foodconsumer.org: Update: Influenza Activity --- United States, August 30--October 31, 2009

Update: Influenza Activity --- United States, August 30--October 31, 2009
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admin on 11/13/2009 01:20:00
 
The 2009 pandemic influenza A (H1N1) virus emerged in the United States in April
2009 (1) and has since spread worldwide. Influenza activity resulting from this
virus occurred throughout the summer and, by late August, activity had begun to
increase in the southeastern United States (2). Since August, activity has
increased in all regions of the United States. As of the week ending October 31,
nearly all states were reporting widespread disease. Since April 2009, pandemic
H1N1 has remained the dominant circulating influenza strain. This report
summarizes U.S. influenza activity* from August 30, 2009, defined as the
beginning of the 2009--10 influenza season, through October 31, 2009.
Viral Surveillance
During August 30--October 31, World Health Organization (WHO) and National
Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating
laboratories in the United States tested 163,123 respiratory specimens for
influenza viruses, 48,585 (30%) of which were positive (Figure 1). Of the 48,483
(99.8%) specimens positive for influenza A, 32,867 (68%) were subtyped by
real-time reverse transcription--polymerase chain reaction (rRT-PCR) or by virus
culture. A total of 32,814 (99.8%) of these were 2009 pandemic influenza A
(H1N1) viruses, 18 (0.1%) were seasonal influenza A (H1), and 35 (0.1%) were
influenza A (H3) viruses.
CDC has antigenically characterized 239 pandemic influenza A (H1N1)viruses
collected since September 1. A total of 238 (99.6%) of the 239 pandemic
influenza A (H1N1) viruses tested were antigenically related to the
A/California/7/2009 (H1N1)pdm reference virus selected by WHO as the 2009
pandemic influenza A (H1N1) vaccine virus; one virus (0.4%) tested showed
reduced titers with antisera produced against A/California/7/2009.
Antiviral Resistance of Influenza Virus Isolates
CDC conducts surveillance for resistance of circulating influenza viruses to
influenza antiviral medications: adamantanes (amantadine and rimantadine) and
neuraminidase inhibitors (zanamivir and oseltamivir). Since September 1, a total
of 256 pandemic influenza A (H1N1) virus isolates collected in the United States
have been tested for resistance to the neuraminidase inhibitors. All but four
were susceptible to oseltamivir, bringing the total number of such resistant
isolates to 14 since April 2009. Twelve of the 14 patients from whom the
resistant isolates were collected had documented exposure to oseltamivir through
treatment or chemoprophylaxis. Exposure to oseltamivir has yet to be determined
for one patient, and another patient had no documented oseltamivir exposure. All
256 tested viruses were sensitive to the neuraminidase inhibitor zanamivir.
Since September 1, one influenza A (H3N2) virus isolate and 152 pandemic
influenza A (H1N1) virus isolates also have been tested for resistance to
adamantanes (amantadine and rimantadine); all of these virus isolates were
resistant to the adamantanes.
State-Specific Activity Levels
During the first week of the influenza season (August 30--September 5), 11
states, clustered mainly in the South, reported widespread activity. By the
following week, that number had more than doubled to 26 states. In subsequent
weeks, more states reported increased activity. As of the week ending October
31, widespread influenza activity† was reported by all but two states
(Mississippi and Hawaii). In contrast, during the 2008--09 influenza season, no
state reported widespread influenza activity before the week ending January 10,
2009.
Outpatient Illness Surveillance
The weekly percentage of outpatient visits for influenza-like illness (ILI)§
reported by the U.S. Outpatient ILI Surveillance Network (ILINet) increased from
3.5% in the week ending September 5 to 7.7% in the week ending October 31
(Figure 2). ILI activity has remained above the national baseline of 2.3% during
this entire period.¶ Since the week ending October 3, all 10 surveillance
regions have reported a percentage of outpatient visits for ILI at or above
their region-specific baseline levels. These percentages are all substantially
elevated compared with data recorded in previous years over the same period.
Influenza-Associated Hospitalizations
Laboratory-confirmed influenza-associated hospitalizations are monitored using a
population-based surveillance network that includes the 10 Emerging Infections
Program (EIP) sites and six new sites.** During September--October, cumulative
influenza hospitalization rates for persons aged 65 years, 1.0 by EIP and 1.1 by
the new sites.
On August 30, CDC and the Council of State and Territorial Epidemiologists
(CSTE) instituted modified case definitions for aggregate reporting of
influenza-associated hospitalizations and deaths. This cumulative state-level
reporting is referred to as the Aggregate Hospitalization and Death Reporting
Activity (AHDRA).†† During August 30--October 31, a total of 17,838
hospitalizations associated with laboratory-confirmed influenza virus infections
were reported to CDC through AHDRA. On average, 31 states each week reported
laboratory-confirmed hospitalizations during that period.
Pneumonia- and Influenza-Related Mortality
Influenza-associated deaths are monitored by the 122 Cities Mortality Reporting
System and AHDRA. For the week ending October 31, pneumonia or influenza was
reported as an underlying or contributing cause of death for 7.4% of all deaths
reported through the 122 Cities Mortality Reporting System, above the
week-specific epidemic threshold of 6.7%§§ and the fifth consecutive week
above the epidemic threshold.
During August 30--October 31, 672 deaths associated with laboratory-confirmed
influenza virus infections were reported to CDC through AHDRA. On average, 29
states reported laboratory-confirmed deaths each week during that period. The
672 laboratory-confirmed deaths are in addition to the 593 laboratory-confirmed
deaths from 2009 pandemic influenza A (H1N1) that were reported to CDC from
April through August 30, 2009.
Influenza-Associated Pediatric Mortality
During August 30--October 31, CDC received 85 reports of pediatric deaths
associated with influenza infection (Figure 4). Seventy-three of these cases
were associated with laboratory-confirmed 2009 pandemic influenza A (H1N1)
virus. The remaining 12 pediatric deaths were associated with an influenza A
infection for which the subtype was undetermined.
Of the 85 pediatric deaths reported since August 30, a total of 12 (14%) were
among children aged