During October 1999--May 2000, influenza A(H3N2), A(H1N1), and B viruses were identified in the Northern Hemisphere. Influenza A(H3N2) predominated, but the number of influenza A(H1N1) viruses increased toward the end of the influenza season in the Northern Hemisphere. Since April, influenza A viruses have predominated in the Southern Hemisphere and tropical regions, but influenza B viruses also have been identified. This report summarizes influenza activity in the United States and worldwide from April 2000 through October 2000.

United States

The WHO Collaborating Center for Reference and Research at CDC conducts active national surveillance for influenza from October through May (1). Although formal weekly reporting is discontinued during summer months, WHO collaborating laboratories can report influenza viruses during the summer to CDC and submit these viruses for antigenic characterization. Since March, influenza A(H1N1) viruses have been the most frequently isolated influenza viruses in the United States. Influenza A(H1N1) viruses were identified each month from April through July and were isolated from an outbreak in July among children and staff at a summer camp in Texas. Influenza A(H1N1) viruses were identified during October in California, Florida, and Texas. Influenza A(H3N2) vi ruses were isolated from sporadic cases during April, from one immunocompromised patient in June, from one imported case in an immune suppressed person in August in Massachusetts, and from three cases in October (one each in California, Hawaii, and Kentucky). Additional influenza A viruses (unsubtyped) were identified in California and Texas during September and in Utah in October. Influenza B viruses were identified each month through May. During August--October, influenza B viruses were identified in Alaska, California, Nevada, Oklahoma, and Washington.

Worldwide

From April through October, influenza A(H1N1), A(H3N2), and B viruses were reported from Asia; influenza A viruses were reported more frequently than influenza B viruses. In Africa, influenza A(H1N1) viruses were reported more frequently than A(H3N2) viruses from April through August, but all subtyped influenza A viruses reported during September were A(H3N2). In Canada, both influenza A and B viruses were reported each month from April through July; most of the viruses reported during June--July were influenza type B. During September--October, influenza A and B viruses were reported in Canada, and influenza A viruses were reported from Mexico. Influenza type A and B viruses also were isolated in Europe during September--October. In South America, influenza A(H1N1) viruses predominated, but influenza A(H3N2) and B viruses were isolated. In Oceania, influenza type A viruses were more commonly isolated than influenza type B; both A(H3N2) and A(H1N1) subtypes circulated.

Characterization of influenza virus isolates

The WHO Collaborating Center for Reference and Research on Influenza at CDC analyzes isolates received from laboratories worldwide. Of the 205 influenza A(H1N1) isolates that were collected and antigenically characterized during April--October, 173 (84%) were similar to A/New Caledonia/20/99, the H1N1 component of the 2000--01 influenza vaccine, 31 (15%) were similar to A/Bayern/07/95, and one (0.5%) showed reduced titers with A/New Caledonia/20/99 antisera. Although A/Bayern-like viruses are antigenically distinct from the A/New Caledonia-like viruses, the A/New Caledonia/20/99 vaccine strain produces high titers of antibody that cross-react with A/Bayern/07/95-like viruses. Of the 205 antigenically characterized H1N1 viruses, 136 were from South or Central America, 42 from the United States, 18 from Asia, seven from Australia, New Zealand, and New Caledonia, and two from Africa.

Of the 65 influenza A(H3N2) viruses antigenically characterized, 60 (92%) were well inhibited by antiserum to the recommended vaccine strain, A/Moscow/10/99. Thirty-four of the antigenically characterized H3N2 viruses were from South America, 17 from Asia, five from Australia, New Zealand, and New Caledonia, four from the United States, two each from Canada and Africa, and one from Europe.

Of the 53 antigenically characterized influenza B viruses, 52 (98%) were antigenically similar to the recommended vaccine strain, B/Beijing/184/93. Seventeen of the influenza B viruses were from Asia, 15 from the United States, 10 from South America, nine from Australia, New Zealand, and New Caledonia, and one each from Africa and Europe.

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. M Tashiro, MD, WHO Collaborating Center for Reference and Research on Influenza, National Institute of Infectious Diseases, Tokyo, Japan. S Lea, MD, C Burgoon, DVM, Waco-McLennan County Health Dept, Waco; M Gaglani, MD, G Herschler, Scott & White Hospital, Temple; D Haught, MSN, Baylor College of Medicine, Austin; L Dobin, D Berkman, Greene Family Camp, McLennan County; N Pascoe, J Morgan, MD, MA Patterson, D Romnes, D Bergmire-Sweat, MPH, Texas Dept of Health. WHO collaborating laboratories. 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(H1N1), A(H3N2), and B viruses circulated in the Southern Hemisphere during the winter season. Influenza activity in the Southern Hemisphere was less extensive than the preceding Southern and Northern Hemisphere influenza seasons when a larger proportion of the circulating influenza viruses were A(H3N2) viruses. The identification of sporadic influenza cases and isolated influenza outbreaks during the summer and fall months is not unusual. Recent isolates from the Northern Hemisphere have been predominantly influenza A(H1N1) and influenza B viruses. However, surveillance information is not a reliable predictor of future influenza activity. The type(s)/subtype(s) of influenza virus that will circulate, the timing of onset and peaking, and the severity of the upcoming season in the Northern Hemisphere cannot be predicted. Persons at increased risk for influenza-related complications should receive annual influenza vaccination to reduce their chances for influenza infection and the severity of the illness should they become infected (2--4).

In February of each year, the World Health Organization (WHO) recommends influenza virus strains for inclusion in the following season's Northern Hemisphere influenza vaccine. The regulatory authorities in each country then determine the actual viruses to be used for vaccine production. Frequently, the regulatory authorities in a country will substitute an antigenically equivalent virus for one or more of the WHO recommended viruses because of better growth or processing properties. In the United States, the Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee is responsible for the selection of vaccine strains to be used by U.S. vaccine manufacturers. For the 2000--01 influenza season, WHO has recommended A/New Caledonia/20/99-like (H1N1), A/Moscow/10/99-like (H3N2), and B/Beijing/184/93-like viruses for inclusion in the Northern Hemisphere influenza vaccine (5). U.S. vaccine manufacturers used the antigenically equivalent stains A/Panama/2007/99 (H3N2) for the A/Moscow/10/99-like strain and B/Yamanashi/166/98 for the B/Beijing/184/93-like strain. Most viruses isolated since April, both in the United States and worldwide, are well matched to the current vaccine strains.

CDC collects and reports U.S. influenza surveillance data during October--May. This information is updated weekly and is available through the CDC voice information system, telephone (888) 232-3228, or the fax information system, telephone (888) 232-3299, by requesting document number 361100, or on the Influenza Branch World-Wide Web site at http://www.cdc.gov/ncidod/diseases/flu/weekly.htm.

References

1. CDC. Influenza activity---United States, 1999--2000 season. MMWR 1999;48:1039--42.
2. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2000;49(no. RR-3).
3. CDC. Delayed supply of influenza vaccine and adjunct ACIP influenza vaccine recommendations for the 2000--01 influenza season. MMWR 2000:49;619--22.
4. CDC. Updated recommendations from the Advisory Committee on Immunization Practices in response to delays in supply of influenza vaccine for the 2000--01 season. MMWR 2000:49;888--92.
5. World Health Organization. Recommended composition of influenza virus vaccines for use in the 2000--01 season. Wkly Epidemiol Rec 2000;75:61--5.

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