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Measurement of vaccine effectiveness against pandemic influenza vaccine: CDC perspective. VRBPAC November 2012 Joseph Bresee Epidemiology and Prevention Branch Influenza Division National Center for Immunization and Respiratory Diseases CDC. Review of human H5N1 Cases.

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Measurement of vaccine effectiveness against pandemic influenza vaccine: CDC perspective


November 2012

Joseph Bresee

Epidemiology and Prevention Branch

Influenza Division

National Center for Immunization and Respiratory Diseases


review of human h5n1 cases
Review of human H5N1 Cases

May - Dec. 1997, Hong Kong: 18 cases, 6 deaths

Jan - Feb. 2003, Hong Kong: 2 cases, 1 death

November 2003 to present:

608 confirmed cases (15 countries)*

359 deaths (59% mortality)

Sporadic cases (children, young adults)

Wintertime seasonality – poultry and human cases

Risk factors

Direct/close contact with sick/dead poultry

Visiting a live poultry market

Some cases without identified exposure source

Late presentation/hospitalization/delayed treatment

Case clusters

Most are due to poultry exposures

Limited human-to-human H5N1 virus transmission

2nd generation (most clusters)

Two clusters of 3rd generation transmission (Pakistan, Indonesia)

Ungchusak et al., NEJM 2005; Kandun et al. NEJM 2006; Wang et al., Lancet 2008;

WER 2009; WHO, unpublished data

*Reported to WHO as of August 10, 2012

2012 human h5n1 cases
2012 Human H5N1 Cases*

N = 30 cases (6 countries)

Egypt (10)

Indonesia (8)

Cambodia (3)

China (2)

Vietnam (4)

Bangladesh (3)

37.5% fewer infections than in 2011 (Jan-Sept)

Case fatality proportion: 63% (19/30)

Median age overall: 22 years (range: 1 - 45 years) [n = 23]

*Reported to WHO as of August 10, 2012

Number of H5N1 human cases reported to WHO



Entire 2011


Jan-Aug 2012




48 63


38 50

83 100

0 0

0 50

100 50

100 100







Viet Nam



Geographic distribution of H5N1 human cases since January 2012





Clade 2.2.1




Clade 1.1

Clade 1.1



Countries reporting human H5N1 cases

Provinces with human H5N1

Source: WHO, CDC

  • Epidemiology and clinical characteristics largely unchanged in recent years
    • Sporadic human illness caused by infection with HPAI H5N1 virus continues to occur and be identified
    • Children, young adults
    • Exposure to sick/dead poultry continues to be major risk factor
    • Seasonality continues (colder periods)
  • Relatively fewer cases in 2012 compared previous years
  • Confirmed cases are an underestimate of human H5N1 virus infections
  • Risk remains
    • Humans exposed, especially in endemic countries
    • Viruses continue to evolve
cdc s influenza division program to measure influenza vaccine effectiveness in the u s
CDC’s Influenza Division program to measure influenza vaccine effectiveness in the U.S.

6 – 59 mo.

OP, Hosp.

Adults >18 y Hosp.

Adults > 50 y Hosp.



6 – 23 m


New Vaccine Surv. Network

Emerging Infections Program

Special studies

US VE Network - 2

US VE Network - 1

ACIP recommended groups


Marshfield Clinic, WI

ACIP recommended groups - MAARI

All Ages



HCWs, Peds. ICU, Pregnant

Household VE











Influenza season

us flu ve network
US Flu VE Network
  • Purpose
  • Estimate VE for prevention of healthcare visits due to influenza, by age group and type/subtype
  • Not designed to develop product-specific estimates


Five medical systems and geographic regions:

  • Group Health Cooperative (WA)
  • Marshfield Clinic (WI)
  • Scott & White Healthcare (TX)
  • University of Michigan (MI)
  • University of Pittsburgh (PA)

EnrolleesChildren and adults with medically

attended acute respiratory illness (MAARI)

us flu ve network methods
US Flu VE networkMethods
  • Design: Prospective case-control study
    • Cases: MAARI with RT-PCR influenza
    • Controls: MAARI but negative for influenza
  • Vaccination status: confirmation through record review pending
  • Immunization: defined by receipt of at 1+ dose of vaccine 14 or more days before onset of respiratory symptoms
  • Analysis: VE = (1-adjusted OR) x 100%, estimated with logistic regression models, with assessment for potential confounding by
    • age, sex, race, ethnicity
    • date of symptom onset
    • days between symptom onset and testing for influenza
    • insurance status, self-reported health status
    • Self-report of medical conditions that increase the

risk of influenza-related complications

US Flu VE Network Evaluating monovalent H1N1 vaccine effectiveness during the pandemic, 10/2009 – 04/2010

Adjusted pandemic vaccine effectiveness for all four licensed vaccines and for all ages combined was 56% (95% CI = 23%-75%)

--low numbers prevented demonstrating significant protection for many subgroup analyses (age, vaccine type)

Griffin, M et al. (2010) . Plos ONE 6 (8): e23085

emerging infections program influenza surveillance
Emerging Infections Program Influenza Surveillance
  • Population-based surveillance for laboratory-confirmed influenza-associated hospitalizations
  • Children (<18 years) and adults
  • Current network covers
    • 10 states: CA, CO, CT, GA, MD, MN, NM, NY, OR, and TN
    • Represents ~7% of the US population
  • Conducted studies to measure VE for influenza-positive hospitalizations from 2005-2011
  • By age groups
Estimating VE against hospitalizations in EIP during the pandemic, 2009-2010

EIP Adult H1N1 Hospitalizations

Successful in calculating significant and consistent overall VE

Low enrollment limited ability to estimate VE among subgroups (or vaccine types)

Average enrollment start date

(≥10% vaccine coverage)

maine school located influenza vaccination campaign during the 2009 hin1 pandemic
Maine School-Located Influenza Vaccination Campaign during the 2009 HIN1 pandemic
  • Maine distributed most of their 2009 H1N1pdm MIV through school-located influenza vaccine campaigns
  • US CDC and Maine CDC collaborated on a state-wide evaluation of VE against absenteeism
    • VE was 33% (95% CI = 26-40) against absences of 3+ days a
  • VE against lab-confirmed influenza in Cumberland county
    • VE was 69% (95% CI = 12-89) b

(a) Graitcer, SB et al. (2012) . Vaccine 30: 4835-4841..

(b) Uzicanin, A et al. (2012) . JID 206: 1059-1067

key points related to influenza vaccine effectiveness program at cdc
Key points related to Influenza Vaccine Effectiveness Program at CDC
  • Robust system for measuring VE
    • Most seasons, able to estimate overall VE as well as age group-specific VE
  • Able to formulate rapid (intra-season) VE estimates
    • Important for seasonal and pandemic communications
  • Geographically dispersed sites – so mitigates risk of geographic variability in disease occurrence
  • During 2009 H1N1 pandemic
    • Sites mobilized quickly and effectively despite early onset of disease
    • Able to initiate special project to address VE in specific populations to augment understanding of vaccine performance
    • Late arrival of vaccine compared to disease limited ability to conduct subgroup analyses
can the us ve system measure product specific ve during a pandemic
Can the US VE system measure product-specific VE during a pandemic?


  • Temporal overlap between disease and vaccination will limit power


  • Robust system for measuring VE – so, with high AR, can get VE
  • High mortality associated with H5 pandemic may reduce ability to fully mobilize sites
  • Mobilized US VE sites and other platforms for 2009 H1N1 pandemic successfully
  • Geographic location of sites might not reflect distribution of product in a way that makes product VE estimates possible
  • Geographically dispersed sites – so should capture cases
  • Currently, 30% of product information is left blank, reducing power to address this question
  • Capture vaccine product information
  • CDC plans to estimate overall and age-group-specific VE for pandemic vaccine during the next pandemic
    • Using existing seasonal VE program platforms, primarily
  • CDC can’t guarantee that product-specific VE estimates will be derived from its work during the next pandemic
    • Sample size available:
      • number of sites/cases in current systems
      • late delivery of vaccines compared with disease
      • Limited ability to expand during a severe pandemic
      • Limited ability to maintain a larger system in advance of a pandemic
      • Human resource challenges during a severe pandemic
    • Distribution of vaccine products uncertain
  • Will continue to plan for pandemic VE work, and this conclusion could be modified in the future