Update: Influenza Activity --- United States, September 28--November 29, 2008

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During September 28--November 29, 2008, influenza activity remained low in the United States. Of the few influenza viruses characterized thus far this season, most are antigenically related to the strains included in the 2008--09 influenza vaccine. Oseltamivir-resistant influenza A (H1N1) viruses have been detected, but currently available data are insufficient to predict their prevalence for the 2008--09 season. This report summarizes U.S. influenza activity* since the last update (1) and reviews new influenza vaccine recommendations for the current season.

Viral Surveillance

During September 28--November 29, 2008, approximately 150 World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System collaborating laboratories in the United States tested 24,657 respiratory specimens for influenza viruses; 365 (1.5%) were positive (Figure 1). Of these, 282 (77.3%) were influenza A viruses, and 83 (22.7%) were influenza B viruses. One hundred twenty-eight (45.4%) of the 282 influenza A viruses were subtyped; 112 (87.5%) of these were influenza A (H1) viruses, and 16 (12.5%) were influenza A (H3) viruses. Influenza-positive tests have been reported from 26 states in eight of the nine surveillance regions since September 28.

Antigenic Characterization

WHO collaborating laboratories in the United States are requested to submit a subset of their influenza-positive respiratory specimens to CDC for further antigenic characterization. CDC has antigenically characterized 30 influenza viruses collected by U.S. laboratories during the 2008--09 season, including 20 influenza A (H1N1), three influenza A (H3N2), and seven influenza B viruses. Twenty-seven of the 30 viruses were antigenically related to the components included in the 2008--09 influenza vaccine (A/Brisbane/59/2007-like (H1N1), A/Brisbane/10/2007-like (H3N2), and B/Florida/04/2006-like). The other three influenza B viruses belong to the B/Victoria/02/87 lineage.

Antiviral Resistance of Influenza Virus Isolates

With limited influenza activity in the United States, few viruses have been available for antiviral resistance testing. Since September 28, 2008, 39 influenza viruses from 11 states have been tested for antiviral resistance; of the viruses tested, 28 (71.8%) were collected from only two states. Preliminary data show that 24 of the 25 influenza A (H1N1) isolates tested were resistant to oseltamivir; all H1N1 isolates were sensitive to zanamivir. All five influenza A (H3N2) and the nine influenza B isolates tested were sensitive to oseltamivir and zanamivir. Twenty-five influenza A (H1N1) isolates and five influenza A (H3N2) isolates were tested for adamantane resistance. All influenza A (H1N1) isolates were sensitive to adamantanes, and all influenza A (H3N2) isolates tested were resistant to adamantanes. The adamantanes are not effective against influenza B viruses.

Currently, data on antiviral resistance, and information on which influenza virus types or subtypes will circulate, are insufficient to provide an indication of the prevalence of antiviral resistance at a national or regional level during this season. CDC has solicited a representative sample of viruses from WHO collaborating laboratories in the United States for resistance testing throughout the season, and more specimens are expected as influenza activity increases.

Novel Influenza A Viruses

One case of human infection with a novel influenza A virus was reported from Texas during the week ending November 15, 2008. A child aged 14 years was infected with swine influenza A (H1N1) in October 2008 after several reported swine exposures. The child recovered from the illness, and no contacts of the child were reported to be ill.

State-Specific Activity Levels

For the week ending November 29, 2008, influenza activity was reported as sporadic in Puerto Rico and 22 states, and one state (Hawaii) reported local activity. Twenty-seven states and the District of Columbia reported no activity. No states have reported regional or widespread activity this season.

Outpatient Illness Surveillance

Since September 28, 2008, the weekly percentage of outpatient visits for influenza-like illness (ILI)^§ reported by approximately 1,500 U.S. sentinel providers in 50 states, New York City, Chicago, and the District of Columbia that comprise the U.S. Outpatient ILI Surveillance Network (ILINet), has ranged from 0.9% to 1.3% (Figure 2). This is below the national baseline of 2.4%. In addition, all nine surveillance regions reported percentages below their respective region-specific baselines.^¶

Pneumonia- and Influenza-Related Mortality

For the week ending November 29, 2008, pneumonia and influenza (P&I) was reported as an underlying or contributing cause of death for 6.7% of all deaths reported to the 122 Cities Mortality Reporting System. This is below the epidemic threshold of 7.1% for that period. Since September 28, 2008, the weekly percentage of deaths attributed to P&I ranged from 6.0%--6.7%, remaining below the epidemic threshold.**

Influenza-Associated Pediatric Hospitalizations

Pediatric hospitalizations associated with laboratory-confirmed influenza infections are monitored by two population-based surveillance networks, the Emerging Infections Program (EIP) and the New Vaccine Surveillance Network (NVSN). No influenza-associated pediatric hospitalizations have yet been reported by either network this season.

Influenza-Related Pediatric Mortality

No influenza-related pediatric deaths have been reported for the 2008--09 season.

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, A Fiore, MD, L Gubavera, PhD, J Bresee, MD, A Klimov, PhD, N Cox, PhD, Influenza Div, National Center for Immunization and Respiratory Diseases; S Doshi, MD, EIS Officer, CDC.

Editorial Note:

During September 28--November 29, 2008, the United States experienced a low level of influenza activity which is typical for this time of year and similar to the past four influenza seasons. The peak of influenza activity has come before January in only five of the past 20 seasons; February or March has been the peak month in 11 of those 20 seasons (CDC, unpublished data, 2008). Influenza vaccine first became available in August, allowing persons to be vaccinated before influenza activity began. Vaccination efforts should continue during December given the most common timing of peak influenza activity, and providers should offer influenza vaccine throughout the influenza season (which can persist as late as April or May) to protect as many persons from influenza infection and its complications as possible.

Most of the U.S. influenza viruses identified and characterized thus far in the 2008--09 season are antigenically similar to the components included in the 2008--09 influenza vaccine. However, these viruses were isolated in few states and early in the influenza season; CDC will test more viruses as flu activity increases and more samples become available. The season has not progressed enough to determine which influenza virus type or subtype will predominate this season.

On average, influenza is estimated to cause approximately 226,000 hospitalizations and 36,000 deaths per year in the United States. Annual vaccination remains the best method for preventing influenza and its potentially severe complications. The Advisory Committee on Immunization Practices (ACIP) recently expanded its recommendations for influenza vaccination to include all children aged 6 months--18 years. In addition, influenza vaccine should be administered to other persons at high risk for influenza-related complications, close contacts of those at high risk (including health-care workers), and anyone else who wants to decrease their risk for influenza (2).

CDC conducts surveillance for resistance of circulating influenza viruses to licensed antiviral medications: adamantanes (amantadine and rimantadine) and neuraminidase inhibitors (zanamivir and oseltamivir). Antiviral resistance testing is not commercially available to guide clinical management of individual patients. Influenza A (H1N1) viruses that have a genetic mutation conferring oseltamivir resistance appeared and circulated during the 2007--08 Northern Hemisphere influenza season (3), and during the 2008 Southern Hemisphere season, with some Southern Hemisphere countries reporting that a majority of tested A (H1N1) viruses were resistant to oseltamivir (4). To date, oseltamivir-resistant A (H1N1) viruses from all countries that have submitted specimens to CDC have been sensitive to zanamivir, and most have been susceptible to the adamantanes. All tested influenza A (H1N1), influenza A (H3N2), and influenza B viruses have been sensitive to zanamivir. Most recent influenza A (H3N2) viruses circulating worldwide are resistant to adamantanes, and adamantanes are not effective against influenza B infections. The prevalence of oseltamivir resistance this season will depend on the level of influenza activity, the proportion of resistance among influenza A (H1N1) viruses and the proportion of A (H1N1) viruses among all circulating influenza viruses. At this time, too few specimens from a limited geographic area have been tested to accurately estimate either proportion; thus the prevalence of oseltamivir resistance for the 2008--09 season cannot be estimated accurately.

Enhanced surveillance for oseltamivir-resistant viruses is ongoing at CDC. Alternatives for antiviral treatment in the context of widely circulating oseltamivir-resistant viruses have been suggested. These treatment options, which might include preferential use of zanamivir or therapy with a combination of antivirals for certain patients, have been outlined in the ACIP 2008 influenza recommendations.Currently, the neuraminidase inhibitors oseltamivir and zanamivir remain the recommended medications for treatment and chemoprophylaxis of influenza.

Clinicians should remain alert for changes in recommendations that might occur as the 2008--09 influenza season progresses. Recommendations regarding the use of antiviral medications might be revised if surveillance data indicate a substantial and widespread increase in the prevalence of oseltamivir-resistant influenza viruses in the United States.

Vaccination remains the cornerstone of influenza prevention efforts. Influenza vaccination can prevent influenza infections from strains that are sensitive or resistant to antiviral medications; the influenza A (H1N1) viruses found to be oseltamivir resistant are antigenically similar to the components included in the 2008--09 vaccine. December 8--14 is National Influenza Vaccination Week. Health-care providers are encouraged to take advantage of heightened awareness of the benefits of influenza vaccination and to increase vaccination efforts during this week to reach persons who have not yet been vaccinated.

CDC continues to conduct surveillance to provide up-to-date recommendations regarding prevention and treatment of influenza. Influenza surveillance reports for the United States are posted online weekly during October--May and are available at http://www.cdc.gov/flu/weekly/fluactivity.htm. Additional information regarding influenza viruses, influenza surveillance, influenza vaccine, and avian influenza is available at http://www.cdc.gov/flu.

This report is based, in part, on data contributed by participating state and territorial health departments and state public health laboratories, WHO collaborating laboratories, National Respiratory and Enteric Virus Surveillance System collaborating laboratories, the U.S. Influenza Sentinel Provider Surveillance System, and the 122 Cities Mortality Reporting System; WHO National Influenza Centers, WHO Global Influenza Programme, Geneva, Switzerland; A Kelso, PhD, I Barr, PhD, WHO Collaborating Center for Reference and Research on Influenza, Parkville, Australia; A Hay, PhD, WHO Collaborating Center for Reference and Research on Influenza, National Institute of Medical Research, London, England; and M Tashiro, MD, WHO Collaborating Center for Reference and Research on Influenza, National Institute of Infectious Diseases, Tokyo, Japan.

References

* The CDC influenza surveillance system collects five categories of information from 10 data sources. Viral surveillance: U.S. World Health Organization collaborating laboratories, the National Respiratory and Enteric Virus Surveillance System, and novel influenza A virus case reporting. Outpatient illness surveillance: U.S. Influenza Sentinel Provider Surveillance Network and the U.S. Department of Veterans Affairs/U.S. Department of Defense BioSense Outpatient Surveillance System. Mortality: 122 Cities Mortality Reporting System and influenza-associated pediatric mortality reports. Hospitalizations: Emerging Infections Program and New Vaccine Surveillance Network. Summary of geographic spread of influenza: state and territorial epidemiologist reports.

Levels of activity are 1) no activity; 2) sporadic: isolated laboratory-confirmed influenza cases or a laboratory-confirmed outbreak in one institution, with no increase in activity; 3) local: increased ILI, or at least two institutional outbreaks (ILI or laboratory-confirmed influenza) in one region with recent laboratory evidence of influenza in that region; virus activity no greater than sporadic in other regions; 4) regional: increased ILI activity or institutional outbreaks (ILI or laboratory-confirmed influenza) in at least two but less than half of the regions in the state with recent laboratory evidence of influenza in those regions; and 5) widespread: increased ILI activity or institutional outbreaks (ILI or laboratory-confirmed influenza) in at least half the regions in the state with recent laboratory evidence of influenza in the state.

^§ Defined as a temperature of >100.0°F (>37.8°C), oral or equivalent, and cough and/or sore throat, in the absence of a known cause other than influenza.

^¶ The national and regional baselines are the mean percentage of visits for ILI during noninfluenza weeks for the previous three seasons plus two standard deviations. A noninfluenza week is a week during which <10% of specimens tested positive for influenza. National and regional percentages of patient visits for ILI are weighted on the basis of state population. Use of the national baseline for regional data is not appropriate.

** The seasonal baseline proportion of P&I deaths is projected using a robust regression procedure in which a periodic regression model is applied to the observed percentage of deaths from P&I that were reported by the 122 Cities Mortality Reporting System during the preceding 5 years. The epidemic threshold is 1.645 standard deviations above the seasonal baseline.

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