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Karin Galil, MD, MPH, Mark J. Pletcher, MD, MPH, Barbara J. Wallace, MSPH, MD, Jane Seward, MBBS, MPH, Pamela A. Meyer, PhD, Andrew L. Baughman, PhD, MPH and Melinda Wharton, MD, MPH

Karin Galil, Jane Seward, Pamela A. Meyer, Andrew L. Baughman, and Melinda Wharton are with the Centers for Disease Control and Prevention, Atlanta, Ga. At the time of this study, Mark J. Pletcher was with the Association of Schools of Public Health, Atlanta, Ga. Barbara J. Wallace is with the New York State Department of Health, Albany.

Correspondence: Requests for reprints should be sent to Karin Galil, MD, MPH, CDC Mail Stop E-61, 1600 Clifton Rd, Atlanta, GA 30333 (e-mail: kgalil{at}cdc.gov).

	INTRODUCTION

TOP INTRODUCTION METHODS RESULTS DISCUSSION References

 
Before widespread vaccination, varicella infection resulted in more than 11 000 hospitalizations¹ and 100 deaths annually in the United States.² Since vaccine licensure in 1995, varicella incidence has decreased markedly.³ Mathematical models have predicted that vaccination will shift the peak incidence of disease to older ages as the incidence declines.⁴ Because older persons are at increased risk of morbidity and mortality from varicella,^5,6 it is essential to monitor these outcomes as vaccination coverage increases.^7,8 The Centers for Disease Control and Prevention (CDC) encourages state health departments to search death certificates and hospital discharge databases to estimate the number of varicella deaths. In this study, we assessed the validity and utility of these data sources.

	METHODS

TOP INTRODUCTION METHODS RESULTS DISCUSSION References

 
New York State death certificates and a statewide hospital discharge database (Statewide Planning and Research Cooperative System, or SPARCS) were searched for varicella codes (International Classification of Diseases, 9th Revision [ICD-9] 052.X and International Classification of Diseases, Ninth Revision, Clinical Modification [ICD-9-CM] 052.X, respectively) for 1989 through 1995. Copies of hospital records (physician attestation sheet, history and physical examination, discharge summary) and death certificates were requested for all deaths identified. Deaths were excluded if the person was not a New York State resident or died outside the state. New York City residents were excluded for logistical reasons. We matched records in the 2 databases on decedents’ dates of birth and death, sex, and names. Two authors (M. J. P., J. S.) reviewed death certificates to confirm the presence of varicella and hospital records to classify the cause of death, on the basis of all available data, as varicella, herpes zoster, varicella-zoster virus (VZV) infection (unclassifiable), or unrelated to VZV infection.

The positive predictive value (PPV) of a varicella code in a data source was defined as the proportion of deaths identified by the data source that were classified as varicella after medical record review. The PPV in each database search was calculated 3 ways: (1) by excluding unclassifiable VZV deaths; (2) by assuming that unclassifiable VZV deaths were from herpes zoster; and (3) by assuming that unclassifiable VZV deaths were from varicella. The PPV was calculated separately for persons aged younger than 50 years and those aged 50 years and older, because varicella is rare in older adults. Exact binomial confidence limits were calculated for each PPV.⁹ Capture–recapture analysis¹⁰ was used to estimate the total number of varicella deaths during the study period. Confidence intervals were calculated by the goodness-of-fit method.¹¹ The completeness of each data source was calculated as the number of verified deaths identified by each database as a proportion of the estimated total number of varicella deaths by capture–recapture analysis. We used the Fisher exact test¹² to compare PPV estimates and the McNemar test¹² to compare the completeness estimates.

	RESULTS

TOP INTRODUCTION METHODS RESULTS DISCUSSION References

 
The search of the death certificates and the SPARCS database yielded 64 potential varicella deaths (Table 1). Hospital records were available for 54 deaths (84%); 5 deaths occurred outside of nonfederal hospitals and 5 records were not found. We identified 45 deaths by searching death certificates (36 hospital records found) and 42 deaths by searching SPARCS (41 hospital records found). Classifications of cause of death by database are shown in Table 2.

PPV of a Varicella Code
The PPV of a varicella code was greater on a death certificate than in SPARCS and was higher in persons aged younger than 50 years than in those aged 50 years and older (Table 2). The PPVs were not substantially altered under our three assumptions, namely that they could not be classified and were excluded, that they were all assumed to be due to varicella, and that they were all assumed to be due to herpes zoster.

Capture–Recapture Analysis
Of the 30 varicella deaths, 9 (30%) were identified exclusively from a search of death certificates, 4 (13%) exclusively from SPARCS, and 17 (57%) from both sources. Excluding deaths without medical records, the capture–recapture analysis yielded an estimate of 32 (95% confidence interval [CI] = 30, 38) varicella deaths. The search was 81% complete (26 of 32) for the death certificates and 66% complete (21 of 32) for the SPARCS hospital discharge data (P = .27).

	DISCUSSION

TOP INTRODUCTION METHODS RESULTS DISCUSSION References

 
Monitoring varicella mortality is important for evaluating the impact of a national vaccination program. The results of our capture–recapture analysis suggest that the search of death certificates in New York State identified more varicella deaths (81% vs 66%) and that the data source had a higher PPV than did the search of SPARCS hospital discharges and the SPARCS data. The greater completeness of data in death certificates reflects the fact that only deaths among inpatients in nonfederal hospitals are recorded in SPARCS. The PPV was a function of age; whereas the validity of a reported varicella death in a person aged younger than 50 years was approximately 80%, the validity of such a report in a person aged 50 years or older was less than 43% (i.e., what was coded as a varicella death had an equal or greater likelihood of being due to herpes zoster).

Study limitations include the difficulty in distinguishing varicella and disseminated herpes zoster, the small number of persons in the study (particularly persons aged 50 years and older), and a possible underestimation of total varicella deaths through capture–recapture analysis. Because the same physician may have coded the discharge diagnoses and completed the death certificate, the assumption of 2 independent data sources for capture–recapture analysis may have been violated.¹⁰ Medical records were unavailable for 10 deaths (16%). Assuming that these deaths were similar to those we reviewed, the total number of varicella deaths estimated by capture–recapture analysis might have been as high as 42, with correspondingly lower estimates of completeness.

This study suggests that death certificates are useful for monitoring varicella deaths for persons aged younger than 50 years. Hospital discharge records were neither more accurate nor more complete than death certificates and may be more difficult to search. In states with automated hospital discharge databases, it may be preferable to search both sources of information to derive a better estimate of the number of varicella deaths.

	Footnotes

 
K. Galil contributed to the collection, analysis, and interpretation of data and to the writing and revision of the manuscript. M. J. Pletcher, B. J. Wallace, J. Seward, and P. A. Meyer contributed to the study design and data collection; M. J. Pletcher and P. A. Meyer also contributed to data analysis, and B. J. Wallace and J. Seward also were involved in revising the manuscript. A. L. Baughman contributed to data analysis and interpretation and to revising the manuscript. M. Wharton contributed to the study design and revising the manuscript.

Peer Reviewed

Human Participant Protection
No human subjects participated in this study.

Accepted for publication July 5, 2001.

	References

TOP INTRODUCTION METHODS RESULTS DISCUSSION References

 
1. Galil K, Lin F, Seward J. Hospitalizations for varicella in the United States, 1988–1995 [abstract 571]. In: Programs and Abstracts of the 37th Annual Meeting of the Infectious Diseases Society of America Meeting (Philadelphia). Alexandria, Va: Infectious Diseases Society of America; 1999:139.

2. Meyer PA, Seward JF, Jumaan AO, Wharton M. Varicella mortality: trends before vaccine licensure in the United States, 1970–1994. J Infect Dis. 2000;182:383–390.[Medline]

3. Seward JF, Watson BM, Peterson CL, et al. Varicella disease after introduction of varicella vaccine in the United States, 1995–2000. JAMA. 2002;287:606–611.[Abstract/Free Full Text]

4. Halloran ME. Epidemiologic effects of varicella vaccination. Infect Dis Clin North Am. 1996;10:631–655.[Medline]

5. Amler RW. The health impact of varicella. In: 17th Immunization Conference Proceedings (Atlanta). Atlanta, Ga: Centers for Disease Control; 1982:137–138.

6. Preblud SR. Age-specific risks of varicella complications. Pediatrics. 1981;68:14–17.[Abstract/Free Full Text]

7. Centers for Disease Control and Prevention. Evaluation of varicella reporting to the National Notifiable Disease Surveillance System—United States, 1972–1997. MMWR Morb Mortal Wkly Rep. 1999;48:55–58.[Medline]

8. Council of State and Territorial Epidemiologists. Inclusion of Varicella-Related Deaths in the National Public Health Surveillance System. Atlanta, Ga: Council of State and Territorial Epidemiologists; 1998. Position statement ID-10.

9. Rosner B. Fundamentals of Biostatistics. 3rd ed. Boston, Mass: PWS-KENT Publishing Co; 1990:170–173.

10. Hook EB, Regal RR. Capture–recapture methods in epidemiology: methods and limitations. Epidemiol Rev. 1995;17:243–264.[Free Full Text]

11. Regal RR, Hook EB. Goodness-of-fit based confidence intervals for estimates of the size of a closed population. Stat Med. 1984;3:287–291.[Medline]

12. Fleiss JL. Statistical Methods for Rates and Proportions. 2nd ed. New York, NY: John Wiley & Sons; 1981.

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