A small outbreak of chicken pox was traced back to a single school bus in Michigan, and highlights the importance giving kids the chicken pox vaccine, according to a new report.

Health officials in rural Muskegon County, Michigan, were alerted to a suspected case of the chicken pox in an 8-year-old last December after the child was sent home from school, according to the report of the outbreak from the Centers for Disease Control and Prevention (CDC), published Sept. 8.

Shortly afterward, three more kids came down with the illness, the researchers wrote. The link among the cases appeared to be the school bus that all four children rode to and from school each day, according to the report. The kids didn't interact socially in school or have classes or lunch together, the researchers noted.

Chicken pox outbreaks are rare, because most children are now vaccinated against the virus. At the school in the report, 95 percent of the students were fully vaccinated, meaning they had received both doses of the vaccine, as recommended, the researchers said. Another 3 percent of students at the school had received one shot, the researchers noted.

No other cases of chicken pox were diagnosed in the school for the rest of the year, according to the report.

The health officials investigating the outbreak revealed that the first child to get sick had not been vaccinated, and that the child's four siblings (also unvaccinated) had all also recently been kept home from school because of a rash that looked like chicken pox.

Among the three children who likely caught the virus on the school bus, one had been fully vaccinated, according to the report. Full vaccination is between 88 and 98 percent effective at preventing chicken pox, according to the CDC.

The other two children, who were siblings, had not been vaccinated, the investigators wrote. Those children later spread chicken pox to their 17-month-old sibling, according to the report.

Chicken pox is caused by the varicella zoster virus. The same virus also causes shingles in adults. In this case, a shingles infection turned out to be the source of the chicken pox infection: The first child's parent had a shingles infection several weeks before the children developed chicken pox, according to the report.

Before the chicken pox vaccine was developed, the virus spread easily from kid to kid, especially in schools. However, now that most kids are vaccinated against the virus, it may be increasingly likely that kids, particularly those who are unvaccinated, will get the virus from adults who have shingles, rather than from other kids, the researchers wrote.

The researchers mentioned only one previous report, which occurred in China, of chicken pox spreading on a school bus. Because school buses put kids in close proximity to each other, however, doctors and public health officials investigating similar outbreaks might want to consider these vehicles as a possible risk factor, the report said.

Originally published on Live Science.

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