2009 H1N1 Influenza Pandemic: Field and ...

SUPPLEMENT ARTICLE

2009 H1N1 Influenza Pandemic: Field and Epidemiologic Investigations in the United States at the Start of the First Pandemic of the 21st Century David L. Swerdlow,1 Lyn Finelli,2 and Carolyn B. Bridges2 1National Center for Immunization and Respiratory Diseases and 2Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia

The detection of any newly emerging threat requires initial assessments of the risk and parameters of disease spread, population susceptibility, and clinical spectrum and severity. Although properties of influenza viruses have been extensively studied over the past century, the spectrum of severity and susceptibility has varied widely among the different pandemic strains and can even vary substantially among seasonal influenza strains. The epidemiologic patterns of the 3 prior twentieth-century pandemics have been distinctly different from one another, including with respect to overall disease burden, case fatality ratios, and the most severely affected age and risk groups. The 2 most recent prior pandemics,

The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention. Correspondence: David L. Swerdlow, M.D., 1600 Clifton Rd, MS-A20, Atlanta, GA 30333 ([email protected]). Clinical Infectious Diseases 2011;52(S1):S1–S3 Published by Oxford University Press on behalf of the Infectious Diseases Society of America 2011. 1058-4838/2011/52S1-0001$37.00 DOI: 10.1093/cid/ciq005

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In April, 2009, CDC identified a novel influenza A virus detected from 2 children with febrile respiratory illness in southern California. The virus quickly emerged and spread globally and by 5 May, confirmed cases had been reported from 41 US states and 21 countries worldwide. Since the virus had never been identified before, little was known about the characteristics of the virus and how the pandemic would progress—would it be severe, how efficient would viral transmission be, would transmission be sustainable, what would the spectrum of illness, factors associated with severe disease, and causes of death be, and what risk groups would be most affected? Field investigations and epidemiologic studies in the United States and elsewhere were critical in helping answer these questions and characterizing the virus and the pandemic. This supplement will report results from field and epidemiologic investigations conducted in the United States since April 2009.

which took place in 1957 and 1968, were both caused by novel avian-human reassortant influenza viruses, and in both pandemics, the pathogen was identified first in Asia, and some information was available from other countries about the disease spectrum before these pandemics spread widely in the United States. [1]. However, with the 2009 influenza A (H1N1) (pH1N1) pandemic, no virologic, clinical, or epidemiologic information was available before its initial detection in the United States in April 2009. The virus was documented to spread very rapidly throughout the world, with laboratoryconfirmed cases detected in .70 countries by early June 2009, just 6 weeks after the detection of the first 2 cases in Southern California [2]. Thus, many epidemiologic questions, such as length of viral shedding, the ability to efficiently spread among people, and severity of disease, required rapid evaluation to inform decisionmaking about the use of mitigation efforts and other counter measures, such as antiviral mediations (Table 1). Initial investigations were published within the first few weeks of identification of the new strain, including initial genetic characterization of the virus [3], clinical 2009 H1N1 Influenza Pandemic

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Table 1. Epidemiologic Questions at the Initiation of the Pandemic Epidemiologic question Severity of the pandemic Efficiency of viral transmission Community attack rate Secondary household attack rate Generation time (reflects incubation period) Viral shedding Spectrum of illness Factors associated with severe illness and causes of death Identification of high-risk groups Underlying conditions, co-morbidities Age groups Pregnancy Health care workers Seasonal vaccine effectiveness

Figure 1. Confirmed and probable 2009 pandemic influenza A (H1N1) cases by report date, United States, as of 23 July 2009 (n 5 43,771). S2

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and epidemiologic description of initial cases [4], and population susceptibility [5]. Data from existing influenza surveillance systems, critical collaborations between the Centers for Disease Control and Prevention (CDC), state and local health departments, and clinicians, were closely monitored, and other surveillance systems were enhanced. Field investigations and epidemiologic investigations were initiated in the United States and elsewhere in collaboration with state and local health departments, clinicians, and academic medical centers to help characterize the virus and the pandemic (Figure 1). In this supplement, we include reports of surveillance, field investigations, and epidemiologic investigations conducted in response to the pandemic in the United States. A unique

perspective on the response to the 2009 H1N1 pandemic is presented by Dr. David Sencer, the CDC Director during the US swine flu vaccination campaign in 1976. Planning for and responding to the pandemic is reviewed by several members of CDCs pH1N1 pandemic response leadership team (Schuchat). The overall epidemiology of the epidemic in the United States is described by Jhung et al. and a review of pandemic influenza surveillance is provided by Brammer et al. The critical role of new diagnostic tools that were used to detect the first cases of pH1N1 infection in California and throughout the pandemic is reviewed by Jernigan et al. and a comparison of epidemiologic and virologic characteristics of pH1N1 with previous pandemic strains is reviewed by Kasowski et al. The clinical characterization of illness is reviewed in several articles including a case series of laboratory-confirmed 2009 pH1N hospitalizations describing risk factors and co-morbid conditions associated with severe illness (Skarbinski), pH1N1-associated deaths in the United States from April through July 2009 (Fowlkes), and a review of pH1N1-associated pediatric mortality (Cox). Articles describing the burden of illness using statistical modeling methods include Meltzer et al, which presents estimates of disease burden based on extrapolation of hospitalizations reported to the Emerging Infections Program (an important activity that informed policy throughout the pandemic); a novel method to track deaths in real time, based on the 122 cities deaths surveillance system, which was also important for determining severity and informing policy during the pandemic (Armstrong); and a description of the first multi-site community surveys, which helped determine community influenza-like illness (ILI) attack rates during the spring (Reed). Janusz et al describe one of the first investigations of the pandemic following the first school

children and young adults, limited impact relative to seasonal influenza and past pandemics among older adults, and identification of some previously unrecognized and some rediscovered highrisk groups, such as the increased risk of severe disease among the morbidly obese and Native American/Alaskan Native populations. Although the overall health impact of the 2009 H1N1 pandemic was lower than that of the 3 prior pandemics, substantial numbers of hospitalizations and deaths occurred, especially among younger adults and children. The 2009 H1N1 pandemic highlighted again the wide range of epidemiologic pictures possible during influenza pandemics and the need for strong influenza surveillance systems, laboratory capacity for influenza diagnosis, and rapid risk assessment to respond most effectively to an emerging threat. Field and epidemiologic investigations helped answer the critical epidemiologic questions posed at the start of the pandemic, and the lessons learned from these investigations can be used to help the nation and world prepare for future influenza pandemics, no matter the severity, as well as outbreaks of other pathogens—both naturally occurring and bioterrorismassociated.

Acknowledgments Supplement sponsorship. Published as part of a supplement entitled ‘‘The 2009 H1N1 Influenza Pandemic: Field and Epidemiologic Investigations,’’ sponsored by the Centers for Disease Control and Prevention. Potential conflicts of interest. All authors: no conflicts.

References 1. Potter CW. Chronicles of influenza pandemics. In Nicholson KG, Webster RG, Hay AJ eds. Textbook of influenza. Malden, MA: Blackwell Science, 1998; 3–17. 2. World Health Organization. http://www.who.int/csr/disease/swineflu/ updates/en/index.html. Accessed 15 April 2010. 3. Garten RJ, Davis CT, Russell CA et al. Antigenic and genetic characteristics of swine-origin 2009 A(H1N1) influenza viruses circulating in humans. Science 2009; 325:197–201. 4. Dawood FS, Jain S, Finelli L, et al. Emergence of a novel swine-origin influenza A (H1N1) virus in humans. N Engl J Med 2009; 360:2605–2615. 5. Hancock K, Veguilla V, Lu X, et al. Cross-reactive antibody responses to the 2009 pandemic H1N1 influenza virus. N Engl J Med 2009; 361: 1945–1952.

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closure due to pH1N1 in Chicago, and they assess the usefulness of door-to-door surveys to assess severity of the new virus, community attack rates, and overall impact of the pandemic. Laboratory-based characteristics of the virus are described in 2 reports of viral shedding duration as determined by polymerase chain reaction and culture detection methods (Bhattarai and Suryaprasad) and the sensitivity and specificity of rapid test diagnostics (Lucas). Transmission dynamics and community mitigation efforts are described in several articles, including a combined analysis of 7 field and epidemiologic investigations to determine the relative infectiousness of ill individuals at each point during the course of their infection. This analysis is important for assessing the duration of time that individuals with ILI should remain isolated to reduce spread of influenza (Donnelly). In addition, an assessment of risk factors associated with ILI was assessed during investigation of the first university outbreak of the pandemic (Guh), which was also the setting for an assessment of nonpharmaceutical interventions (Mitchell). Household transmission and nonpharmaceutical interventions were assessed during a school outbreak early in the pandemic in San Antonio, Texas (Loustalot), and a description of an outbreak at an elementary school in rural Pennsylvania at the outset of the pandemic provides insight into the dynamics of school outbreaks, attack rates, and transmission (Marchbanks). A combined review of 7 school-based outbreak investigations describes the lessons learned from these investigations (Iuliano). Results from a survey conducted in June 2009 in New York describes the effectiveness of school closure in limiting social interaction, as well as its economic impact on households (Borse). Information from this assessment will be important when developing guidelines to prevent spread of pandemic influenza. A model for assessing disease burden in the workplace was conducted among staff of the surveillance team at the CDC (Gindler), and a survey of emergency department surge preparedness in Atlanta, Georgia, is presented (Sugarman). Finally, the pandemic particularly affected specific populations—3 articles describe pH1N1 influenza among persons infected with human immunodeficiency virus (Peters), Alaskan Natives and Pacific Islanders (Wenger), and health care personnel in the United States (Wise). Together, these investigations and surveillance reports paint a picture of a rapidly spreading virus with greatest impact in