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Using ESSENCE-FL and a serosurvey to estimate total influenza infections,

2009

Richard S. Hopkins, MD, MSPHKate Goodin, MPHMackenzie Weise, MPHAaron Kite-Powell, MSJanet Hamilton, MPH

CSTE annual meetingOmaha, Nebraska

June, 2012

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Outline

Quick review of 2009 H1N1 epidemic in FL Missing information during response: cumulative number

of infected people Seroprevalence survey – Cox et al

Number infected, by age group at end of pandemic Total ED visits for ILI, by age group ILI illnesses reported by BRFSS respondents, by age

group Ratio of infections to ED visits Ratio of infections to reported illnesses in BRFSS How these ratios can be used going forward

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Percentage of Visits for Influenza-like Illness (ILI) Reported by the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet), National

Summary 2008-2009 and Previous Two Seasons

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3

1- First cases detected

2- PCR testing available at CDC

3- Vaccine available

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Florida Influenza Surveillance Systems

ESSENCE ILI chief complaint hospital admissions for ILI through EDs

Florida Pneumonia and Influenza Mortality Surveillance System (FPIMSS) (modeled on 122 Cities system)

Notifiable disease reporting (through Merlin) Novel flu Pediatric influenza deaths

County influenza activity code reporting Influenza and ILI outbreak reporting (through EpiCom) Florida ILINet sentinel surveillance Bureau of Laboratories viral surveillance

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Influenza-Like Illness in ESSENCE

--Timely, accurate characterization of 75% of all emergency department room visits --Added discharge disposition as a field, 87 hospitals now reporting

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County Flu Activity Reporting

Week 3, 2010Week 17, 2009 Week 37, 2009

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Bureau of Laboratories Viral Surveillance

Number of Influenza-Positive Specimens Tested by the Florida Bureau of Laboratories (BOL) by Subtype by Lab Event Date* Week 40, 2008 to Week 15, 2010 as Reported in Merlin by 2:00 p.m. April 20, 2010

0

100

200

300

400

500

600

40 42 44 46 48 50 52 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 1 3 5 7 9 11 13 15

Week

Num

ber

of P

ositi

ve S

peci

men

s

A (H1N1) Novel

A (H3) Seasonal

A (H1) Seasonal

A (Unspecified)

B

--Novel H1N1 consistently sensitive to oseltamivir and zanamivir--Almost all influenza circulating in Florida was H1N1

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What we were missing

In September-November 2009, the DOH incident commander wanted to know answers to questions like:

How big is this going to get? Have we reached the peak yet? How long is it going to last? Will the burden on EDs and hospitals remain

tolerable?

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Influenza modeling

A key parameter for any influenza epidemic is R: the average number of new infections resulting from each infection

For any given value of R, you can calculate the final cumulative % infected – higher R means higher cumulative %

R can be estimated from epidemiologic data early in the epidemic (slope of curve)

We could use a tool that would tell us how close we are to the eventual predicted prevalence

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Seroprevalence study

We requested this from CDC Chad Cox led CDC team that visited FL Chose Tampa Bay area as project site because

of good availability of blood bank specimens for adults – all specimens from a four-day range

Getting leftover blood specimens for children was more of a challenge, resulting in spread-out specimen collection range

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Adult samples collected

Pediatric collection time frame

* Includes Hillsborough, Manatee, Pinellas and Pasco counties

Percentage of ED Visits for Influenza-like Illness, Tampa Bay* Florida, 2009-2010

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Summary

Location – Tampa Bay (Pinellas, Pasco, Hillsborough and Manatee counties)

Sample Population – Anonymous blood samples from clinical laboratories and blood bank

Number Sampled - 876 total samples, 219 from children aged <18 years

Testing type – HI assay ≥20-40

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2009 H1N1 Seroprevalence by Age Group, UnadjujstedTampa Bay Florida, Nov-Dec 2009

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Adjusted prevalences

In the final analysis, the prevalence of antibody was adjusted to account for the small proportion who would have received vaccine before serum was obtained

Also adjusted for estimated sensitivity of the assay in detecting antibody in infected people

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Estimated number of infected peopleApplying seroprevalence from Tampa Bay to whole state

Age group Population % infected with pH1N1

2009 Florida population

Estimated number infected with pH1N1

< 5 years 28% 1,136,803 318,305

5 to 17 years 53% 3,013,571 1,597,193

18 to 24 years 47% 1,685,626 792,244

25 to 49 years 19% 6,082,023 1,155,584

50 to 64 years 11% 3,598,367 395,820

> 64 years 9 % 3,302,610 297,235

Total 25% 18,819,000 4,704,750

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Estimating total ED visits for ILIAge group % of ED visits at

ESSENCE EDs# of ILI visits at ESSENCE EDs

Adjusted estimate of ILI visits

< 5 years 0.7313 27,490 37,591

5 to 17 years 0.7162 37,985 53,038

18 to 24 years 0.7176 15,524 21,634

25 to 49 years 0.7186 31,683 44,089

50 to 64 years 0.7289 7,441 10,209

> 64 years 0.7379 3,303 4,476

Total 0.7232 123,426 170,674

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Ratios of pH1N1 infections to ED visits for ILIAge group

< 5 years 8.5 to 15 to 17 years 30.0 to 118 to 24 years 36.5 to 125 to 49 years 26.0 to 150 to 64 years 39.0 to 1> 64 years 66.0 to 1Total 27.5 to 1

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Additional observations

22.3% of ED visits were for children under age 5, while only 6.7% of estimated infections were in that age group. Were children sicker than adults? Were parents were more likely to take children to ED

than themselves? For persons aged 50 to 64 years, 6.0% of ED

visits and 8.4% of infections were in that age group.

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Similar calculations from BRFSS

During October through December, FL BRFSS asked respondents about ILI, defined as fever and (cough or sore throat)

Date of onset was assigned based on date of interview and respondent’s time interval (last week, last 2 weeks, last 3 to 4 weeks)

Ratios calculated for seroprevalence to reported ILI

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Ratios of pH1N1 infections to BRFSS report of ILIAge group

< 5 years no data5 to 17 years no data18 to 24 years 13.1 to 125 to 49 years 10.7 to 150 to 64 years 3.6 to 1> 64 years 11.6 to 1Total 8.7 to 1

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Why such high ratios?

Most infections were apparently quite mild Most ill people do not make a healthcare visit,

much less visit an ED

For our purposes, it doesn’t matter – antibodies resulting from mild infections are probably just as good at stopping the spread of a pandemic as those from severe illness

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Epidemic curve from ESSENCE data alone

ILI visits to participating EDs, adjusted for non-participating EDs, ESSENCE-FL, 2009-2010

0

2000

4000

6000

8000

10000

12000

14000

2009-0

1

2009-0

5

2009-0

9

2009-1

3

2009-1

7

2009-2

1

2009-2

5

2009-2

9

2009-3

3

2009-3

7

2009-4

1

2009-4

5

2009-4

9

2010-0

1

2010-0

5

2010-0

9

2010-1

3

2010-1

7

2010-2

1

2010-2

5

2010-2

9

2010-3

3

2010-3

7

Week

Co

un

t

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New estimates of total infections by week using these ratios

Estimated Number of People Ill with Influenza Based on Adjusted ILI Visit Counts and Seroprevalence Proportions with 95% Confidence Limits, All Ages Combined

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

Week

Co

un

t

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How will we use these results?

We intend to publish them in the peer-reviewed literature

We hope others will do similar analyses If faced with another influenza epidemic or

pandemic, we can use these ratios to estimate the cumulative number of infections

Combined with mathematical models, the cumulative # of infections can help tell us whether we are near or past the peak of the epidemic

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Acknowledgements

County health department epidemiologists

Hospitals participating in ESSENCE

CDC seroprevalence study team, led by Dr. Chad Cox