Earlier quoted context omitted.
What's "effective" mean in this context? The anthrax letters in the US in 2001 certainly dominated news cycles.
The most effective bioterror attack in the USA was https://en.wikipedia.org/wiki/1984_Rajneeshee_bioterror_atta... and it killed zero people, affecting 751 people in total. Bioterror is expensive and difficult. For prions specifically, the main "problem" (which is a good thing overall for us) is that they're naked proteins, and they denature relatively quickly when outside of a protective environment. Spray them onto…
That is not how terror is defined though. The amount affected by terrorism is the amount of people that get terrorised by the act, not the amount that gets sick, dies or know someone who did. Using your definition 9/11 only affected aprox. 10.000 people but in reality if cased mass hysteria and fear in the US (and elsewhere) that is still there today.