Influenza activity - United States, 2014-15 season and composition of the 2015-16 influenza vaccine.

During the 2014-15 influenza season in the United States, influenza activity increased through late November and December before peaking in late December. Influenza A (H3N2) viruses predominated, and the prevalence of influenza B viruses increased late in the season. This influenza season, similar to previous influenza A (H3N2)-predominant seasons, was moderately severe with overall high levels of outpatient illness and influenza-associated hospitalization, especially for adults aged ≥65 years. The majority of circulating influenza A (H3N2) viruses were different from the influenza A (H3N2) component of the 2014-15 Northern Hemisphere seasonal vaccines, and the predominance of these drifted viruses resulted in reduced vaccine effectiveness. This report summarizes influenza activity in the United States during the 2014-15 influenza season (September 28, 2014-May 23, 2015) and reports the recommendations for the components of the 2015-16 Northern Hemisphere influenza vaccine.

Update: Influenza activity--United States, September 28, 2014-February 21, 2015.

Influenza activity in the United States began to increase in mid-November, remained elevated through February 21, 2015, and is expected to continue for several more weeks. To date, influenza A (H3N2) viruses have predominated overall. As has been observed in previous seasons during which influenza A (H3N2) viruses predominated, adults aged ≥65 years have been most severely affected. The cumulative laboratory-confirmed influenza-associated hospitalization rate among adults aged ≥65 years is the highest recorded since this type of surveillance began in 2005. This age group also accounts for the majority of deaths attributed to pneumonia and influenza. The majority of circulating influenza A (H3N2) viruses are different from the influenza A (H3N2) component of the 2014-15 Northern Hemisphere seasonal vaccines, and the predominance of these antigenically and genetically drifted viruses has resulted in reduced vaccine effectiveness. This report summarizes U.S. influenza activity* since September 28, 2014, and updates the previous summary.

Pneumonia among adults hospitalized with laboratory-confirmed seasonal influenza virus infection-United States, 2005-2008.

BACKGROUND: Influenza and pneumonia combined are the leading causes of death due to infectious diseases in the United States. We describe factors associated with pneumonia among adults hospitalized with influenza. METHODS: Through the Emerging Infections Program, we identified adults ≥ 18 years, who were hospitalized with laboratory-confirmed influenza during October 2005 through April 2008, and had a chest radiograph (CXR) performed. Pneumonia was defined as the presence of a CXR infiltrate and either an ICD-9-CM code or discharge summary diagnosis of pneumonia. RESULTS: Among 4,765 adults hospitalized with influenza, 1392 (29 %) had pneumonia. In multivariable analysis, factors associated with pneumonia included: age ≥ 75 years, adjusted odds ratio (AOR) 1.27 (95 % confidence interval 1.10-1.46), white race AOR 1.24 (1.03-1.49), nursing home residence AOR 1.37 (1.14-1.66), chronic lung disease AOR 1.37 (1.18-1.59), immunosuppression AOR 1.45 (1.19-1.78), and asthma AOR 0.76 (0.62-0.92). Patients with pneumonia were significantly more likely to require intensive care unit (ICU) admission (27 % vs. 10 %), mechanical ventilation (18 % vs. 5 %), and to die (9 % vs. 2 %). CONCLUSIONS: Pneumonia was present in nearly one-third of adults hospitalized with influenza and was associated with ICU admission and death. Among patients hospitalized with influenza, older patients and those with certain underlying conditions are more likely to have pneumonia. Pneumonia is common among adults hospitalized with influenza and should be evaluated and treated promptly.

Complications among adults hospitalized with influenza: a comparison of seasonal influenza and the 2009 H1N1 pandemic.

BACKGROUND: Persons with influenza can develop complications that result in hospitalization and death. These are most commonly respiratory related, but cardiovascular or neurologic complications or exacerbations of underlying chronic medical conditions may also occur. Patterns of complications observed during pandemics may differ from typical influenza seasons, and characterizing variations in influenza-related complications can provide a better understanding of the impact of pandemics and guide appropriate clinical management and planning for the future. METHODS: Using a population-based surveillance system, we compared clinical complications using International Classification of Diseases, Ninth Revision (ICD-9) discharge diagnosis codes in adults hospitalized with seasonal influenza (n = 5270) or 2009 pandemic influenza A(H1N1) (H1N1pdm09; n = 4962). RESULTS: Adults hospitalized with H1N1pdm09 were younger (median age, 47 years) than those with seasonal influenza (median age, 68 years; P < .01), and differed in the frequency of certain underlying medical conditions. Whereas there was similar risk for many influenza-associated complications, after controlling for age and type of underlying medical condition, adults hospitalized with H1N1pdm09 were more likely to have lower respiratory tract complications, shock/sepsis, and organ failure than those with seasonal influenza. They were also more likely to be admitted to the intensive care unit, require mechanical ventilation, or die. Young adults, in particular, had 2-4 times the risk of severe outcomes from H1N1pdm09 than persons of the same ages with seasonal influenza. CONCLUSIONS: Although H1N1pdm09 was thought of as a relatively mild pandemic, these data highlight the impact of the 2009 pandemic on the risk of severe influenza, especially among younger adults, and the impact this virus may continue to have.

Comparing clinical characteristics between hospitalized adults with laboratory-confirmed influenza A and B virus infection.

We challenge the notion that influenza B is milder than influenza A by finding similar clinical characteristics between hospitalized adult influenza-cases. Among patients treated with oseltamivir, length of stay and mortality did not differ by type of virus infection.

Patients hospitalized with laboratory-confirmed influenza during the 2010-2011 influenza season: exploring disease severity by virus type and subtype.

BACKGROUND: The 2010-2011 influenza season was dominated by influenza A(H3N2) virus, but influenza A(H1N1) pdm09 (pH1N1) and B viruses cocirculated. This provided an opportunity to explore within-season predictors of severity among hospitalized patients, avoiding biases associated with season-to-season differences in strain virulence, population immunity, and healthcare seeking. METHODS: Population-based, laboratory-confirmed influenza hospitalization surveillance data were used to examine the association between virus type/subtype and outcomes in children and adults. Multivariable analysis explored virus type/subtype, prompt antiviral treatment, medical conditions, and age as predictors for severity (intensive care unit admission or death). RESULTS: In children, pH1N1 (adjusted odds ratio [aOR], 2.19; 95% confidence interval [CI], 1.11-4.3), chronic metabolic disease (aOR, 5.23; 95% CI, 1.74-15.69), and neuromuscular disorder (aOR, 4.84; 95% CI, 2.02-11.58) were independently associated with severity. In adults, independent predictors were pH1N1 (aOR, 2.21; 95% CI, 1.66-2.94), chronic lung disease (aOR, 1.46, 95% CI, 1.12-1.89), and neuromuscular disorder (aOR, 1.68; 95% CI, 1.11-2.52).Antiviral treatment reduced the odds of severity among adults (aOR, 0.47; 95% CI, .33-.68). CONCLUSIONS: During the 2010-2011 season, pH1N1 caused more severe disease than H3N2 or B in hospitalized patients. Underlying medical conditions increased severity despite virus strain. Antiviral treatment reduced severity among adults. Our findings underscore the importance of influenza prevention.

Outbreak of variant influenza A(H3N2) virus in the United States.

BACKGROUND: Variant influenza virus infections are rare but may have pandemic potential if person-to-person transmission is efficient. We describe the epidemiology of a multistate outbreak of an influenza A(H3N2) variant virus (H3N2v) first identified in 2011. METHODS: We identified laboratory-confirmed cases of H3N2v and used a standard case report form to characterize illness and exposures. We considered illness to result from person-to-person H3N2v transmission if swine contact was not identified within 4 days prior to illness onset. RESULTS: From 9 July to 7 September 2012, we identified 306 cases of H3N2v in 10 states. The median age of all patients was 7 years. Commonly reported signs and symptoms included fever (98%), cough (85%), and fatigue (83%). Sixteen patients (5.2%) were hospitalized, and 1 fatal case was identified. The majority of those infected reported agricultural fair attendance (93%) and/or contact with swine (95%) prior to illness. We identified 15 cases of possible person-to-person transmission of H3N2v. Viruses recovered from patients were 93%-100% identical and similar to viruses recovered from previous cases of H3N2v. All H3N2v viruses examined were susceptible to oseltamivir and zanamivir and resistant to adamantane antiviral medications. CONCLUSIONS: In a large outbreak of variant influenza, the majority of infected persons reported exposures, suggesting that swine contact at an agricultural fair was a risk for H3N2v infection. We identified limited person-to-person H3N2v virus transmission, but found no evidence of efficient or sustained person-to-person transmission. Fair managers and attendees should be aware of the risk of swine-to-human transmission of influenza viruses in these settings.

Increase in rates of hospitalization due to laboratory-confirmed influenza among children and adults during the 2009-10 influenza pandemic.

BACKGROUND: The Emerging Infections Programs (EIP) network has conducted population-based surveillance for hospitalizations due to laboratory-confirmed influenza among children since 2003, with the network expanding in 2005 to include adults. METHODS: From 15 April 2009 through 30 April 2010, the EIP conducted surveillance among 22.1 million people residing in 10 states. Incidence rates per 100 000 population were calculated using US Census Bureau data. Mean historic rates were calculated on the basis of previously published and unpublished EIP data. RESULTS: During the 2009 pandemic of influenza A virus subtype H1N1 infection, rates of hospitalizations due to laboratory-confirmed influenza were 202, 88, 49, 31, 27, 36, 28, and 27 episodes per 100 000 among persons aged <6 months, 6-23 months, 2-4 years, 5-17 years, 18-49 years, 50-64 years, 65-74 years, and ≥75 years, respectively. Comparative mean rates from previous influenza seasons during which EIP conducted surveillance were 153, 53, 20, 6, 4, 8, 20, and 56 episodes per 100 000 among persons aged <6 months, 6-23 months, 2-4 years, 5-17 years, 18-49 years, 50-64 years, 65-74 years, and ≥75 years, respectively. CONCLUSIONS: During the pandemic, rates of hospitalization due to laboratory-confirmed influenza among individuals aged 5-17 years and 18-49 years increased 5-fold and 6-fold, respectively, compared with mean rates from previous influenza seasons. Hospitalization rates for other pediatric and adult age groups increased, compared with mean rates from previous influenza seasons, whereas the rate among individuals aged ≥75 years decreased.

Reduced influenza antiviral treatment among children and adults hospitalized with laboratory-confirmed influenza infection in the year after the 2009 pandemic.

Influenza antiviral treatment is recommended for all persons hospitalized with influenza virus infection. During the 2010-2011 influenza season, antiviral treatment of children and adults hospitalized with laboratory-confirmed influenza declined significantly compared with treatment during the 2009 pandemic (children, 56% vs 77%; adults, 77% vs 82%; both P < .01).

Influenza-associated hospitalizations by industry, 2009-10 influenza season, United States.

In response to pandemic (H1N1) 2009, data were collected on work status and industry of employment of 3,365 adults hospitalized with laboratory-confirmed influenza during the 2009-10 influenza season in the United States. The proportion of workers hospitalized for influenza was lower than their proportion in the general population, reflecting underlying protective characteristics of workers compared with nonworkers. The most commonly represented sectors were transportation and warehousing; administrative and support and waste management and remediation services; health care; and accommodation and food service.

Seasonal and 2009 pandemic influenza A (H1N1) virus infection during pregnancy: a population-based study of hospitalized cases.

We sought to describe characteristics of hospitalized reproductive-aged (15-44 years) women with seasonal (2005/2006 through 2008/2009) and 2009 pandemic influenza A (H1N1) virus infection. We used population-based data from the Emerging Infections Program in 10 US states, and compared characteristics of pregnant (n = 150) and nonpregnant (n = 489) seasonal, and pregnant (n = 489) and nonpregnant (n = 1088) pandemic influenza cases using χ(2) and Fisher's exact tests. Pregnant women represented 23.5% and 31.0% of all reproductive-aged women hospitalized for seasonal and pandemic influenza, respectively. Significantly more nonpregnant than pregnant women with seasonal (71.2% vs 36.0%) and pandemic (69.7% vs 31.9%) influenza had an underlying medical condition other than pregnancy. Antiviral treatment was significantly more common with pandemic than seasonal influenza for both pregnant (86.5% vs 24.0%) and nonpregnant (82.0% vs 55.2%) women. Pregnant women comprised a significant proportion of influenza-hospitalized reproductive-aged women, underscoring the importance of influenza vaccination during pregnancy.

Hospital-onset influenza hospitalizations--United States, 2010-2011.

BACKGROUND: Seasonal influenza is responsible for more than 200,000 hospitalizations each year in the United States. Although hospital-onset (HO) influenza contributes to morbidity and mortality among these patients, little is known about its overall epidemiology. OBJECTIVE: We describe patients with HO influenza in the United States during the 2010-2011 influenza season and compare them with community-onset (CO) cases to better understand factors associated with illness. METHODS: We identified laboratory-confirmed, influenza-related hospitalizations using the Influenza Hospitalization Surveillance Network (FluSurv-NET), a network that conducts population-based surveillance in 16 states. CO cases had laboratory confirmation ≤ 3 days after hospital admission; HO cases had laboratory confirmation > 3 days after admission. RESULTS: We identified 172 (2.8%) HO cases among a total of 6,171 influenza-positive hospitalizations. HO and CO cases did not differ by age (P = .22), sex (P = .29), or race (P = .25). Chronic medical conditions were more common in HO cases (89%) compared with CO cases (78%) (P < .01), and a greater proportion of HO cases (42%) than CO cases (17%) were admitted to the intensive care unit (P < .01). The median length of stay after influenza diagnosis of HO cases (7.5 days) was greater than that of CO cases (3 days) (P < .01). CONCLUSION: HO cases had greater length of stay and were more likely to be admitted to the intensive care unit or die compared with CO cases. HO influenza may play a role in the clinical outcome of hospitalized patients, particularly among those with chronic medical conditions.

Preliminary results of 2009 pandemic influenza surveillance in the United States using the Aggregate Hospitalization and Death Reporting Activity.

BACKGROUND: To augment established influenza surveillance systems in the United States, the Centers for Disease Control and Prevention and the Council of State and Territorial Epidemiologists implemented the Aggregate Hospitalization and Death Reporting Activity (AHDRA) in August 2009. The AHDRA was designed to meet increased demands for timely and detailed information describing illness severity during the 2009 H1N1 influenza A (pH1N1) pandemic response. OBJECTIVES: We describe the implementation of AHDRA and provide preliminary results from this new surveillance activity. METHODS: All 50 US states were asked to report influenza-associated hospitalizations and deaths to AHDRA each week using either a laboratory-confirmed or syndromic surveillance definition. Aggregate counts were used to calculate age-specific weekly and cumulative rates per 100,000, and laboratory-confirmed reports were used to estimate the age distribution of pH1N1 influenza-associated hospitalizations and deaths. RESULTS: From August 30, 2009, through April 6, 2010, AHDRA identified 41,689 laboratory-confirmed influenza-associated hospitalizations and 2096 laboratory-confirmed influenza-associated deaths. Aggregate Hospitalization and Death Reporting Activity rates peaked earlier than hospitalization and death rates seen in previous influenza seasons with other surveillance systems, and the age distribution of cases revealed a tendency for hospitalizations and deaths to occur in persons <65 years for age. CONCLUSIONS: Aggregate Hospitalization and Death Reporting Activity laboratory-confirmed reports provided important information during the 2009 pandemic response. Aggregate Hospitalization and Death Reporting Activity syndromic reports were marked by low representativeness and specificity and were therefore less useful. The AHDRA was implemented quickly and may be a useful surveillance system to monitor severe illness during future influenza pandemics.

Racial and ethnic disparities in hospitalizations and deaths associated with 2009 pandemic Influenza A (H1N1) virus infections in the United States.

PURPOSE: Concerns have been raised regarding possible racial-ethnic disparities in 2009 pandemic influenza A (H1N1) (pH1N1) illness severity and health consequences for U.S. minority populations. METHODS: Using data from the Centers for Disease Control and Prevention's Behavioral Risk Factor Surveillance System, Emerging Infections Program Influenza-Associated Hospitalization Surveillance, and Influenza-Associated Pediatric Mortality Surveillance, we calculated race-ethnicity-specific, age-adjusted rates of self-reported influenza-like illness (ILI) and pH1N1-associated hospitalizations. We used χ(2) tests to evaluate racial-ethnic disparities in ILI-associated health care-seeking behavior and pH1N1 hospitalization. To evaluate pediatric deaths, we compared racial-ethnic proportions of deaths against U.S. population distributions. RESULTS: Prevalence of self-reported ILI was lower among Hispanics (6.5%), higher among American Indians/Alaska Natives (16.2%), and similar among non-Hispanic blacks (7.7%) compared with non-Hispanic whites (8.5%). No racial-ethnic differences were identified in ILI-associated health care-seeking behavior. Age-adjusted pH1N1-associated Emerging Infections Program hospitalization rates were higher among all minority populations (range: 8.1-10.9/100,000 population) compared with non-Hispanic whites (3.0/100,000). The proportion of pH1N1-associated pediatric deaths was higher than expected among Hispanics (31%) and lower than expected among non-Hispanic whites (45%) given the proportions of the U.S. population they comprise (22% and 58%, respectively). CONCLUSIONS: Racial-ethnic disparities in pH1N1-associated hospitalizations and pediatric deaths were identified. Vaccination remains the primary intervention for preventing influenza.

Children with asthma hospitalized with seasonal or pandemic influenza, 2003-2009.

OBJECTIVE: To describe the characteristics and clinical courses of asthmatic children hospitalized with seasonal or 2009 pandemic H1N1 influenza and compare complications by influenza type. METHODS: During the 2003-2009 influenza seasons and the 2009 pandemic, we conducted surveillance of 5.3 million children aged 17 years or younger for hospitalization with laboratory-confirmed influenza and identified those with asthma (defined as those aged 2-17 years with a history of asthma in their medical record or a discharge code for acute asthma exacerbation or status asthmaticus). We collected data from medical records on medical history and clinical course; data on asthma severity and control were not routinely collected. RESULTS: During the 2003-2009 influenza seasons, 701 (32%) of 2165 children hospitalized with influenza had asthma; during the 2009 pandemic, 733 (44%) of 1660 children had asthma. The median age of the asthmatic children was 7 years, and 73% had no additional medical conditions. Compared with asthmatic children with seasonal influenza, a higher proportion with 2009 pandemic H1N1 influenza required intensive care (16% vs 22%; P=.01) and were diagnosed with pneumonia (40% vs 46%; P=.04), whereas equal proportions had respiratory failure (5% vs 5%; P=.8) and died (1% vs 1%; P=.4). More asthmatic children with influenza A (seasonal or pandemic) had diagnoses of asthma exacerbations compared with those with influenza B (51% vs 29%; P<.01). CONCLUSIONS: The majority of asthmatic children hospitalized with influenza have no additional medical conditions. Complications such as pneumonia and need for intensive care occur in a substantial proportion, highlighting the importance of influenza prevention through vaccination among asthmatic children.