Why is the Chernobyl disaster considered to be the world's worst-ever civil nuclear incident when only an excess of 15 childhood thyroid cancer deaths were documented as of 2011?

The Chernobyl disaster was called the world's worst-ever civil nuclear incident because it is one of only two nuclear energy accidents rated at seven—the maximum severity—on the International Nuclear Event Scale. The initial emergency response, together with later decontamination of the environment, involved more than 500,000 personnel and cost an estimated 18 billion roubles—roughly US$68 billion in 2019, adjusted for inflation. Even though Chernobyl's health effects to the general population are uncertain, the World Health Organization predict an eventual 9,000 cancer related fatalities in Ukraine, Belarus and Russia.