Page images
PDF
EPUB
[graphic]
[blocks in formation]
[blocks in formation]

nterim Within-Season Estimate of the Effectiveness of Trivalent Inactivated Influenza Vaccine Marshfield, Wisconsin, 2007-08 Influenza Season

During clinical trials, the efficacy of vaccination with activated influenza vaccines for the prevention of serogically confirmed influenza infection has been estimated high as 70%-90% among healthier adults. However, e effectiveness of annual influenza vaccination typically lower during those influenza seasons when a suboptimal atch between the vaccine strains and circulating influza strains is observed. For example, in a 4-year randomed study of influenza vaccine among healthy persons aged 65 years, the predominant strain was drifted from the ccine strain in 2 of the 4 years. Inactivated vaccine effeceness (VE) against culture-confirmed influenza ranged m 71% to 79% when the vaccine and circulating strains ere suboptimally matched to 74% to 79% when the atches were well matched (1). In contrast, a 2-year study inactivated influenza vaccine among healthy adults aged -64 years found no measurable VE during a year when poorly matched strain circulated, but found VE of 86% ainst laboratory-confirmed influenza during the follow3 year when the vaccine and circulating strains were well itched (2). Although laboratory data on the antigenic aracteristics of circulating influenza viruses compared with ccine strains are available during influenza seasons, estiites of VE usually have not been made until months er the conclusion of the season. This report summarizes erim results of a 2008 case-control study to estimate : effectiveness of trivalent inactivated influenza vaccine prevention of medically attended, laboratory-confirmed luenza during the 2007–08 influenza season, when most culating influenza A (H3N2) and B viruses were optimally matched to the vaccine strains. Despite the >optimal match between two of three vaccine strains and culating influenza strains, overall VE in the study popuion during January 21-February 8, 2008, was 44%.

These findings demonstrate that, in any season, assessment of the clinical effectiveness of influenza vaccines cannot be determined solely by laboratory evaluation of the degree of antigenic match between vaccine and circulation strains.

Patients living in a 14 postal-code area surrounding Marshfield, Wisconsin, were eligible to participate in this study. Nearly all residents in this area receive outpatient and inpatient care from Marshfield Clinic health-care providers. Study enrollment began on January 21, 2008, based on laboratory evidence of influenza circulation from both Marshfield Clinic laboratories and the Wisconsin State Laboratory of Hygiene and continued through March 28, 2008. Patients who visited a Marshfield Clinic facility with medically attended illnesses were screened for study eligibility during outpatient or inpatient visits. Patients who reported feverishness, chills, or cough were eligible for enrollment. Those who reported symptoms for 8 or more days were not eligible for enrollment because influenza virus shedding decreases with illness duration, making detection of the virus unlikely after 8 days of symptoms. The majority of ill patients not approached during a

[merged small][merged small][ocr errors][merged small]

DEPARTMENT OF HEALTH AND HUMAN SERVICES
CENTERs for Disease CONTROL AND PREVENTION

The MMWR series of publications is published by the Coordinating Center for Health Information and Service, Centers for Disease Control and Prevention (CDC), U.S. Department of Health and Human Services, Atlanta, GA 30333.

Suggested Citation: Centers for Disease Control and Prevention. [Article title]. MMWR 2008;57: [inclusive page numbers].

Centers for Disease Control and Prevention
Julie L. Gerberding, MD, MPH

Director

Tanja Popovic, MD, PhD

Chief Science Officer James W. Stephens, PhD Associate Director for Science

Steven L. Solomon, MD

Director, Coordinating Center for Health Information and Service
Jay M. Bernhardt, PhD, MPH

Director, National Center for Health Marketing
Katherine L. Daniel, PhD

Deputy Director, National Center for Health Marketing
Editorial and Production Staff

Frederic E. Shaw, MD, JD
Editor, MMWR Series
Teresa F. Rutledge

(Acting) Managing Editor, MMWR Series

Douglas W. Weatherwax
Lead Technical Writer-Editor
Donald G. Meadows, MA
Jude C. Rutledge
Writers-Editors

Peter M. Jenkins

(Acting) Lead Visual Information Specialist Lynda G. Cupell

Malbea A. LaPete

Visual Information Specialists

Quang M. Doan, MBA

Erica R. Shaver

Information Technology Specialists

Editorial Board

William L. Roper, MD, MPH, Chapel Hill, NC, Chairman Virginia A. Caine, MD, Indianapolis, IN

David W. Fleming, MD, Seattle, WA

William E. Halperin, MD, DrPH, MPH, Newark, NJ
Margaret A. Hamburg, MD, Washington, DC
King K. Holmes, MD, PhD, Seattle, WA
Deborah Holtzman, PhD, Atlanta, GA
John K. Iglehart, Bethesda, MD
Dennis G. Maki, MD, Madison, WI
Sue Mallonee, MPH, Oklahoma City, OK
Stanley A. Plotkin, MD, Doylestown, PA
Patricia Quinlisk, MD, MPH, Des Moines, IA
Patrick L. Remington, MD, MPH, Madison, WI
Barbara K. Rimer, DrPH, Chapel Hill, NC
John V. Rullan, MD, MPH, San Juan, PR
Anne Schuchat, MD, Atlanta, GA

Dixie E. Snider, MD, MPH, Atlanta, GA

John W. Ward, MD, Atlanta, GA

clinical encounter were identified the next day by usir electronic diagnosis codes entered by the clinician; thex patients were contacted by telephone and enrolled at hom if they met eligibility criteria, The Marshfield Clinic Re search Foundation institutional review board approved the study.

Nasal or nasopharyngeal swabs were obtained from consenting patients and were tested for influenza A or B inte tion by reverse transcription-polymerase chain reaction (RT-PCR) at the Marshfield Clinic Research Foundatic: using CDC-recommended probes and primers. Viral cur ture was performed on all samples that were RT-PCR pos tive to provide virus isolates for antigenic characterizatie Influenza vaccination status was determined through a immunization information system (Regional Early Chic hood Immunization Network) used by all public and pr vate immunization providers for vaccines administered adults and children. Previous validations have demonstrate that the system captures 96%-98% of influenza vaccine administered to area residents (Marshfield Clinic Resear Foundation, unpublished data, 2005-2007). Trivale. inactivated influenza vaccine from Sanofi-Pasteu ([Fluzone®], Swiftwater, Pennsylvania) was the only infl enza vaccine used by Marshfield Clinic during the 20008 influenza season.

For this case-control study, a case of medically attende influenza was defined as an acute illness in a patient wit feverishness, chills, or cough and documentation of infl enza infection by RT-PCR. Controls were defined a patients with the same symptoms who had a negative R PCR test for influenza. Using persons with actute respira tory symptoms who test negative for influenza as contro is a method that in modeling studies has compared favor ably with cohort studies and traditional case-contro designs for the assessment of vaccine effectiveness (5 Patients were categorized as immunized if they had received influenza vaccine 14 days or more before enrollment; addition, children aged <9 years were categorized a immunized if they had received 2 doses of influenza vas cine. Twenty-three children were excluded because they ha received only 1 of the 2 recommended doses; this subgrou was too small to permit a separate analysis of VE for par immunization.

VE was estimated by using logistic regression to com pare patients with laboratory-confirmed influenza with patients who tested negative for influenza. The likelihood of receiving influenza vaccination in this population associated with a propensity to seek health care, and use o

*Available at http://www.recin.org/default.asp.

-negative controls helped adjust for this source of bias estimating VE for medically attended influenza illness. mparisons of this study design to traditional cohort and e-control methods for assessing VE have been published ently (3). For this analysis, the enrolled patients were egorized into two groups: persons for whom influenza cine was recommended by the Advisory Committee on munization Practices (ACIP) for the 2007-08 season <ed on age or an existing chronic medical condition† that reased their risk for influenza-related complications (i.e., ACIP recommended group), and healthy children and lts aged 5-49 years (i.e., the healthy group). Logistic regression models were adjusted for age, week of ollment, and presence of a chronic medical condition. e last variable was not included in the models restricted healthy patients aged 5-49 years. VE was estimated as J × [1 adjusted odds ratio]) and was interpreted as o if the percentage was negative. The first 59 influenza us isolates obtained during the study were submitted to >C for detailed antigenic characterization. During January 21-February 8, 2008, a total of 1,779 ients were assessed for study eligibility after a clinical counter for acute respiratory illness or febrile illness. A al of 850 (48%) did not meet eligibility criteria; 773 %) of exclusions resulted from absence of feverishness,

-

efined as existing if the patient had two or more health-care visits with relevant ternational Classification of Diseases, Ninth Revision, Clinical Modification agnosis codes during 2007. Diagnosis codes were based on ACIP criteria, cluding cardiac, pulmonary, renal, neurological/musculoskeletal, metabolic, rebrovascular, immunosuppressive, circulatory system, and liver disorders; abetes mellitus; and malignancies.

chills, or cough or an illness duration 8 days or longer. Of the 929 eligible patients, 639 (69%) consented to the study and were tested for influenza infection. Final enrollment for this interim analysis was reduced to 616 patients after exclusion of 23 partially immunized children who had received only 1 of 2 recommended vaccine doses.

Influenza was detected by RT-PCR in 191 (31%) enrollees; 75% of influenza infections were type A. Distribution by sex was similar for patients who tested positive and patients who tested negative for influenza (Table 1); however, the median age was higher for patients who tested positive (21 years) than those who tested negative (10 years). Approximately 19% of patients who tested positive and 39% of those who tested negative had been vaccinated against influenza.

The overall interim estimate of VE was 44% (Table 2); the estimate was higher among persons in the healthy group aged 5–49 years (54%). The overall estimate of VE for prevention of medically attended influenza A infections was 58%. No VE was observed for prevention of medically attended influenza B infections.

Subtyping by RT-PCR performed at CDC demonstrated that 40 of 41 influenza A specimens were influenza A (H3N2) viruses; the remaining specimen was an H3N2 and B virus mixture. Preliminary data on antigenic characterizations were available for nine influenza A (H3N2) viruses and 18 of 20 influenza B viruses. Two of nine influenza A (H3N2) viruses were A/Wisconsin/67/2005-like, the H3N2 component of the 2007-08 Northern Hemisphere vaccine; the other seven were A/Brisbane/10/2007like (H3N2) viruses, a strain that is drifted from the

BLE 1. Number and percentage of patients with medically attended acute respiratory illness who were enrolled* in a study and ¿ted for influenza, by selected characteristics — Marshfield, Wisconsin, January 21–February 8, 2008

[blocks in formation]

atients who reported having feverishness, chills, or cough for <8 days were eligible for enrollment.

y reverse transcription-polymerase chain reaction. efined as existing if the patient had two or more health-care visits with relevant International Classification of Diseases, Ninth Revision, Clinical lodification diagnosis codes during 2007. Diagnosis codes were based on Advisory Committee on Immunization Practices (ACIP) criteria, including ardiac, pulmonary, renal, neurological/musculoskeletal, metabolic, cerebrovascular, immunosuppressive, circulatory system, and liver disorders; abetes mellitus; and malignancies.

« PreviousContinue »