It's very simple...really!
THAT - when referring to things
WHO - when referring to people
e.g. An effective vaccine would be one 𝘁𝗵𝗮𝘁 could match up to the mutative prowess of the virus. (vaccine is an object/thing)
e.g. The efforts of the scientists 𝘄𝗵𝗼 developed vaccines against COVID-19 within such a short time span are truly commendable. ('who' refers to 'scientists' i.e., people, not 'efforts')
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