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?

Context: The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the No. 4 reactor in the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR in the Soviet Union. Called the world's worst-ever civil nuclear incident, it is one of only two nuclear energy accidents rated at seven—the maximum severity—on the International Nuclear Event Scale, the other being the 2011 Fukushima nuclear disaster in Japan. 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. Chernobyl's health effects to the general population are uncertain. An excess of 15 childhood thyroid cancer deaths were documented as of 2011. A United Nations committee found that to date fewer than 100 deaths have resulted from the fallout. Determining the total eventual number of exposure related deaths is uncertain based on the linear no-threshold model, a contested statistical model. Model predictions of the eventual total death toll in the coming decades vary. The most widely cited studies by the World Health Organization predict an eventual 9,000 cancer related fatalities in Ukraine, Belarus and Russia.

Answer: 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.