It’s not you. It should indeed have been “trial” and it is a typo. Inside the article the word trial appears: “…mid-point survey by the trial’s organisers…”

It’s not you. It should indeed have been “trial” and it is a typo. Inside the article the word trial appears: “…mid-point survey by the trial’s organisers…” Sloppiness by the Economist. Not surprising as their best editors have probably gradually been canceled or diversified out.