccine for the 1997-98 influenza season contains A/Bayern/07/95-like(H1N1), A/Wuhan/359/95-like(H3N2), and B/Beijing/184/93-like antigens. U.S. vaccine manufacturers will use the antigenically equivalent strains A/Johannesburg/82/96(H1N1), A/Nanchang/933/95(H3N2), and B/Harbin/07/94 because of their growth properties and suitability for vaccine production.

The Advisory Committee on Immunization Practices recommends targeting influenza vaccination programs toward persons at increased risk for influenza-associated complications that can result in hospitalization or death. Groups at increased risk include 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 and 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. Because persons who are clinically or subclinically infected can transmit influenza virus to high-risk persons, 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 the likelihood of acquiring influenza and for whom vaccination is not contraindicated (2).

Use of influenza vaccine in the United States has increased substantially in recent years and annual vaccine production has kept pace with this demand. Based on data from CDC's National Health Interview Survey, the proportion of persons aged greater than or equal to 65 years who reported having received influenza vaccine during the previous year increased from 32.9% in 1989 to 55.3% in 1994 (3,4). Data from the Behavioral Risk Factor Surveillance System (BRFSS), a state-based random-digit-dialed telephone survey of the civilian, noninstitutionalized adult population, indicate that the median estimated percentage of persons aged greater than or equal to 65 years who reported having received influenza vaccine during the previous year increased from 49.9% in 1993 to 59.2% in 1995 (4,5). From 1989 to 1995, annual vaccine production increased from approximately 28 million doses to approximately 73 million doses (Food and Drug Administration, unpublished data, 1997). Projected influenza vaccine supply for the 1997-98 influenza season is expected to meet projected increased demand.

In the United States, the optimal time for organized influenza vaccination campaigns is October through mid-November. However, beginning in September, health-care providers should offer influenza vaccine to persons at high risk for influenza who are assessed for routine care or are hospitalized. After mid-November, health-care providers should continue to offer influenza vaccine to high-risk persons until and even after influenza activity has begun in the community. Because timing of influenza activity varies from year to year and among regions and local communities, local influenza surveillance reports can be useful to health-care providers in determining the period through which continuing influenza vaccination is beneficial.

Information about influenza surveillance is available through the toll-free CDC Voice Information Service (influenza update, recorded message) by telephone (888) 232-3228) or fax (888) 232-3299 (document no. 361100) or through 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, 1996-97 season, and composition of the 1997-98 influenza vaccine. MMWR 1997;46:325-30.

2. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1997;46(no. RR-9).

3. CDC. Influenza and pneumococcal vaccination coverage levels among persons aged greater than or equal to 65 years -- United States, 1973-1993. MMWR 1995;44:506-7,513-5.

4. CDC. Pneumococcal and influenza vaccination levels among adults aged greater than or equal to 65 years -- United States, 1993. MMWR 1996;45:853-9.

5. Powell-Griner E, Anderson JE, Murphy W, Coordinators for the Behavioral Risk Factor Surveillance System. State- and sex-specific prevalence of selected characteristics -- Behavioral Risk Factor Surveillance System, 1994 and 1995. In: CDC surveillance summaries (August 1). MMWR 1997;46(no. SS-3):23,25.

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

+------------------------------------------------------------------- -----+ | Erratum: Vol. 46, No. 35 | |             | | In the article "Update: Influenza Activity -- Worldwide, | | March-August 1997," on page 817, the sentence beginning on line 7 | | should read: "In Asia, of the 65 influenza B viruses associated with | | sporadic and outbreak activity since March, 21 (32%) were | | characterized as B/Beijing/184/93-like, and 44 (68%) were | | B/Victoria/02/87-like." | +------------------------------------------------------------------- -----+

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