MMWR: H1N1 Update: Influenza Activity, USA, August 30 - October 31, 2009

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 were substantially elevated for this time of year and exceeded or were approaching the end-of-season cumulative rates for the last three seasons.

Preliminary cumulative rates of laboratory-confirmed, influenza-associated hospitalizations reported for children aged 0--4 years were 3.1 per 10,000 population by EIP and 7.3 per 10,000 population by the new sites (Figure 3). Rates for other age groups were as follows: 5--17 years, 1.5 by EIP and 2.9 by the new sites; 18--49 years, 1.2 by EIP and 1.2 by the new sites; 50--64 years, 1.3 by EIP and 1.2 by the new sites; and > 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 < 2 years, nine (11%) were among children aged 2--4 years, 30 (35%) were among children aged 5--11 years, and 34 (40%) were among children aged 12--17 years. Seventy-eight (92%) of the 85 decedents had a medical history reported. Of the 78, 56 (72%) had one or more medical conditions associated with an increased risk for influenza-related complications (3).

Since April 26, CDC has received 145 reports of pediatric deaths associated with influenza infection. Of these, 129 (89%) cases were associated with laboratory-confirmed 2009 pandemic influenza A (H1N1) virus. The remaining 16 pediatric deaths were associated with seasonal influenza or an influenza A virus for which the subtype was undetermined. In comparison, during the preceding five influenza seasons, the total number of reported pediatric influenza deaths ranged from 46 to 153, with an average of 82 deaths each year.

Reported by: WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza. L Brammer, MPH, S Epperson, MPH, L Blanton, MPH, R Dhara, MPH, T Wallis, MS, L Finelli, DrPH, T Fiore, MD, L Gubareva, PhD, J Bresee, MD, L Kamimoto, MD, X Xu, MD, A Klimov, PhD, C Bridges, MD, N Cox, PhD, Influenza Div, National Center for Immunization and Respiratory Diseases, CDC; C Cox, MD, EIS Officer, CDC.

Editorial Note:

During August 30--October 31, influenza activity was substantially above historic levels in all U.S. surveillance systems. By mid-October, nearly all states reported geographically widespread influenza activity.

Nationwide, the percentage of visits to health-care providers for ILI was higher than that observed at the peak of any seasonal influenza season since ILINet was implemented in its current form in 1997. Influenza-associated hospitalization rates continued to trend upward in all age groups, substantially above historical rates from the same time period during previous years.

The widespread occurrence of pandemic H1N1 influenza in the United States highlights the importance of understanding and appropriately using available tools for prevention and treatment of influenza.

Particularly important in reducing the impact of pandemic H1N1 infections are recommendations for the use of influenza A (H1N1) 2009 monovalent vaccines and a continued emphasis on early, empiric antiviral treatment of hospitalized patients and others who are ill and at greater risk for influenza-related complications.

Severe outcomes among children, continue to be prominent during the 2009 influenza A (H1N1) pandemic. A total of 145 pediatric deaths associated with influenza infection have been reported since April 26. In comparison, 82 deaths were reported on average during the previous five influenza seasons.

Pediatric hospitalization rates are higher than those of any other age group and are particularly high among children aged < 5 years. These epidemiologic data provide support for ACIP recommendations that include persons aged 6 months-24 years in the initial target groups for vaccination using the influenza A (H1N1) 2009 monovalent vaccine now available (3).

In addition, vaccination providers should vaccinate persons who live with or care for infants aged <6 months because young infants themselves cannot be vaccinated. Other target groups for initial supplies of influenza A (H1N1) 2009 monovalent vaccine include pregnant women, health-care and emergency medical services personnel, and persons aged 25--64 years who are at higher risk for more severe disease because of chronic health disorders or compromised immune systems (3).

The supply of influenza A (H1N1) 2009 monovalent vaccines will continue to increase rapidly through November and December.^¶¶ However, these vaccines are not yet available to all persons who might benefit from vaccination. In the absence of widespread immunity based on vaccination, early empiric antiviral treatment of persons who are severely ill or at high risk for influenza-related complications can reduce the number of severe illnesses from pandemic H1N1.

Observational studies of hospitalized patients with seasonal influenza and pandemic H1N1 influenza have suggested that mortality is reduced among hospitalized patients who received antiviral medications (4 - 5). However, the use of antiviral treatment for hospitalized patients remains suboptimal, as highlighted in recent studies indicating that 21% - 25% of hospitalized patients with laboratory-confirmed pandemic H1N1 did not receive antiviral medications and, among those who did, treatment was often delayed until 1--2 days after admission (6 - 7).

Antiviral medications active against influenza are widely available, and early empiric treatment with oseltamivir or zanamivir of hospitalized persons and others who are severely ill or at high risk for influenza-related complications is recommended (8).

In addition, peramivir, an investigational intravenous neuraminidase inhibitor medication, has recently been made available under an Emergency Use Authorization by the Food and Drug Administration. Peramivir is available for treatment of certain adult and pediatric patients with suspected or laboratory-confirmed pandemic H1N1 (9,10).

The current dominant influenza virus by far is 2009 pandemic influenza A (H1N1); seasonal influenza viruses continue to circulate at low levels in the United States and elsewhere. However, influenza circulation patterns remain unpredictable, and seasonal influenza viruses might circulate more widely later in the influenza season.

CDC will continue to monitor changes in geographic spread, type, and severity of 2009 pandemic influenza A (H1N1) and will issue weekly online FluView reports.*** Additional detailed information regarding 2009 pandemic influenza A (H1N1) also is available online.^†††

Acknowledgments

This report is based, in part, on data contributed by participating state and territorial health departments and state public health laboratories, World Health Organization collaborating laboratories, National Respiratory and Enteric Virus Surveillance System collaborating laboratories, the U.S. Outpatient ILI Surveillance Network, the Emerging Infections Program, the Aggregate Hospitalization and Death Reporting Activity, the Influenza Associated Pediatric Mortality Surveillance System, and the 122 Cities Mortality Reporting System.