A truck carrying 900 doses of flu vaccine was stolen outside a Milwaukee school where vaccinations were being given, rendering them unusable, authorities said.

The truck was found abandoned a half hour after being stolen Thursday night and a 38-year-old man was arrested. None the truck's contents were missing, police said.

However, since the vaccine was not in possession of the Milwaukee Health Department for the half hour, the doses would have to be returned to the manufacturer, the Milwaukee Journal Sentinel reported.

Police said the truck's driver was collecting medical supplies at the school, and left the engine running so the vaccine would stay cool in a refrigeration unit.

"It appeared to be a crime of opportunity," police Capt. Michael Massa said.

Milwaukee health officials said the return of the vaccine doses would not affect the city's vaccination program, and that sufficient supplies were on hand, the Journal Sentinel said.