We had data from December 2019 until April 2020 showing that it spread quickly and can kill you.
We also had data that strong action can stop an outbreak (i.e. Aviation flu, Swine Flu, Ebola, etc). Sure when we took those actions early enough to curtail the outbreak it leads to people thinking the actions were useless (see Y2K debate) but it doesn't actually mean the actions were useless. People only notice when the dam collapses, not when it doesn't.
We also had data that strong action can stop an outbreak (i.e. Aviation flu, Swine Flu, Ebola, etc). Sure when we took those actions early enough to curtail the outbreak it leads to people thinking the actions were useless (see Y2K debate) but it doesn't actually mean the actions were useless. People only notice when the dam collapses, not when it doesn't.