During May-October 2004, influenza A (H3N2) viruses circulated worldwide
and were associated with mild-to-moderate levels of disease activity. Influenza
A (H1N1)* and B viruses were reported less frequently. In North America, isolates
of influenza A (H3N2), A (H1N1), and B were identified sporadically. This
report summarizes influenza activity in the United States and worldwide during
May-October 2004.† Influenza activity in North America typically peaks
Until recently, in the United States, national influenza surveillance
was conducted by four systems that operated during October-May. One of these
systems consists of approximately 1,000 sentinel health-care providers, who
regularly report data to CDC on patient visits for influenza-like illness
(ILI). In addition, during 2004, approximately 350 sentinel providers continued
to submit weekly reports during May-September. A second system consists of
approximately 120 U.S.-based World Health Organization (WHO) and National
Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories;
these laboratories report the number of respiratory specimens tested and the
number and types of influenza viruses identified throughout the year.
For the 2004-05 influenza season, CDC has added two new surveillance
systems: one that tracks naturally reported pediatric deaths associated with
laboratory-confirmed influenza infections and another that tracks hospitalizations
associated with laboratory-confirmed influenza infections in children aged
<18 years. The latter system, which will continue at a minimum of nine
sites through CDC’s Emerging Infections Program, augments CDC’s
ongoing surveillance at the three National Vaccine Surveillance Network sites
of children aged <5 years hospitalized with fever or respiratory illness.
During May 23–October 2, the weekly percentage of patient visits
to sentinel providers for ILI ranged from 0.4% to 0.8%. WHO and NREVSS collaborating
laboratories tested 11,916 respiratory specimens; 54 (0.5%) were positive
for influenza. Of the positive results, 29 (54%) were influenza B viruses,
14 (26%) were influenza A (H3N2) viruses, and 11 (20%) were influenza A viruses
that were not subtyped. Both influenza A and B viruses were reported during
late May-September 2004.
During October 3-16, influenza activity occurred at low levels in the
United States. Since October 3, WHO and NREVSS collaborating laboratories
in the United States have tested 1,414 respiratory specimens; eight (0.6%)
were positive. Of these, six were influenza A viruses, and two were influenza
B viruses. The proportion of patient visits to sentinel providers for ILI
and the proportion of deaths attributed to pneumonia and influenza were below
baseline levels. During the week ending October 16, nine states and New York
City reported sporadic influenza activity, and 40 states and the District
of Columbia reported no influenza activity.
During May-July, influenza A (H3N2) viruses predominated in Africa (Madagascar,
Senegal, and South Africa). In Asia, influenza A (H3N2) viruses predominated
in China, Hong Kong, and Thailand and also were reported in Japan. Influenza
A (H3N2) viruses were responsible for regional outbreaks in Taiwan in August
In Oceania (Australia, New Caledonia, and New Zealand), influenza A
(H3N2) viruses predominated and were associated with multiple nursing home
outbreaks in Australia and New Zealand in August and September. In South America,
influenza A (H3N2 and non-subtyped) viruses predominated in Argentina, Brazil,
Chile, Peru, and Uruguay. Influenza A (H3N2) viruses were associated with
widespread outbreaks in Argentina, Chile, and Paraguay during May-June.
During May-July, influenza A (H1N1) viruses predominated in the Philippines
and also were reported in China, Japan, New Caledonia, Peru, and Thailand.
Influenza B viruses were reported in South America (Argentina, Brazil, Chile,
Colombia, and Peru), Asia (China, Japan, and Korea), Africa (South Africa),
and North America (United States). Influenza B viruses were associated with
widespread outbreaks in Brazil during May-June.
WHO’s Collaborating Center for Surveillance, Epidemiology, and
Control of Influenza, located at CDC, analyzes influenza virus isolates received
from laboratories worldwide. During May-October, 236 influenza A (H3N2) viruses
(110 from Latin America, 100 from Asia, 24 from North America [including 10
from the United States], one from Africa, and one from Oceania) were collected
and characterized antigenically. A total of 208 (88.1%) were A/Fujian/411/02-like
and similar to A/Wyoming/03/2003, the A (H3N2) component of the 2004-05 influenza
vaccine; 28 (11.9%) had reduced titers to A/Wyoming/03/2003. The eight influenza
A (H1N1) viruses (one from Canada, three from Hong Kong, two from Singapore,
and two from the United Kingdom) collected during May-September and characterized
antigenically at CDC were similar to A/New Caledonia/20/99, the A (H1N1) component
of the 2003-04 influenza vaccine.
Influenza B viruses circulating worldwide can be divided into two antigenically
distinct lineages: B/Yamagata/16/88 and B/Victoria/2/87. Before 1991, B/Victoria
lineage viruses circulated worldwide; from late 1991 to early 2001, no viruses
of the B/Victoria lineage were identified outside Asia. However, since March
2001, B/Victoria-lineage viruses have been identified in many countries outside
Asia, including the United States. Viruses of the B/Yamagata lineage began
circulating worldwide in 1990 and continue to be identified.3 The
type-B component of the 2004-05 influenza vaccine (B/Shanghai/361/2002-like)
belongs to the B/Yamagata lineage. Of the 73 influenza B isolates collected
during May-September and characterized antigenically at CDC, 54 belonged to
the B/Yamagata lineage, and 19 belonged to the B/Victoria lineage.
Of the B/Yamagata lineage viruses, 50 (92.6%) were B/Shanghai/361/2002-like,
and four (7.4%) had reduced titers to B/Shanghai/361/2002. Twenty-one of the
B/Yamagata lineage viruses were from North America (including 16 from the
United States), 25 were from South America, five were from Asia, two were
from Oceania, and one was from Europe.
Since December 2003, nine countries (Cambodia, China, Indonesia, Japan,
Laos, Malaysia, South Korea, Thailand, and Vietnam) have reported outbreaks
of avian influenza A (H5N1) infection affecting poultry and, in some countries,
other animals. As of October 25, a total of 44 laboratory-confirmed cases
of avian influenza A (H5N1) virus infection in humans had been reported in
Vietnam and Thailand in 2004.4 Of these 44
patients, 32 died. The cases occurred in association with recurring H5N1 outbreaks
among poultry in those countries.
Four human H5N1 cases occurred in Vietnam (three in children and one
in a young adult) during July-September. In Thailand, four cases occurred
in September and one case in October. The cases were associated with severe
respiratory illness, with persons requiring hospitalization; all but one patient
died. The cumulative case-fatality proportion for confirmed H5N1 cases since
January 2004 is 73% (Vietnam: 27 cases, 20 deaths; Thailand: 17 cases, 12
WHO Collaborating Center for Surveillance, Epidemiology, and Control
of Influenza; K Teates, MPH, L Brammer, MPH, A Balish, T Wallis, H Hall, A
Klimov, PhD, K Fukuda, MD, N Cox, PhD, Div of Viral and Rickettsial Diseases,
National Center for Infectious Diseases; M Katz, MD, EIS Officer, CDC.
During May-October 2004, influenza A (H3N2) viruses were the most frequently
reported virus subtype worldwide; however, influenza A (H1N1) and influenza
B viruses also circulated. At this time, neither the influenza virus subtype
that will predominate in the United States nor the severity and timing of
the 2004-05 season can be predicted.
The ongoing widespread epizootic of highly pathogenic H5N1 viruses in
Asia remains a major concern. Since December 2003, nine Asian countries have
reported H5N1 poultry outbreaks, with human cases reported from two of these
countries. No evidence of sustained person-to-person transmission has been
identified to date, although a probable instance of limited person-to-person
transmission in a family cluster was identified recently in Thailand. CDC
continues to recommend enhanced surveillance for suspected H5N1 cases among
travelers with severe unexplained respiratory illness returning from H5N1-affected
countries. Additional information about avian influenza is available at http://www.phppo.cdc.gov/han/archivesys/viewmsgv.asp?alertnum=00209.
Influenza surveillance reports for the United States are published weekly
during October-May and are available through CDC’s voice (telephone,
888-232-3228) and fax (telephone, 888-232-3299, document number 361100) information
systems and at http://www.cdc.gov/flu/weekly/fluactivity.htm. Additional
information about influenza viruses, influenza surveillance, and the influenza
vaccine is available at http://www.cdc.gov/flu.
This report is based on data contributed by WHO collaborating laboratories;
National Respiratory and Enteric Virus Surveillance System laboratories; Sentinel
Providers Influenza Surveillance System; WHO National Influenza Centers, Communicable
Diseases, Surveillance and Response, WHO, Geneva, Switzerland. A Hay, PhD,
WHO Collaborating Centre 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.
*Includes both the A (H1N1) and A (H1N2) influenza virus types. Although
H1N2 viruses have not been identified since February 2004, not all isolated
H1 viruses have been tested for the subtype of their neuraminidase. Thus,
this subtype might continue to circulate in some parts of the world. Influenza
A (H1N2) viruses appear to have resulted from reassortment of the genes of
the circulating influenza A (H1N1) and A (H3N2) subtypes. Because the hemagglutinin
proteins of the A (H1N2) viruses are similar to those of the circulating A
(H1N1) viruses, and the neuraminidase proteins are similar to the circulating
A (H3N2) viruses, the 2004-05 influenza vaccine should provide protection
against A (H1N2) viruses.
†As of October 16, 2004.
Update: Influenza Activity—United States and Worldwide, May-October
2004. JAMA. 2005;293(2):155-156. doi:10.1001/jama.293.2.155