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(AP) — Months before people started getting vaccinated for COVID-19, endangered black-footed ferrets in northern Colorado were injected with an experimental coronavirus vaccine.

Kaiser Health News reports that about 120 animals at the National Black-footed Conservation Center near Fort Collins got the vaccine in late summer. There haven’t been any cases of COVID-19 there but ferrets are feared to be highly vulnerable to the disease.

Experts say vaccinating vulnerable species against the disease is important for humans too. When animals contract the virus from humans, it can mutate as it spreads rapidly, posing a new threat if it spills back to people.

Ferrets were rescued from the brink of extinction after some were discovered in Wyoming four decades ago.