An outbreak of chickenpox has affected at least one elementary school in Wisconsin, according to health officials in the state.

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In a statement posted to Facebook, the Rusk County Public Health Department in Ladysmith, Wis., announced the outbreak at Ladysmith Elementary School. At least five cases have been confirmed, including four cases of breakthrough chickenpox, which occurs in people who have been vaccinated against the disease.

Symptoms of breakthrough chickenpox are often milder, with few to none of the blisters or red spots that typically come with the disease. That said, those infected with breakthrough chickenpox are still contagious.

Chickenpox is a highly contagious virus that spreads when an infected person coughs, sneezes or even talks. The virus can also spread to an unvaccinated person if they come into contact with an infected person’s chickenpox blisters. Symptoms typically include fever, headache, lethargy, and an itchy rash.

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Health department officials said they are offering chickenpox vaccines for free to anyone who has not yet received the shot.