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Influenza Activity --- United States and Worldwide, June 13--September 25, 2010

From June 13 to September 25, 2010, the United States experienced low levels of influenza activity. During this period, typical seasonal patterns of influenza activity occurred in the Southern Hemisphere; in addition, influenza activity was observed in the tropical regions, with a mix of 2009 influenza A (H1N1), influenza A (H3N2), and influenza B viruses cocirculating. This report summarizes influenza activity in the United States and worldwide since the update published on July 30, 2010 (1).

United States

In the United States, CDC collaborates with federal, state, and local partners to collect influenza information via multiple surveillance systems. During the summer period of June 13--September 25, 2010, influenza surveillance systems that were in operation included: 1) World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories, which conduct viral surveillance; 2) the U.S. Outpatient Influenza-Like Illness Surveillance Network (ILINet), which reports outpatient visits for influenza-like illness (ILI)*; 3) the BioSense Program, which contains chief complaint and discharge diagnosis data on emergency department visits due to ILI; 4) pneumonia and influenza deaths from the 122 Cities Mortality Reporting System; 5) influenza-associated pediatric deaths from the Influenza-Associated Pediatric Mortality Reporting System; and 6) reports of novel influenza A virus cases from the National Notifiable Disease Surveillance System (NNDSS).

During June 13--September 25, WHO and NREVSS collaborating laboratories analyzed a total of 25,833 respiratory specimens from the United States. Of these specimens, 326 (1.3%) tested positive for influenza; 261 (80%) were influenza A viruses, and 65 (20%) were influenza B viruses. Of the influenza A viruses, 185 (71%) were subtyped; 130 (70%) were H3, and 55 (30%) were 2009 influenza A (H1N1). The percentage of specimens positive for influenza varied slightly over time, with <1% of tested specimens positive each week until late July and 1.0%--2.6% of tested specimens positive from late July through late September. The largest proportion of positive samples came from the southeastern United States (U.S. Department of Health and Human Services Region 4: Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, and Tennessee) (40%), followed by western states (Region 9: Arizona, California, Hawaii, and Nevada) (14%), and the Midwestern states (Region 5: Illinois, Indiana, Missouri, Minnesota, Ohio, and Wisconsin) (10%).

The weekly percentage of outpatient visits to ILINet providers for ILI during June 13--September 25 ranged from 0.7% to 1%, which is below the national baseline of 2.3%. This was consistent with data provided by the BioSense Program, which indicated a low level of ILI visits in emergency departments (0.7%--1.7%). Mortality attributed to pneumonia and influenza, as reported by the 122 Cities Mortality Reporting System, was below the epidemic threshold§ throughout the period covered by this report except for 3 nonconsecutive weeks. No influenza-associated pediatric deaths were reported to the Influenza-Associated Pediatric Mortality Reporting System, and no human cases of novel influenza A were reported to NNDSS for the period June 13--September 25.

In early July, two outbreaks of influenza A (H3) were reported in Iowa. The first outbreak caused ILI in four of 13 members of a college sports team; three of these four were found to be positive for influenza A by rapid test, and two of three were later confirmed to have influenza A (H3) infection by polymerase chain reaction (PCR). The second outbreak involved nine of 12 children and one parent with ILI in a child care setting; two children were rapid-test positive for influenza A, and one was PCR-positive for influenza A (H3) infection. None of these patients had a direct history of recent travel, and no epidemiologic links were identified between the two Iowa outbreaks. The Maryland Department of Health and Mental Hygiene reported an outbreak of influenza B during the first half of August, which involved children visiting the United States as part of an international exchange program. Approximately 35 of 400 children had ILI, and six sought health care at a local hospital. Influenza B was identified in eight cases.


From June 13 to September 25, influenza A (H3), 2009 influenza A (H1N1), and influenza B were identified worldwide. Seasonal influenza A (H1) viruses were reported only rarely. Reports by the WHO Global Influenza Surveillance Network (2) showed that influenza B was the viral type most commonly identified until early July, when 2009 influenza A (H1N1) became the predominant strain. Since late August, influenza A (H3) has been the viral subtype most commonly identified. The WHO Global Surveillance Network reported that the predominant viral type or subtype identified from Asia (44% of analyzed specimens) was H3, followed by 2009 influenza A (H1N1) (32% of specimens) and influenza B (13%), but this varied by country. In Africa, influenza B (44%) viruses were the most common, followed by influenza A (H3) (31%). In South America, 2009 influenza A (H1N1) (32% of specimens) and influenza B (32%) predominated. In North America, 69% of isolates were identified as influenza A (H3) viruses. The 2009 influenza A (H1N1) virus predominated in Oceania (63% of specimens). In Europe a small number of cases were reported, and most specimens (59%) were influenza B, followed by 2009 influenza A (H1N1) (19%).

Antigenic Characterization of Influenza Virus Isolates

Virus isolates from around the world are received and analyzed at the WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, located at CDC. Seventy-nine 2009 influenza A (H1N1) viruses were collected and analyzed from June 13 to September 25 (16 from the United States, 33 from South America, 13 from Asia, 15 from Africa, and two from Oceania); all but one (99%) were antigenically similar to A/California/7/2009 (2009 influenza A H1N1), which is the H1N1 component of the 2010--11 Northern Hemisphere vaccine recommended by WHO. Of the 101 influenza A (H3) viruses characterized (24 from the United States, one from North America, 42 from South America, seven from Asia, and 27 from Africa), 97 (96%) were antigenically similar to A/Perth/16/2009, the H3N2 component of the 2010--11 Northern Hemisphere vaccine, and four (4%) showed reduced titers with antisera produced against A/Perth/16/2009. Finally, of 45 influenza B isolates characterized at CDC during this period, 34 (76%) belong to the B/Victoria lineage (12 from North America [including nine from the United States], 16 from South America, and six from Asia), and 30 of 34 (88%) were antigenically similar to B/Brisbane/60/2008, which is the influenza B component of the 2010--11 Northern Hemisphere vaccine (B/Victoria lineage). The remaining 11 influenza B viruses (three from the United States, three from South America, and five from Asia) belong to the B/Yamagata lineage.

Resistance Profiles of Influenza Virus Isolates

The WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza at CDC tested isolates collected during June 13--September 25, 2010, for resistance to antiviral medications. Of 232 isolates tested for neuraminidase inhibitor resistance, 178 were received from 18 foreign countries (52 were 2009 influenza A [H1N1], 85 were H3N2, and 41 were influenza B), and 54 were collected from the United States (17 were 2009 influenza A [H1N1], 25 were H3N2, and 12 were influenza B). None of the 232 tested viruses were found to be resistant to either oseltamivir or zanamivir. Of 180 isolates tested for adamantane resistance (139 foreign isolates and 41 isolates from the United States), 100% were found to be resistant to adamantanes.

Human Infection with Avian Influenza A (H5N1) Virus Isolates

During June 13--September 25, six cases of human infection with avian influenza A (H5N1) were reported to WHO from Egypt and Indonesia; five of these cases were fatal (3). Since December 2003, a total of 505 human cases (resulting in 300 fatalities) have been reported from 15 countries in Asia and Africa. No human cases of avian influenza (H5N1) infection have been identified from North America or South America.

Reported by

WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza. M Jhung, MD, L Brammer, MPH, S Epperson, MPH, L Blanton, MPH, R Dhara, MPH, L Finelli, DrPH, L Gubareva, PhD, A Klimov, PhD, T Wallis, MS, M Okomo-Adhiambo, PhD, J Bresee, MD, N Cox, PhD, National Center for Immunization and Respiratory Diseases; A Storms, MD, EIS Officer, CDC.

Editorial Note

During June 13--September 25, 2010, influenza A (H3), 2009 influenza A (H1N1), and influenza B were present worldwide and also were detected in the United States. Despite the overall low levels of influenza virus circulation throughout the summer in the United States, there have been clusters of H3N2 disease, as well as sporadic cases of infection with 2009 influenza A (H1N1) and influenza B viruses. These outbreaks are typical of sporadic outbreaks of influenza during the summer months.

Although it is difficult to predict which influenza virus strains will predominate during the upcoming influenza season, antigenic characterization of viral isolates submitted during the summer demonstrated that the majority are antigenically similar to the influenza vaccine candidates included in the 2010--11 Northern Hemisphere vaccine. Vaccination continues to be the best method for preventing influenza and its associated complications. Vaccine manufacturers project ample supplies of influenza vaccine in the United States for the 2010--11 influenza season. Guidelines for influenza vaccination, published in July 2010, were revised this year by the Advisory Committee on Immunization Practices (ACIP) and call for annual vaccination of all persons aged ≥6 months (4). Because children aged <6 months are too young for influenza vaccination but are one of the groups at highest risk for influenza-related hospitalization, vaccination of household contacts and caregivers of young children is recommended to reduce their risk for influenza illness (4). Vaccination of pregnant women might also provide protection to infants aged <6 months in addition to protecting women during pregnancy and the postpartum period (5,6).

On August 5, 2010, ACIP recommended that trivalent inactivated influenza vaccine (brand name Afluria) manufactured by CSL Biotherapies, not be administered to children aged 6 months--8 years (7). This recommendation was based on findings of an increased risk for fever in children aged <9 years and febrile seizures among children aged <5 years in Australia and New Zealand associated with receipt of CSL Southern Hemisphere trivalent inactivated vaccine (TIV). If no other age-appropriate, licensed seasonal influenza vaccine is available for a child aged 5--8 years who has a medical condition that increases their risk for influenza complications, Afluria may be given, and providers should discuss the benefits and risks of influenza vaccination with the parents or caregivers before administering Afluria (7). No cause has been identified to date to explain these adverse reactions in children who received CSL TIV. Safety of seasonal influenza vaccine will continue to be monitored through the season.

Antiviral medications for treatment and prevention of influenza continue to be an important adjunct to vaccination as part of efforts to reduce serious morbidity and mortality related to influenza (8). Because all H3N2 and 2009 influenza A (H1N1) viruses submitted for testing are resistant to adamantanes (amantadine and rimantidine), adamantane use is not recommended. However, influenza B, influenza A (H3N2), and 2009 influenza A (H1N1) viruses remain susceptible to neuraminidase inhibitors (oseltamivir and zanamivir), and these drugs are recommended agents for influenza prophylaxis and treatment when indicated.

Additional information on influenza testing, use of antiviral medications, influenza infection control guidelines, and vaccination recommendations are available online from CDC at Beginning October 15, 2010, weekly influenza surveillance reports also will be available online at


This report is based, in part, on data contributed by state and territorial health departments and state public health laboratories; WHO collaborating laboratories; the US Outpatient Influenza-Like Illness Surveillance Network; the 122 Cities Mortality Reporting System; Maryland Dept of Health and Mental Hygiene; Iowa Dept of Public Health; National Influenza Centers, Communicable Diseases, Surveillance, and Response, World Health Organization, Geneva, Switzerland; J McCauley, PhD, R Daniels, PhD, WHO Collaborating Center for Reference and Research on Influenza, National Institute of Medical Research, London, England; A Kelso, PhD, I Barr, PhD, WHO Collaborating Center for Reference and Research on Influenza, Parkville, Australia; and M Tashiro, MD, T Odagiri, PhD, WHO Collaborating Center for Reference and Research on Influenza, National Institute of Infectious Diseases, Tokyo, Japan.


  1. CDC. Update: influenza activity---United States, 2009--10 season. MMWR 2010;59:901--8.
  2. World Health Organization. Global Influenza Surveillance Network. FluNet. Available at Accessed October 1, 2010.
  3. World Health Organization. Confirmed human cases of avian influenza A (H5N1). Geneva, Switzerland: World Health Organization; 2010. Available at Accessed October 1, 2010.
  4. CDC. Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2010. MMWR 2010;59(No. RR-8).
  5. Zaman K, Roy E, Arifeen SE, et al. Effectiveness of maternal influenza immunization in mothers and infants. N Engl J Med 2008;359:1555--64.
  6. Eick AA, Uyeki TM, Klimov A, et al. Maternal influenza vaccination and effect on influenza virus infection in young infants. Arch Pediatr Adolesc Med 2010. Epub ahead of print (October 4, 2010). Available at Accessed October 5, 2010.
  7. CDC. Update: recommendations of the Advisory Committee on Immunization Practices (ACIP) regarding use of CSL seasonal influenza vaccine (Afluria) in the United States during 2010--11. MMWR 2010;59:989--92.
  8. Harper SA, Bradley JS, Englund JA, et al. Seasonal influenza in adults and children--diagnosis, treatment, chemoprophylaxis, and institutional outbreak management: clinical practice guidelines of the Infectious Diseases Society of America. Clin Infect Dis 2009;48:1003--32.

* 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 baseline was calculated as the mean percentage of patient visits for ILI during noninfluenza weeks for the preceding three influenza seasons, plus 2 standard deviations. Noninfluenza weeks are those in which <10% of laboratory specimens are positive for influenza. Wide variety in regional data precludes calculating region-specific baselines; therefore, applying the national baseline to regional data is inappropriate. National and regional percentages of patient visits for ILI are weighted on the basis of state population.

§ The epidemic threshold is 1.645 standard deviations above the seasonal baseline. The seasonal baseline is projected using a robust regression procedure that applies a periodic regression model to the observed percentage of deaths from pneumonia and influenza during the preceding 5 years.

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