HLTH 4200 Week 9 Outbreak Narrative Example

Chickenpox at Maple Grove Elementary, September Through November: An Outbreak Narrative Told in the Order the Investigator Learned It, With Time, Place and Person Kept Apart

Student Name

Walden University

HLTH 4200: Principles of Epidemiology

Instructor Name

Month Day, Year

Chickenpox at Maple Grove Elementary, September Through November: An Outbreak Narrative Told in the Order the Investigator Learned It, With Time, Place and Person Kept Apart

How the Investigation Began

On Monday, October 6, the school nurse at Maple Grove Elementary, a composite public school with 480 students in kindergarten through fifth grade, called the county health department. Three second-grade students in two classrooms had been sent home the previous week with itchy, blistering rashes, and a parent had reported that a pediatrician diagnosed chickenpox. The health department opened an investigation that afternoon. This narrative follows what the investigator learned, in the order it was learned, and then sets out the findings by time, place and person.

The Case Definition

The investigator first set a case definition, adapted from one used in a published school outbreak investigation: an acute, generalized rash of flat spots, raised bumps and fluid-filled blisters, without another apparent cause, in a student or staff member at Maple Grove with onset from September 1 until the outbreak was declared over (Lopez et al., 2006). Rash in a vaccinated person was counted if it began more than 42 days after the last vaccine dose, since a rash soon after vaccination can be caused by the vaccine itself. Cases were confirmed by laboratory testing of a lesion where a sample could be taken, or counted through contact with a laboratory-confirmed student.

What Was Learned, in Order

On October 6 and 7, the investigator reviewed the school's absence records and nurse visit logs for September and early October and found two earlier rashes that had not been reported: a second grader with onset on September 23 and a fourth grader, the second grader's sibling, with onset on September 30. The September 23 case was the earliest identified.

On October 8, letters went to all families asking about rashes since September 1, and the school provided vaccination records for every student. On October 9, a lesion sample from one of the three original cases tested positive for varicella-zoster virus.

Over the next three weeks, the investigator interviewed families of every reported case. New cases continued to be reported through mid-November. The last case had rash onset on November 12. With no new cases in the 42 days after that onset, two maximum incubation periods of 21 days, the outbreak was declared over on December 24.

Time

The shape is that of a propagated outbreak, passed between children across several generations, rather than a single common exposure (Centers for Disease Control and Prevention, 2012). Varicella has an incubation period of about two weeks, and the peaks roughly two weeks apart, from the first case in late September to the peak in mid-October, are consistent with successive generations of transmission. A point-source outbreak would show a single sharp peak within one incubation period.

Table 1

Cases by Week of Rash Onset

Read as an epidemic curve, the counts rise over three weeks, peak in the week of October 13 and decline through mid-November.

Place

The 31 cases were not spread evenly across the school.

Second grade had the highest attack rate, and within second grade, 11 of the 14 cases were in the two classrooms where the first cases appeared. The two second-grade classrooms shared a recess period and a combined reading group. Cases in other grades included siblings of second-grade cases and students who rode the same bus routes.

Table 2

Cases by Grade

No staff member met the case definition.

Person

Attack rates were highest among unvaccinated students and lowest among students with two doses. Using the unvaccinated attack rate as the comparison, the effectiveness of two doses in this outbreak was about (40.0 minus 2.0) ÷ 40.0 = 95%, and of one dose about (40.0 minus 25.0) ÷ 40.0 = 38%, though the small number of unvaccinated students makes both estimates uncertain. Cases among vaccinated students were milder: most had fewer than 50 lesions and no fever.

These findings echo a published investigation of an elementary school outbreak in a highly vaccinated population, which found that one dose of varicella vaccine did not prevent school outbreaks and helped prompt the recommendation for a second dose (Lopez et al., 2006; Marin et al., 2007).

Table 3

Cases by Varicella Vaccination Status

Vaccination status was taken from school records; all two-dose students had received their second dose at least 42 days before the outbreak.

What the Narrative Order Reveals

Telling the outbreak in the order it was learned shows something the final tables hide. The first two cases, on September 23 and 30, were found only by reviewing records after the nurse's call; they had not been reported when they occurred. Had the earlier cases been reported promptly, vaccination clinics might have begun two weeks sooner, before the peak in mid-October. The order of learning, not only the order of onset, determines when control measures can start.

Control Measures

On October 10, the health department recommended that students with varicella stay home until all lesions had crusted, that families of unvaccinated and one-dose students be offered vaccination and that the school notify families of students with weakened immune systems. Vaccination clinics were held at the school on October 14 and 21.

Conclusion

Told in the order it was learned, the outbreak began with three reported cases and grew, through record review and active case finding, to 31 cases from September 23 to November 12. The epidemic curve shows person-to-person spread over several generations; the cases clustered in two second-grade classrooms and among their siblings; and attack rates were highest among unvaccinated and one-dose students. Kept apart, the time, place and person findings each answer a different question about how the outbreak moved.

References

Centers for Disease Control and Prevention. (2012). Principles of epidemiology in public health practice: An introduction to applied epidemiology and biostatistics (3rd ed.). U.S. Department of Health and Human Services. https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/index.html

Lopez, A. S., Guris, D., Zimmerman, L., Gladden, L., Moore, T., Haselow, D. T., Loparev, V. N., Schmid, D. S., Jumaan, A. O., & Snow, S. L. (2006). One dose of varicella vaccine does not prevent school outbreaks: Is it time for a second dose? Pediatrics, 117(6), e1070-e1077. https://doi.org/10.1542/peds.2005-2085

Marin, M., Güris, D., Chaves, S. S., Schmid, S., & Seward, J. F. (2007). Prevention of varicella: Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recommendations and Reports, 56(RR-4), 1-40. https://www.cdc.gov/mmwr/preview/mmwrhtml/rr5604a1.htm