once/when

[diamondcutter]

Each disease can only officially be classified as an epidemic once a certain number of deaths have occurred from the disease.

Source: English, Book 3, a textbook for senior high school students in China, Pearson Education and Beijing Normal University Press

I’d like to know whether the conjunction “once” means “when” in the sentence above.

 

[Barque]

Yes. Or you could read it as "upon a certain number of deaths having occurred ..."

 

[diamondcutter]

The use of “once” in the sentence in the OP sounds a little odd to me because it’s the first time I’ve seen that usage. I’d like to know if it’s a common usage.

 

[jutfrank]

Yes, it is.

 

[Rover_KE]

You can also think of it as meaning 'as soon as'.