The Current Season: A Review of the 2003-04 Influenza Season

Nancy J. Cox, Ph.D.Chief, Influenza Branch

National Center for Infectious DiseasesCenters for Disease Control and

Prevention

Timeline of Emergence of Timeline of Emergence of Influenza Viruses in HumansInfluenza Viruses in Humans

1918 1957 1968 1977 1997

1998/9

2003

H1

H1

B

H2

H7H5H5

H9

SpanishInfluenza

AsianInfluenza

RussianInfluenza

AvianInfluenza

Hong KongInfluenza

H3

Pan

dem

icvaccin

es

Reg

ula

r vaccin

es

Influenza Virus Types & Subtypes in Humans (Trivalent Vaccine)

• Type A– Seasonal epidemics caused by H3N2, H1N1, and

H1N2 subtypes– Pandemics (caused by new subtypes)

• Type B– No subtypes– Seasonal epidemics only

Influenza VirusInfluenza Virus

Antigenic Change

• Antigenic ‘drift” occurs in HA and NA– Continual development of new strains secondary to

genetic mutations/seasonal epidemics– A viruses >> B viruses

• Antigenic “shift” occurs in HA and NA– Associated with pandemics – Appearance of novel influenza A viruses bearing new

HA or HA & NA

135

135135

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262

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Model of the influenza A H3 HA structure

Top view Side view

N. HemisphereTemperate

(Japan & US. West Coast)

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Tropical(Singapore & S. China)

S. HemisphereTemperate(Australia)

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JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Seasonal Occurrence of Influenza(Reichelderfer PS, et al. Current Topics in Medical Virology, 1988)

Influenza Disease

• National morbidity and mortality data

• Comparison with other influenza seasons

• Mortality in children

• The circulating viruses

What Was Different About the 2003-04 Influenza Season?

• Sustained increases in influenza activity were reported unusually early

• Reports of influenza-related pediatric deaths were prominently reported by national media

• Vaccine manufacturers had sold almost all vaccine by mid-December, a time when demand continued

• There was a less than optimal match between the predominant circulating viruses (A/Fujian/411/02-like) and the corresponding vaccine component, A/Panama/2007/99

Virologic Surveillance

• Virologic data are the foundation of influenza surveillance– Specific for influenza– Detect changes in circulating strains– Detect novel influenza viruses– Used for vaccine strain selection– Monitor vaccine match

WHO/NREVSS Collaborating LaboratoriesNational Summary, 2003-04

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40 42 44 46 48 50 52 1 3 5 7 9 11 13 15 17 19

Week

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of Is

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A(H3N2)

A(H1N1) and A(H1N2)

A(Unsubtyped)

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Percent Positive

Percentage of Specimens Testing Positive for Influenza National Summary, 2003-04

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40 42 44 46 48 50 52 1 3 5 7 9 11 13 15 17 19

Week

% o

f Vis

its fo

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%pos. 2003-04 %pos. 1999-2000* %pos 2002-03

Percentage of Visits for Influenza-like IllnessReported by Sentinel Providers

National Summary, 2003-04

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40 42 44 46 48 50 52 1 3 5 7 9 11 13 15 17 19

Week

% o

f V

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% ILI 2003-04 %ILI 1999-2000* %ILI 2002-03 national baseline

Weekly Influenza Activity Estimates Reportedby State & Territorial Epidemiologists

Week ending December 20, 2003 - Week 51

No Report Sporadic Regional Widespread No Activity Local Activity

Weekly Influenza Activity Estimates Reportedby State & Territorial Epidemiologists

Week ending January 24, 2004 - Week 3

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% o

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Pneumonia and Influenza Mortality for 122 U.S. Cities

Week Ending 01/24/04

2000 2001 2002 2003

Epidemic Threshold

Seasonal Baseline

10 20 30 40 50 10 20 30 40 50 10 20 30 40 50 10 20 30 40 50

Summary of 2003-04 Influenza Activity in the U.S. as of Week 3

• Influenza activity has declined in all regions• For week 51 of 2003, widespread influenza activity

was reported by more states than during any week during the past 10 years

• Influenza-related deaths rose above the “epidemic threshold” for the first time during week 51 and has stayed above the baseline for 5 consecutive weeks

• Reports suggest that highest ILI attack rates were among children and young adults

Influenza-Associated Deaths Among Children < 18 Years of Age

• 121 influenza-associated deaths (lab confirmed) reported to CDC

• Median age, 3.8 years (2 weeks to 17 years)• 72 (60%) < 5 years of age• 33 (27%) 6-23 months of age• 26 (21%) had underlying medical conditions• Available vaccination histories indicate 57

unvax and 2 vax according to recs

Is 2003-04 Different in Impact Among Children?

• Influenza-associated deaths not reportable conditions in the U.S. and annual average number of influenza deaths unknown.

• During 1990-1999, and estimated annual average of 92 respiratory and circulatory deaths occurred among children < 5 years; estimate based on mathematical modeling, not on counting lab confirmed fatalities.

• Studies to determine if hospitalization increased in children are ongoing.

The Viruses: 2003-04 Influenza Season

• 573 influenza viruses from U.S. characterized antigenically by CDC’s WHO Collaborating Center

• 565 were H3N2 viruses (2 H1s and 6 Bs)– 106 (18.8%) were well inhibited by antiserum to

the A/Panama vaccine strain– 459 (81.2%) were similar to the

A/Fujian/411/2002 drift variant and not well inhibited by antiserum to A/Panama

Why Wasn’t an A/Fujian-like Virus Included in the 2003-04 Influenza Vaccine?

• The A/Fujian/411/02 (H3N2) antigenic variant was identified at CDC on January 31, 2003.

• European and U.S. regulatory authorities have required that influenza viruses used to produce vaccine be isolated and passaged only in eggs or cells such as primary chick kidney cells for safety reasons.

• An A/Fujian-like (H3N2) egg-derived vaccine candidate was first available in April and its use required further characterization and preparation of reagents, steps that would have delayed vaccine availability.

• Time constraints and uncertainty about antigenic differences among circulating strains led to a decision retain A/Panama/2007/99 (H3N2) in the 2003-04 vaccine.

Considerations for Recommendations

• Are there new antigenic variants?

• Are new variants spreading?

• Are current vaccines able to induce antibodies to the new variants?

• Are any new variants useful for vaccine production?

Influenza Vaccine

• Vaccine uptake

• Vaccine production and distribution

• Vaccine effectiveness– What we know from past studies– What we know about this year

Influenza Vaccination Coverage Estimated per Influenza Season

1988 – 2001, U.S.

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1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

Influenza Season

Per

cent

Vac

cina

ted >64 Yrs.

50-64 High Risk

50-64 All

18-49 High Risk

18-49 All

Source: National Health Interview Survey, Jan.-June interviews, 2002 data preliminary

Influenza Vaccine Production and Distribution, 1999-2003

Year Doses Produced Doses Distributed Number and Proportion of Doses

Produced Not Distributed

1999 77.2 M 76.8 M 0.4 M (0.5%)

2000 77.9 M 70.4 M 7.5 M (9.6%)

2001 87.7 M 77.7 M 10 M (11.4%)

2002 95.0 M 83.0 M 12 M (12.6%)

2003 (projected) 87.1 M 83.2 M 3.9 M (4.5%)

Influenza Vaccine Effectiveness

• Determinants–age and immune status–vaccine match

• Effectiveness by age and status–< 65 years, healthy 70-90% influenza– 65 years, community 30-70% influenza– 65 years, nursing home 30-40% influenza

50-60% hospital.

80% death

Estimates of Vaccine Effectiveness for the 2003-04 Influenza Season

• Study conducted by NIP/CDC among HCW at a hospital in Denver

• Endpoint was effectiveness against influenza like illness (not vs. lab-confirmed influenza more specific endpoint or vs.more serious outcomes, such as hospitalization or death)

• This study was unable to demonstrate vaccine effectiveness vs. influenza-like illness

• Other studies ongoing in the U.S.• One study in France has reported approx. 60%

effectiveness against lab-confirmed influenza

Improvements Underway or Being Discussed

• Enhancing global influenza surveillance (early warning system)- Asia, Central and South America

• Establishing confirmatory tests for hemagglutination inhibition test (virus neutralization)

• Using reverse genetics for making vaccine reference viruses if egg derived virus unavailable

• Conducting annual vaccine effectiveness studies• Requiring reporting of laboratory confirmed deaths

in children

Acknowledgements

• Members of the Influenza Branch

• The WHO National Influenza Centers

• The WHO Collaborating Centers in London, Tokyo and Melbourne

• The WHO Regional Offices

• WHO Headquarters in Geneva

The 2003-04 Influenza Season: The 2003-04 Influenza Season: We’re on a RollWe’re on a Roll