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American Public Health Association. Chickenpox/herpes zoster. In: Heymann D, ed. Control of
Communicable Manual. 19
th
ed. Washington, DC: American Public Health Association; 2009:
109 – 116.
Michigan statistics:
There were 1,889 cases of varicella (chickenpox) reported to MDCH in 2009, representing a 38%
decline from the 3,048 cases reported in 2008. Gender was reported for 1,848 (97.8%), and was
evenly split between females and males. Age was reported for 1877 (99.4%), ranging from 1
month to 78 years, with a median age of 9 years (mean age 9.9 years). The greatest proportion of
cases was reported in the 5-9 year age group (821, 34.5%). Overall, 1,074 (56.9%) indicated prior
receipt of at least 1 dose of varicella vaccine.
In 2008, there were 3,048 cases of varicella (chickenpox) reported to MDCH; this represents a
27% decline from the number reported in 2007. Gender was reported for 2,989 (98.1%), cases
were evenly split between males and females. Age was reported for 3,009 (97.7%); the median
age was 8 years (mean 9.5), with cases ranging from 19 days to 93 years. The largest number of
cases was reported in the 5-9 year-old age group (1,481, 48.1% of cases), followed by the 10-19
year-old age group (967, 31.4% of cases). Overall, 1,737 (57%) cases indicated prior receipt of
varicella vaccine.
In 2007, a total of 4,191 cases of chickenpox were reported, a decline of 19% from the previous
year. Gender was reported for 4,073 (97%); there were slightly more males (male-to-female ratio
1.03:1). Age was reported for 4,157 (99%), and ranged from 2 months to 107 years, with a median
of 8 years. The largest number of cases was reported in the 5-9 year-old age group (2,292, 55% of
cases), followed by the 10-19 year-old age group (1,242, 30% of cases). Severity of illness was
approximated by estimates of the number of lesions as reported in one of four categories: less than
50, 50 – 249, 250 – 500, and more than 500. Such categorical estimates of number of lesions were
reported for 3,336 (79.6%) cases. Of these, 3,216 (96%) had information on prior vaccination
history. Previous receipt of varicella vaccination was associated with milder illness (risk ratio
0.2383, 95% confidence limits 0.2057;0.2762, p<0.005; for purposes of this analysis, cases with
fewer than 50 lesions were considered mild, and cases with 50 or more were considered moderate-
to-severe). This was similar to a finding from 2006 Michigan data and supports conclusions from
various studies suggesting that breakthrough varicella disease occurs in some vaccinated persons
but tends to be a milder illness than cases in unvaccinated persons.
In September of 2005 Michigan implemented individual case reporting for varicella. Previously,
surveillance was based on aggregate weekly case counts from schools, day-care centers, and
providers. Thus 2006 provided the first complete year of individual case-based varicella data.
Although overall the reported incidence of varicella in Michigan has declined by approximately
85% since the introduction of varicella vaccine in 1995, the level of reported disease continues to
be on the order of several thousand cases each year. In an effort to minimize the burden on public
health resources, data collection efforts are focused on the collection of basic demographic
variables and three additional key data elements: age, varicella vaccination history, and severity of
illness.
In 2006, a total of 5,200 cases of varicella (confirmed or probable) were reported; this represents a
30% increase over the 4,004 cases reported in 2005. It is unclear if this reflects a true increase in
occurrence or an improvement in the reporting of cases. Of the 5,200 cases, age was reported for
5,172 (99.5%) and gender was reported for 5,133 (98.7%). The largest number of cases was