Influenza Activity --- United States and Worldwide, May 18--September 19, 2008

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During May 18--September 19, 2008, influenza A (H1), influenza A (H3), and influenza B viruses were detected worldwide and were identified sporadically in the United States. This report summarizes influenza activity in the United States and worldwide since the last update (1) and reviews the new influenza vaccine recommendations for the upcoming season. Influenza viruses circulating this summer appear antigenically similar to the strains included in the 2008--09 influenza vaccine. Recent antiviral resistance data are limited, but oseltamivir resistance among influenza A (H1N1) viruses might persist during the 2008--09 influenza season.

United States

In the United States, CDC uses 10 different systems for national influenza surveillance (2). Seven of these systems are operated year-round and provided data for this report.*

During May 18--September 19, 2008, World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System collaborating laboratories in the United States tested 19,774 specimens collected from the United States for influenza viruses; 147 (<1%) were positive. Of these, 81 (55%) were influenza A viruses, and 66 (45%) were influenza B viruses. Of the influenza A viruses, five (6%) were influenza A (H1) viruses, 17 (21%) were influenza A (H3) viruses, and 59 (73%) were not subtyped. Twenty-two states, representing the nine public health surveillance regions, reported influenza viruses. Among positive tests, 66% were reported from Hawaii and Florida: 43 (29%) from Hawaii and 54 (37%) from Florida. The majority of the viruses were reported during late May through July. Only 28 viruses (20 influenza A and eight influenza B viruses) were reported in August and early September.

During May 18--September 19, 2008, the weekly percentage of visits to U.S. sentinel providers for influenza-like illness remained below the national baseline of 2.2% (range: 0.5%--0.9%) according to data from the U.S. Influenza Sentinel Provider Surveillance Network. The weekly percentage of visits for acute respiratory illness to sentinel providers from the BioSense Surveillance System also remained below the national baseline of 3.2% (range: 1.3%--1.8%).

Data from the 122 Cities Mortality Reporting System indicate that the percentage of deaths attributed to pneumonia and influenza was below the epidemic threshold throughout the summer. A total of 86 influenza-associated deaths were reported during the 2007--08 influenza season; only one of these deaths occurred since May 18, 2008. No human cases of novel influenza A have been reported to the National Notifiable Diseases Surveillance System during the summer months.

Worldwide

During May 18--September 19, 2008, influenza A (H1), influenza A (H3), and influenza B viruses were detected worldwide. In Africa, influenza A (H1) viruses predominated. In Asia, influenza A (H1), A (H3), and B viruses were detected, and the predominant virus subtype varied by country. In South America, influenza A (H1) and influenza B viruses were detected. In North America, Europe, and Oceania, influenza A (H1), A (H3), and B viruses were detected sporadically.

Antigenic Characterization of Influenza Virus Isolates

The WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, located at CDC, analyzes influenza virus isolates received from laboratories worldwide. Of 55 influenza A (H1N1) viruses that were collected and analyzed during May 18--September 19, one came from the United States, 44 from South America, five from Europe, and five from Asia. Fifty-three (96%) were antigenically similar to A/Brisbane/59/2007, the H1N1 component of the 2008--09 influenza vaccine for the Northern Hemisphere. Of the 15 influenza A (H3) viruses that were characterized, one came from the United States, seven from Latin America, one from Europe, and six from Asia. All 15 were antigenically similar to A/Brisbane/10/2007, the H3N2 component of the 2008--09 influenza vaccine.

Circulating influenza B viruses can be divided into two antigenically distinct lineages that have been detected worldwide since March 2001, represented by B/Yamagata/16/88 and B/Victoria/02/87 viruses. Of the 28 influenza B isolates collected during May 18--September 19 and characterized at CDC, 23 (82%) (one from the United States, 19 from South America, one from Europe, and two from Asia) belong to the B/Yamagata lineage. All of the B/Yamagata-lineage viruses are similar to B/Florida/04/2006, the recommended influenza B component for the 2008--09 influenza vaccine for the Northern Hemisphere. The remaining five influenza B viruses (one from Europe and four from South America) belong to the B/Victoria lineage.

Resistance Profiles of Influenza Virus Isolates

During the 2007--08 influenza season (September 30, 2007--May 17, 2008), the prevalence of oseltamivir-resistant influenza A (H1N1) viruses was 10.9%; no resistance to oseltamivir was detected among influenza A (H3N2) or influenza B viruses during that season (1). During May 18--September 19, 2008, the WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza at CDC, a member of the WHO Global Influenza Surveillance Network, received 187 isolates that were collected during this period and analyzed them for neuraminidase resistance. Of these isolates, 185 were sensitive to zanamivir, and two required additional testing. Of the 86 influenza A (H1N1) viruses received from 14 countries and analyzed for oseltamivir resistance, 40 (46.5%) were resistant. Only five of the influenza A (H1N1) viruses that were analyzed were from the United States; two of these viruses were resistant to oseltamivir. None of the 27 influenza A (H3) viruses analyzed for neuraminidase resistance were resistant to oseltamivir.

Human Infections with Avian Influenza A (H5N1) Viruses

During May 18--September 19, 2008, 12 persons with avian influenza A (H5N1), nine of whom died, were reported to WHO from Indonesia, Egypt, and Bangladesh (2). Since December 1, 2003, 387 human cases of avian influenza A (H5N1) have been reported from Asia and Africa. No human cases have been identified in North America or South America.

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 Gubareva, PhD, J Bresee, MD, A Klimov, PhD, N Cox, PhD, Influenza Div, National Center for Immunization and Respiratory Diseases; F Dawood, MD, EIS Officer, CDC.

Editorial Note:

During May 18--September 19, 2008, influenza A (H1), influenza A (H3), and influenza B viruses were detected worldwide. The influenza virus type and subtype that will predominate, the severity of influenza-related disease activity, and the level of antiviral resistance during the 2008--09 influenza season cannot be forecast in advance of the influenza season. However, of the isolates submitted for antigenic characterization during May through early September from Northern and Southern Hemisphere countries, the majority were antigenically similar to the viruses contained in the 2008--09 influenza vaccine for the Northern Hemisphere.

Annual influenza vaccination remains the best method for preventing influenza and its potentially severe complications. When vaccine strains are well matched to influenza viruses circulating during the influenza season, vaccine effectiveness typically exceeds 50% and can be as high as 70%--90% in healthy adults. Data from an interim within-season vaccine effectiveness study during the 2007--08 influenza season showed that overall vaccine effectiveness for prevention of medically attended, laboratory-confirmed influenza infection was 44%, despite a suboptimal match between two of the three vaccine strains and the predominant circulating strain (3). Vaccine effectiveness against influenza A (H3N2), the subtype most frequently associated with increases in influenza-related complications and deaths, was 58% (3). These data demonstrate that influenza vaccination can offer substantial benefit, even in years where the match between circulating strains and vaccine strains is suboptimal.

The Advisory Committee on Immunization Practices (ACIP) recently expanded its recommendations for influenza vaccination to include all persons aged 6 months--18 years. Vaccine providers should begin vaccinating all persons in this population during the 2008--09 influenza season, if feasible, but this recommendation should be fully implemented no later than the 2009--10 influenza season (5). In addition, vaccination efforts should continue to be targeted toward persons who are at increased risk for influenza complications, including 1) children aged 6 months--4 years, 2) adults aged >50 years, 3) children and adults of any age who are immunosuppressed or have other chronic medical conditions that might predispose them to influenza-related complications, 4) persons who reside in nursing homes or chronic care facilities, or 5) females who will be pregnant during the influenza season (4). Household and other close contacts of persons at greater risk for influenza infection, including health-care workers and contacts and out-of-home caregivers for all children aged <5 years, also should be vaccinated (4). Health-care providers should begin offering influenza vaccination as soon as vaccine becomes available and should continue vaccination efforts throughout the influenza season (4).

During the 2007--08 influenza season, an increase in the prevalence of influenza A (H1N1) viruses resistant to the neuraminidase inhibitor oseltamivir was first observed (5). However, no oseltamivir resistance among influenza A (H3N2) or influenza B viruses was demonstrated, and the overall prevalence of oseltamivir resistance among circulating influenza viruses in the United States was low. Resistance data from the United States for the summer months of 2008 and data from Southern Hemisphere countries are limited, and the extent to which these data forecast the prevalence of oseltamivir resistance among influenza A (H1N1) viruses during the 2008--09 influenza season in the United States is uncertain. Influenza A (H1N1) viruses resistant to oseltamivir have been sensitive to zanamivir. Adamantanes (rimantidine and amantadine) are not recommended currently for treatment or chemoprophylaxis because the prevalence of resistance to these antiviral medications remains high among influenza A (H3N2) viruses (4).

Enhanced surveillance for oseltamivir-resistant viruses is ongoing at CDC and will continue during the 2008--09 influenza season. At this time, oseltamivir and zanamivir remain the medications recommended for treatment and chemoprophylaxis of influenza (4). Randomized controlled clinical trials conducted before the emergence of oseltamivir-resistant influenza viruses have demonstrated that neuraminidase inhibitors such as oseltamivir and zanamivir reduce the duration and severity of illness if started within 48 hours of illness onset (6) and are approximately 80% effective in preventing illness among close contacts of patients with influenza (7). Multiple observational studies have found that treatment with oseltamivir improves outcomes associated with influenza complications among hospitalized patients with laboratory-confirmed influenza, including a reduction in mortality (8), and hospital length of stay (9). Clinicians providing care for patients during the influenza season, especially those requiring hospitalization for respiratory illness, should consider influenza as a possible cause of illness and evaluate the potential benefits of treating influenza with neuraminidase inhibitors. Recommendations for use of neuraminidase inhibitors might be revised as the 2008--09 influenza season progresses if surveillance data indicate an increase in the prevalence of oseltamivir-resistant influenza viruses in the United States.

Vaccination to prevent influenza is the cornerstone of prevention efforts, and influenza vaccination can prevent infection regardless of whether circulating viruses are sensitive or resistant to antiviral medications. To reduce the burden of influenza in the United States, CDC continues to recommend a three-pronged approach: 1) influenza vaccination, 2) use of neuraminidase inhibitor antiviral medications when indicated for treatment or prevention, and 3) use of other measures to decrease the spread of influenza, including promotion of hand hygiene, respiratory hygiene, cough etiquette, and staying home from work and school when ill.

Influenza surveillance reports for the United States are posted on the Internet weekly during October--May 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

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