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Varicella Surveillance in Public Elementary Schools --- Multnomah County, Oregon, 2002--2004

Varicella vaccination of school-aged children reduces the number of varicella cases and lost days of school. In 1996, the Advisory Committee on Immunization Practices (ACIP) recommended routine vaccination of all children aged 12--18 months, catch-up vaccination of all susceptible children before age 13 years, and vaccination of susceptible persons who have close contact with persons at high risk for serious complications and susceptible persons at high risk for exposure (1). In 1999, ACIP updated these recommendations to include vaccination requirements for child care and school entry (2). Since 2000, in accordance with ACIP recommendations, varicella vaccination requirements have been phased in for Oregon children who have not had varicella before starting out-of-home child care, kindergarten, or seventh grade; elementary school children will be fully covered by school year (SY) 2006--07. To monitor changes in varicella incidence, Oregon Health Services (OHS) and Multnomah Education Service District (MESD) started routine, individual, case-based varicella surveillance in Multnomah County public elementary schools (kindergarten through 5th grade) beginning SY 2002--03. This report describes the surveillance system, the incidence of varicella during SY 2002--03 and SY 2003--04, and the results of active surveillance for unidentified cases during SY 2002--03. The findings indicate that the number of varicella cases has decreased in Oregon and that establishing public elementary school--based varicella surveillance is feasible and useful.

In 2002, data were collected from 37,850 public elementary school students, representing 77% of Multnomah County children aged 5--10 years; the remaining 23%, who attended private schools, were home-schooled, or did not attend school, were excluded from the surveillance system (3). Multnomah County has 109 public elementary schools served by 26 school nurses employed by MESD and supervised from a central office. A case of varicella was defined as a first occurrence of acute, generalized pruritic maculopapulovesicular rash, without other apparent cause and persisting longer than 24 hours, in a public elementary school student and at least one of the following: 1) physician diagnosis of varicella after examination during an office visit; 2) school nurse diagnosis of varicella after examination at school; 3) clinic nurse diagnosis of varicella based on signs and symptoms described by parents during a telephone conversation, but without examination; or 4) parental description of varicella, when physicians or nurses did not examine the student with suspected varicella.

For individual, case-based surveillance, school nurses sent OHS investigators vaccination and demographic data for students absent (according to their parents) or sent home (by a school nurse) with suspected varicella. OHS investigators interviewed parents of these students to collect data regarding previous occurrences of varicella and current disease characteristics, identifying cases by using the case definition. In addition, during SY 2002--03, OHS investigators conducted active surveillance of students exposed to cases of varicella in their classrooms by interviewing their parents to detect additional students with varicella not reported by their schools; vaccination and demographic data for exposed classmates were provided by MESD.

During SY 2002--03, a total of 24 nurses from 45 public elementary schools reported 130 students with suspected varicella. After investigation, 114 (88%) students were determined to have illnesses consistent with the varicella case definition. Of these students, 52 (46%) had varicella diagnosed by parental descriptions of rashes, 39 (34%) by physician diagnoses, 17 (15%) by school nurse diagnoses, and six (5%) by clinic nurse diagnoses based on parental descriptions. The 16 remaining students with suspected varicella included six lost to follow-up, one with a previous occurrence of varicella, and nine with other conditions diagnosed by physicians, predominantly insect bites; all 16 were vaccinated.

During SY 2002--03, the incidence of varicella peaked during October--January, when 75 (66%) cases occurred and steadily declined in subsequent months (February--May). Active surveillance for unidentified cases detected 11 additional cases among 1,479 students exposed to 96 reported students with varicella in 67 classrooms (sensitivity: 90% [96/107]).

During SY 2003--04, a total of 25 nurses from 31 public elementary schools reported 93 students with varicella; 82 (88%) students had illnesses consistent with the varicella case definition. Of these students, 36 (44%) had varicella diagnosed by physician diagnoses, 23 (28%) by parental descriptions of rashes, 16 (20%) by clinic nurse diagnoses based on parental descriptions, and seven (8%) by school nurse diagnoses. The 11 remaining students included nine lost to follow-up and two with other conditions diagnosed by physicians; all 11 were vaccinated.

Overall, for both SY 2002--03 and 2003--04, approximately 60% of varicella cases occurred as single cases in classrooms, and 40% occurred during a classroom outbreak (defined as two or more varicella cases in a classroom with onset dates <21 days apart). Among students, cases occurred most commonly in those aged 7 years (22%), followed by those aged 6 years (19%), aged 8 years (15%), aged 9 years (14%), aged 10 years (12%), and aged 5 years (10%). Sixty-nine percent of students with varicella had been vaccinated, and 28% lacked evidence of immunity, having had neither a history of vaccination nor disease; vaccination or disease history of the remaining 3% was unknown. Among vaccinated students, school vaccination records contained vaccination dates for 99.9%. Reporting by school nurses was timely; during both school years, the median time from the day students were first absent or sent home with varicella to the day OHS was notified was 3 days (range: same day to 18 days).

Reported by: S Perkins, J Fratto, Dept of School Health Svcs, Multnomah Education Svcs District, Portland; LE Lee, MPH, PR Cieslak, MD, E Lorber, MD, H Ho, MD, M Kohn, MD, State Epidemiologist, Oregon Health Svcs. LA Zimmerman, MPH, D Guris, MD, National Immunization Program, CDC.

Editorial Note:

Since varicella vaccine licensure in 1995, surveillance has been mostly limited to nationwide reporting of deaths attributed to varicella, active surveillance for individual cases of varicella at sentinel sites (e.g., schools), and aggregate varicella case reporting from certain states (4--6). Although data have documented a substantial decline in varicella incidence, hospitalizations, and deaths, a national population-based varicella reporting system to compare incidence across jurisdictions and over time is needed. Since 1998, the Council of State and Territorial Epidemiologists (CSTE) has twice recommended that state public health agencies establish statewide surveillance systems for detecting individual cases of varicella to evaluate the ongoing impact of vaccination on varicella morbidity (7,8). CSTE set 2005 as the target year for national case-based surveillance, and CDC established guidelines for varicella surveillance in 2002 (9). Varicella surveillance at sentinel sites was considered an acceptable interim measure before establishing statewide surveillance. This report demonstrates the feasibility and usefulness of establishing a school-based varicella surveillance system as an interim measure in school systems with school nurses supervised from a central office. School-based surveillance might be less feasible in other school systems in Oregon that do not have centrally supervised school nurses.

The school-based varicella surveillance system described in this report successfully detected new varicella cases among public elementary school students. School nurses reported 90% of new varicella cases, and 88% were confirmed by physicians, nurses, or parents.

School-based surveillance detected more cases than would have been detected by using physician-based surveillance alone, considering that only 34% of SY 2002--03 cases and 44% of SY 2003--04 cases were diagnosed by physicians. These findings suggest that traditional varicella surveillance systems, which rely on physician reporting only, might substantially underestimate varicella incidence.

Although restricted in scope to the elementary school population in one county, second-year surveillance data indicate that varicella incidence has decreased. Whether this is attributable to increased varicella vaccination coverage or to year-to-year variation in disease occurrence is unclear. Nonetheless, consistent year-to-year operation of a school-based varicella surveillance system can track trends in varicella incidence.

The findings in this report are subject to at least one limitation. Although the varicella surveillance system successfully detected new cases, diagnostic tests were not performed. In areas with high vaccination coverage, varicella that occurs after vaccination is mild, with few lesions, and might be missed. Thus, laboratory confirmation of varicella cases can increase the sensitivity and specificity of varicella case detection (9).

CDC has provided guidelines for individual, case-based varicella reporting and, if feasibility is a problem, recommends that state health departments limit the number of case variables to three: age, varicella vaccination history, and number of lesions (a measure of severity) (9). As incidence declines, collecting additional data (e.g., contact information, severity of disease in the source case, and laboratory confirmation) will be necessary to describe the epidemiology of remaining disease. States are expected to be able to report individual varicella case information via CDC's National Notifiable Disease Surveillance System by the end of 2005.

Acknowledgments

The findings in this report are based, in part, on contributions by J Betten, D Anderson, G Andrews, L Beck, T Cardoza, K Chew, E Demming, C Peterson, K Mahaffey-Dietrich, M Ellis, C Fawcett, I Harris, S Hauser, C Hill, M Howard, R Lazuran, M McLachlan, C Murphy, D Rains, J Schrinsky, K Seal, L Sneed, S Springer, L Stember, M Thrasher, B Tomlinson, G Wade, Dept of School Health Svcs, Multnomah Education Svcs District; AD Sullivan, PhD, JK Poujade, G Oxman, MD, Communicable Disease Office, Multnomah County Dept of Public Health, Portland; R Lehman, MD, C Mosbaek, MS, Oregon Health Svcs. S Schafer, MD, EIS Officer, CDC.

References

  1. CDC. Prevention of varicella: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1996;45 (No. RR-11).
  2. CDC. Prevention of varicella: updated recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1999; 48(No. RR-6).
  3. Portland State University. Annual Oregon population reports and supplements, 2002. Portland, OR: Portland State University, Population Research Center; July 2002.
  4. CDC. Varicella-related deaths---United States, 2002. MMWR 2003;52:54.
  5. 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--11.
  6. CDC. Decline in annual incidence of varicella---selected states, 1990--2001. MMWR 2003;52:884--5.
  7. Council of State and Territorial Epidemiologists. Position statement ID-09: Varicella surveillance and control. Des Moines, IA: Council of State and Territorial Epidemiologists; 1998.
  8. Council of State and Territorial Epidemiologists. Position statement 02-ID-06: Varicella surveillance. Kansas City, MO: Council of State and Territorial Epidemiologists; 2002.
  9. CDC. Varicella [Chapter 14]. In: Wharton M, ed. Vaccine preventable disease. Surveillance manual. 3rd ed. Atlanta, GA: US Department of Health and Human Services, CDC; 2002.

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