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B R I E F R E P O R T

Comparing Clinical CharacteristicsBetween Hospitalized Adults WithLaboratory-Conrmed Inuenza Aand B Virus Infection

Su Su,1,2 Sandra S. Chaves,1 Alejandro Perez,1 Tiffany DMello,1,2

Pam D. Kirley,3 Kimberly Yousey-Hindes,4 Monica M. Farley,5

Meghan Harris,6 Ruta Sharangpani,7 Ruth Lyneld,8 Craig Morin,8

Emily B. Hancock,9 Shelley Zansky,10 Gary E. Hollick,11 Brian Fowler,12

Christie McDonald-Hamm,13 Ann Thomas,14 Vickie Horan,15

Mary Lou Lindegren,16 William Schaffner,16 Andrea Price,17

Ananda Bandyopadhyay,18 and Alicia M. Fry1

1Inuenza Division, Centers for Disease Control and Prevention, and 2Atlanta

Research and Education Foundation, Atlanta, Georgia; 3California Emerging

Infections Program, Oakland;

4

Connecticut Emerging Infectious Program, YaleUniversity, New Haven, Connecticut; 5School of Medicine, Emory University, and

Atlanta Veterans Affairs Medical Center, Atlanta, GA; 6Iowa Department of Public

Health, Des Moines; 7Michigan Department of Community Health, Lansing;8Minnesota Department of Health, St. Paul; 9New Mexico Department of Health,

Santa Fe; 10Emerging Infections Program, New York State Department of Health,

Albany, and 11Department of Medicine, University of Rochester School of Medicine

and Dentistry, New York; 12Ohio Department of Health, Columbus; 13Oklahoma State

Department of Health, Oklahoma City; 14Oregon Public Health Division, Portland;15South Dakota Department of Health, Pierre; 16Vanderbilt University School of

Medicine, Nashville, Tennessee; 17Salt Lake County Health Department, Salt Lake

City, Utah; and 18The Bill & Melinda Gates Foundation, Seattle, Washington

We challenge the notion that inuenza B is milder than in-

uenza A bynding similar clinical characteristics betweenhospitalized adult inuenza-cases. Among patients treated

with oseltamivir, length of stay and mortality did not differ

by type of virus infection.

Keywords. inuenza A and B virus infection; antiviral

treatment; hospitalization; adult.

Infection due to inuenza B virus is often perceived to be milder

than inuenza A virus infection. However, studies have shown

similar clinical features between patients infected with seasonal

inuenza A and B virus in outpatient settings [1, 2] and substan-

tial inuenza B infections among pediatric inuenza-associatedfatalities [3]. In addition, some studies have suggested that

oseltamivir may be less effective at reducing fever in outpa-

tients infected with inuenza B virus compared with inuenza

A virus[4]; very few published studies have compared out-

comes among hospitalized patients, especially among adults.

We used 8 years of data from adults hospitalized with labora-

tory-conrmed inuenza to compare clinical characteristics

between those infected with inuenza A and B viruses and to

compare outcomes among patients treated with antiviral med-

ications by virus type.

METHODS

We used data from 20052006 through 20122013 inuenza sea-

sons collected through the Inuenza Hospitalization Surveillance

Network (FluSurv-NET), a partnership between the Centers for

Disease Control and Prevention (CDC) and state and local health

departments, academic institutions, and their collaborators in

multiple states. Prior to 1 September 2009, the following 10 states

were included in surveillance: California, Colorado, Connecticut,

Georgia, Maryland, Minnesota, New Mexico, New York, Oregon,

and Tennessee. During 1 September 200930 April 2010, the fol-

lowing 5 additional states were included in surveillance: Iowa,

Idaho, Michigan, Oklahoma, and South Dakota. After 1 October

2010, California, Colorado, Connecticut, Georgia, Idaho, Mary-

land, Michigan, Minnesota, New Mexico, New York, Ohio, Okla-

homa, Oregon, Rhode Island, Tennessee, and Utah were includedin surveillance. FluSurv-NET conducts population-based surveil-

lance for laboratory-conrmed inuenza-associated hospitaliza-

tions during the inuenza season (ie, 1 October to 30 April for

regular inuenza season; the 20082009 season, however,

ended on 14 April to account for the emergence of the inuenza

A(H1N1)pdm09 virus in the spring of 2009; the 20092010 sea-

son encompassed 15 April 2009 through 30 April 2010). Patients

were captured in the surveillance system if they resided in the

project catchment area and were hospitalized in one of the sur-

veillance hospitals with a positive inuenza test result as deter-

mined by viral culture, immuno

uorescence antibody staining,rapid antigen test, reverse transcription polymerase chain reac-

tion, or documentation of a positive test result in a patients med-

ical record. Demographic and clinical information were obtained

from medical chart review. The analysis was limited to patients

aged 18 years and excluded possible nosocomial infections.

In addition, we summarized inuenza virus surveillance data

from the World Health Organization (WHO) and National Re-

spiratory and Enteric Virus Surveillance System collaborating

Received 28 January 2014; accepted 9 April 2014; electronically published 18 April 2014.

Correspondence: Sandra S. Chaves, MD, MSc, Centers for Disease Control and Prevention,

1600 Clifton Rd, MS-A20, Atlanta, GA 30333 ( bev8@cdc.gov).

Clinical Infectious Diseases 2014;59(2):2525

Published byOxfordUniversity Presson behalfof theInfectious DiseasesSocietyof America2014.

This work is written by (a) US Government employee(s) and is in the public domain in the US.

DOI: 10.1093/cid/ciu269

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mailto:bev8@cdc.govmailto:bev8@cdc.gov

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laboratories (CDC, unpublished data) for adults (aged 21

years) to describe type-specic distribution of inuenza viruses

circulating during each season included in this analysis.

We used ttest and Wilcoxon rank-sum test to compare con-

tinuous variables (age and median length of stay) and 2 tests

and logistic regression to compare categorical variables (sex,

presence of high-risk medical conditions, mechanical ventila-

tion, intensive care unit [ICU] admission, bacterial coinfection,

prolonged hospital stay [5 days], and death during hospitali-zation) by inuenza type among younger adults (aged 1864

years) and older adults (aged65 years). We also compared

mortality and length of hospitalization among patients who

were treated with oseltamivir by virus type using unconditional

logistic regression and Cox proportional hazards regression

models; for this analysis we excluded the 20082009 season

when there was widespread oseltamivir resistance among sea-

sonal inuenza A viruses [5]. Potential confounders such as

ICU admission, presence of high-risk medical conditions, and

age group were included to produce an adjusted hazard ratio.

All statistical analyses were performed using SAS statistical soft-

ware version 9.3 (Cary, North Carolina).

Collection of human subject data has been determined by the

CDC to be routine public health surveillance and was not sub-

ject to institutional review board approval from human research

protections.

RESULTS

We identied 23 186 (87%) inuenza A and 3579 (13%) inu-

enza B virus associated hospitalizations among adults from

20052006 through 20122013. Inuenza A virusassociated

hospitalizations predominated in every season with the highest

proportion (99%) in the pandemic period and the lowest (66%)

in the 20072008 season (Figure1). The numbers of hospitali-

zations associated with both in

uenza A and B mirrored theprevalence of viruses in the community identied by the na-

tional inuenza virologic surveillance each season. Despite the

greater number of inuenza A hospitalizations, there was no

signicant difference in the overall proportions of hospitaliza-

tions with an ICU admission by virus type for each season.

There were no signicant differences between inuenza A

and B virus infections among hospitalized adults aged 65

years regarding presence of high-risk conditions, median length

of hospitalization, admission to ICU, or death in the hospital.

After adjusting for the presence of high-risk conditions, antivi-

ral treatment, and seasonality, the adjusted odds ratio (aOR) for

ICU admission was 1.05 (95% condence interval [CI], .90

1.20), and for death was 0.89 (95% CI, .701.16). Adults aged

1864 years hospitalized with inuenza A virus infection were

more often admitted to the ICU (OR, 1.36 [95% CI, 1.181.57])

than those with inuenza B; however, the nding became null

Figure 1. Number of laboratory-conrmed inuenza hospitalizations in adults and percentage admitted to intensive care unit (ICU), by virus type andinuenza season, the Inuenza Hospitalization Surveillance Network (FluSurv-NET), and the proportion of inuenza A and B viruses reported to World Health

Organization Inuenza Virus Surveillance, 20052013.

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after adjusting for presence of high-risk conditions, antiviral

treatment, and seasonality (aOR, 1.14 [95% CI, .981.33]). No

signicant difference in other parameters, including length of

stay (P= .47) and death (aOR, 0.88 [95% CI, .751.08]) were de-

tected in this age group.

Overall, 75% of all adults hospitalized with inuenza received

antiviral treatment. The use of antivirals increased from 54%

prepandemic to 82% during and postpandemic. We identied

17 089 and 2200 adults infected with inuenza A and inuenzaB viruses who received oseltamivir during the study period, re-

spectively. Almost all (99%) patients were treated with oseltami-

vir, either alone or in combination with other inuenza antiviral

medications. The median length of hospital stay was 4 (inter-

quartile range [IQR], 37) and 5 (IQR, 37) days among treated

adults

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Disclaimer. The ndings and conclusions in this report are those of the

authors and do not necessarily represent the ofcial position of the Centers

for Disease Control and Prevention (CDC).

Financial support. The Inuenza Hospitalization Surveillance Network

(FluSurv-NET) is a collaboration of state health departments, academic in-

stitutions and local partners, and is funded by the CDC. This work was sup-

ported by the CDC (cooperative agreement numbers CDC-RFA-CK12-1202

and 5U38HM000414).

Potential conicts of interest. All authors: No reported conicts.

All authors have submitted the ICMJE Form for Disclosure of Potential

Conicts of Interest. Conicts that the editors consider relevant to the con-

tent of the manuscript have been disclosed.

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