The success of vaccine makes it harder for officials to track disease, so every student case must be reported.
Minnesota is intensifying its surveillance of chickenpox in schools, but not because the disease is becoming a bigger problem. Quite the opposite.
Case numbers in Minnesota have dropped so low that the state's disease detectives can't track the spread of chickenpox under their old system -- by which most schools reported if they had a multistudent outbreak and only 80 "sentinel" schools reported all individual student infections.
So instead, all schools must now report every single student case of chickenpox.
State health officials have planned the switch for years because they expected that increased use of a chickenpox vaccine would reduce the number of infections.
"It's very good news," said Claudia Miller, an immunization specialist for the Minnesota Department of Health. "It means the two-dose recommendation [of varicella vaccine] has been effective."
Since 2009 Minnesota has required that children receive two doses of vaccine against chickenpox before starting kindergarten unless their parents opt out for religious or other reasons. According to the latest federal survey data, 87 percent of Minnesota children have received their first dose around their first birthdays.
"The vaccine is very effective at preventing severe disease," Miller said.
Some vaccinated children are still infected but tend to experience milder symptoms and fewer than 50 of the red, itchy lesions characteristic of the disease.
Under the old reporting system, the state estimated 600 chickenpox infections among students in the 2011-12 school year. The old system was created when chickenpox was so prevalent that schools didn't have time to report every case. So the state required only that they report outbreaks (defined as five or more cases among students younger than 13 in one school, or three or more cases involving students older than 13).
Under the new system, health officials expect an exact count of 500 to 600 cases in the 2012-13 school year.
Chickenpox is one of several infectious diseases that schools report to the state Health Department, including lab-confirmed influenza and mumps.
Seven student chickenpox infections already have been reported -- four involving fully vaccinated students and two involving students who had received only one dose. Five of the vaccinated students experienced only mild symptoms.
Despite the decline of chickenpox in Minnesota, Miller said it is important to accurately track the infectious disease. Among other things, Miller said it is important to monitor the severity of chickenpox and the frequency of cases involving vaccinated students.
Jeremy Olson 612-673-7744