1 figure omitted
Influenza began circulating in the United States unusually early this
season, and influenza activity nationwide is expected to increase. Cases of
severe disease, including deaths, have been reported in children. This report
summarizes influenza activity in the United States during the weeks ending
October 4–December 6, 2003.* During the week ending December 6, influenza
activity was reported to CDC as widespread in 24 states. The early season
and the unusually high and persistent demand for vaccine have resulted in
a decreasing supply of trivalent inactivated vaccine. Emphasis should be placed
on vaccinating persons at high risk for complications from influenza, including
healthy children aged 6-23 months. Healthy persons aged 5-49 years who wish
to receive vaccine should consider being vaccinated with the intranasally
administered live, attenuated influenza vaccine (LAIV), a substantial supply
of which remains available.
CDC conducts national influenza surveillance by monitoring (1) viruses
through a system of approximately 120 World Health Organization (WHO) and
National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories,
(2) visits for influenza-like illness (ILI)† through the U.S. Influenza
Sentinel Providers Surveillance Network, (3) the percentage of U.S. deaths
attributable to pneumonia and influenza (P&I) reported through the 122
Cities Mortality Reporting System, and (4) estimated levels of influenza activity
reported to CDC by state and territorial epidemiologists. CDC also receives
reports from clinicians and local health officials on influenza outbreaks
and cases nationwide.
For the weeks ending October 4–December 6, WHO and NREVSS collaborating
laboratories in the United States tested 24,906 respiratory specimens for
influenza viruses; 6,751 (27.1%) were positive. During the same period, the
weekly percentages of respiratory specimens testing positive for influenza
viruses increased from 1.4% to 37.1%. During the 2000-01, 2001-02, and 2002-03
influenza seasons, the peak percentages of specimens testing positive for
influenza ranged from 23.2% to 26.4%. During the 1999-00 influenza season,
when influenza A (H3N2) viruses predominated, the peak weekly percentage of
specimens testing positive was 30.9% (CDC, unpublished data, 2003).1
Of the 6,751 positive isolates, 6,716 (99.5%) were influenza A viruses,
and 35 (0.5%) were influenza B viruses. Of the 6,716 influenza A viruses,
1,255 (18.7%) have been subtyped; 1,254 (99.9%) were influenza A (H3N2) viruses,
and one (0.1%) was an influenza A (H1) virus. As of December 6, a total of
47 states and all nine surveillance regions had reported laboratory-confirmed
CDC has characterized antigenically 215 influenza viruses that were
collected and submitted by U.S. laboratories since October 1. Of these, 212
were influenza A (H3N2) viruses, and one was an influenza A (H1) virus. Of
the 212 influenza A (H3N2) viruses, 54 (25%) were similar antigenically to
the vaccine strain A/Panama/2007/99 (H3N2), which is contained in this season's
vaccine, whereas 158 (75%) were similar antigenically to A/Fujian/411/2002,
a drift variant of A/Panama/2007/99.
During the weeks ending October 4–December 6, the weekly percentages
of patient visits‡ to approximately 1,000 sentinel providers nationwide
for ILI increased from 0.9% to 5.1%, which is above the national baseline§
of 2.5%. During the 2000-01, 2001-02, and 2002-03 influenza seasons, the peak
weekly percentages of patient visits for ILI ranged from 3.3% to 4.4%. During
the 1999-00 season, the peak weekly percentage for patient visits for ILI
was 7.1% (CDC, unpublished data, 2003).1
During the week ending December 6, P&I accounted for 7.0% of all
deaths reported through the 122 Cities Mortality Reporting System. The epidemic
threshold∥ for that week was 7.6%. Since the week ending October 4, the
weekly percentage of P&I deaths has been below the epidemic threshold.
The percentage of P&I deaths exceeded the epidemic threshold for zero
weeks during the 2002-03 influenza season, for 9 weeks during the 2001-02
season, and for 10 weeks during the 2000-01 influenza season. During the 1999-00
influenza season, the percentage of P&I deaths exceeded the epidemic threshold
for 15 weeks (CDC, unpublished data, 2003).1
During the week ending December 6, influenza activity¶ was reported
as widespread in 24 states (Alaska, Arizona, Arkansas, Colorado, Idaho, Indiana,
Iowa, Mississippi, Missouri, Montana, Nebraska, Nevada, New Mexico, North
Carolina, Oklahoma, Oregon, Pennsylvania, Rhode Island, Tennessee, Texas,
Utah, Virginia, Washington, and Wyoming), regional in 15 states (Alabama,
California, Connecticut, Florida, Georgia, Illinois, Kansas, Kentucky, Maryland,
Minnesota, New York, North Dakota, Ohio, South Carolina, and West Virginia)
and New York City, and local in six states (Louisiana, Massachusetts, Michigan,
New Jersey, South Dakota, and Vermont) and the District of Columbia. Sporadic
influenza activity was reported in five states (Delaware, Hawaii, Maine, New
Hampshire, and Wisconsin) and Guam.
Pediatric cases. CDC has received reports of severe complications of
influenza occurring in young infants, school-age children, and adolescents.
Complications have included encephalopathy, seizures, dehydration with severe
hypotension, respiratory failure requiring mechanical ventilation, and secondary
bacterial pneumonia, including necrotizing pneumonia with community-associated
methicillin-resistant Staphylococcus aureus (CA-MRSA).
Three deaths (an infant aged 20 months with underlying reactive airways disease,
a previously healthy infant aged 22 months, and a previously healthy child
aged 16 years) have been associated with secondary pneumonia caused by CA-MRSA.
Other influenza-related deaths not related to CA-MRSA in children have occurred.
Fatal cases reported to CDC are being investigated by local and state health
authorities. Laboratory testing has confirmed influenza A virus infection
in these fatal cases; antigenic characterization is pending. The vaccination
status of the majority of the deceased children has not been determined.
Pregnant women. In Texas, 88 pregnant women had laboratory-confirmed
influenza A infections. Symptoms included fever, cough, and profound sinus
tachycardia (i.e., 150-170 beats per minute) that resolved subsequently. One
patient required intensive care for bilateral pneumonia and myocarditis. Of
the 88 patients, two (2.3%) had been vaccinated 2 and 10 days before admission,
respectively. No influenza-associated maternal deaths occurred; one case of
fetal loss occurred but was not attributed to maternal influenza infection.
The majority of the 88 cases were associated with influenza A infection; however,
influenza B viruses also were detected.
S Harper, MD, T Uyeki, MD, E Murray, MSPH, L Brammer, MPH, J Wright,
DVM, K Fukuda, MD, N Cox, PhD, Div of Viral and Rickettsial Diseases; C McDonald,
Div of Healthcare Quality Promotion, National Center for Infectious Diseases;
M Wharton, MD, Epidemiology and Surveillance Div, National Immunization Program,
Influenza seasons can vary substantially in terms of timing and pattern
of onset, peaking, decline, and overall severity. In the United States, the
2003-04 influenza season began unusually early, with community activity first
reported in early October, followed by continued spread of influenza activity
during the weeks ending October 4–December 6. National activity levels
have not yet peaked, and neither the duration of activity nor the season's
eventual magnitude is known. As of December 6, influenza A (H3N2) viruses
predominated in the United States, but different influenza viruses might predominate
later in the season. Influenza seasons dominated by A (H3N2) viruses (e.g.,
those in 1996-97, 1997-98, and 1998-99) typically are associated with high
levels of severe illness and deaths.3 No evidence exists to indicate
that the A/Fujian-like viruses in circulation are more virulent than other
influenza A (H3N2) viruses. However, reports of severe pediatric illnesses
and deaths underscore the severe consequences that influenza infections can
cause in children.4
Cases of sudden death associated with influenza in previously healthy
children also were reported in the United States during the 2002-03 season
(4; CDC unpublished data, 2003). Although the pathophysiology
of sudden deaths associated with influenza in children is unknown, atypical
symptoms (e.g., abdominal pain, absence of fever, and mild respiratory symptoms)
have been reported.
Encephalopathy is another severe and potentially under-recognized complication
of influenza in children.5 One case so far this season has resulted
in the death of a patient (CDC, unpublished data, 2003). Patients might report
high fevers, seizures, headaches, abnormal mental status, and/or confusion
and do not always exhibit classic influenza symptoms. Cases have been reported
among young children and school-aged children, including adolescents. Suspected
cases should be reported to CDC at telephone, 404-639-0277 or 404-639-2893;
fax, 404-639-3866; or e-mail, email@example.com or firstname.lastname@example.org.
Although secondary bacterial pneumonia is a common complication of influenza
infection, S. aureus typically occurs in a minority
of such cases. Clinical and laboratory features of S. aureus pneumonia are similar to other types of community-acquired pneumonia.6,7 Clinicians should be aware that CA-MRSA can be a cause of community-acquired
pneumonia. Treatment for pneumonia after influenza infection should be guided
by bacterial culture results when possible. Aspirin and other salicylate-containing
medications should not be administered to children with fever and respiratory
Pregnant women are at higher risk than nonpregnant women for having
complications secondary to influenza. Pregnant women who will be in their
second or third trimester during influenza season should be vaccinated against
So far this season, influenza A/Fujian/411/2002-like viruses are predominating
in the United States. This strain differs from the influenza A (H3N2) virus
contained in the 2003-04 vaccine (i.e., A/Panama/2007/99). The A/Fujian-like
viruses are antigenic drift variants of the A/Panama strain and were detected
by global surveillance early this year but too late for inclusion in the current
influenza vaccine. Hemagglutination inhibition testing using postinfection
ferret sera indicates that antibodies to the A/Panama vaccine virus cross-react
with A/Fujian-like viruses; therefore, current influenza vaccines should provide
some protection against A/Fujian-like viruses. However, the level of protection
remains uncertain until vaccine effectiveness studies are completed. The vaccine
also contains A/New Caledonia/20/99 (H1N1)-like and B/Hong Kong/330/2001-like
viruses and should protect persons who are vaccinated against these viruses
if they circulate more widely later in the season.
Approximately 83.4 million doses of influenza vaccine, including inactivated
influenza vaccine made by two manufacturers and LAIV made by a third manufacturer,
were produced for the 2003-04 influenza season. All doses of trivalent inactivated
vaccine appear to have been sold by the manufacturers and their major distributors.
Trivalent inactivated vaccine remains available from physicians' offices and
in other settings. As of December 9, a total of 3.9 million doses of LAIV
were available from the manufacturer (Wyeth Pharmaceuticals, Collegeville,
Pennsylvania, telephone 800-358-7443).
To ascertain the availability of influenza vaccine, CDC conducted a
survey of state and urban area immunization programs. As of December 3, a
total of 28 states had redistributed influenza vaccine from health-care providers
and public immunization clinics that had excess supplies to those that needed
vaccine. In addition, 34 states had influenza vaccine inventory that had not
been distributed. However, in an average year, <10% of influenza vaccine
is purchased by state health departments.
Influenza antiviral medications are available for use in adults and
children. Four prescription antiviral medications (i.e., amantadine, rimantadine,
oseltamivir, and zanamivir) are approved for treatment of influenza A virus
infections. Oseltamivir and zanamivir also are approved for treatment of influenza
B. The costs, routes of administration, adverse effects, contraindications,
approved ages, and potential for antiviral resistance differ among the four
drugs. When administered within 48 hours of symptom onset, antiviral treatment
of influenza can reduce the duration of illness by approximately 1 day in
healthy adults.9 Data on the use of any of the four antiviral agents
during pregnancy are not available. Amantadine, rimantadine, and oseltamivir
also are approved for chemoprophylaxis of influenza A virus infections and
can be used for control of institutional influenza outbreaks. When used for
chemoprophylaxis, antivirals can be approximately 70%-90% effective in preventing
illness in healthy adults.9,10 To obtain information about approved
age groups, dosing, and adverse effects, clinicians should consult antiviral
drug package inserts (available from the Food and Drug Administration at http://www.fda.gov/cder/drug/antivirals/influenza/default.htm#drugs).
CDC has published recommendations for prevention and control of influenza
(available at http://www.cdc.gov/mmwr/PDF/rr/rr5208.pdf). Supplemental
recommendations have been released for the 2003-04 influenza season (see sidebar).
Influenza surveillance reports for the United States are published weekly
during October-May and are available from CDC at http://www.cdc.gov/flu or through CDC's voice (telephone, 888-232-3228) and fax (telephone,
888-232-3299, document number 361100) information systems.
This report is based on data contributed by A Tulu, K Hankins, Dallas
County Health and Human Svcs Office; G Wendell, J Sheffield, Parkland Memorial
Hospital, Dallas; J Siegel, Children's Medical Center, Dallas; N Pascoe, S
Avashia, Texas Dept of Health. K Gershman, Colorado State Dept of Public Health
and Environment. Participating state and territorial epidemiologists and state
public health laboratory directors. WHO collaborating laboratories. National
Respiratory and Enteric Virus Surveillance System collaborating laboratories,
U.S. Influenza Sentinel Provider Surveillance System. Div of Public Health
Surveillance and Informatics, Epidemiology Program Office; DJ O'Mara, Immunization
Svcs Div, National Immunization Program, CDC.
References: 10 available
*Data reported as of December 5.
†Temperature of >100.0°F (37.8°C) and cough and/or sore
throat in the absence of a known cause other than influenza.
‡National and regional percentages of patient visits for ILI
are weighted on the basis of state population.
§Calculated as the mean percentage of visits for ILI during noninfluenza
weeks, plus two standard deviations. Wide variability in regional data precludes
calculating region-specific baselines and makes it inappropriate to apply
the national baseline to regional data.
∥The expected baseline proportion of P&I deaths reported by
the 122 Cities Mortality Reporting System is projected by using a robust regression
procedure that applies a periodic regression model to the observed percentage
of deaths from P&I during the previous 5 years; the epidemic threshold
is 1.645 standard deviations above the seasonal baseline percentage.2
¶Levels of activity are (1) no activity,
(2) sporadic—small numbers of laboratory-confirmed
influenza cases or a single influenza outbreak reported but no increase in
cases of ILI, (3) local—outbreaks of influenza
or increases in ILI cases and recent laboratory-confirmed influenza in a single
region of a state, (4) regional—outbreaks of
influenza or increases in ILI cases and recent laboratory-confirmed influenza
in at least two but less than half the regions of a state, and (5) widespread—outbreaks of influenza or increases in ILI cases and
recent laboratory-confirmed influenza in at least half the regions of a state.
Update: Influenza Activity—United States, 2003-04 Season. JAMA. 2004;291(1):34-37. doi:10.1001/jama.291.1.34