oxfordlearnersdictionaries.com:
(1) She was seriously ill as an infant.
my variant:
(2) She was seriously ill as she was an infant.
Is it right to say:
(1) comes from (2) by omitting "she was" (ellipsis).
If not, then why not?
oxfordlearnersdictionaries.com:
(1) She was seriously ill as an infant.
my variant:
(2) She was seriously ill as she was an infant.
Is it right to say:
(1) comes from (2) by omitting "she was" (ellipsis).
If not, then why not?
No, your version, "...as she was an infant", doesn't really work, because it changes the meaning.
As is a word with many meanings. The original sentence says that she was seriously sick when she was an infant. Your variant sounds like she was sick because she was an infant.
"as" has different meanings. "As an infant" means "at the time when she was an infant". But often "as" means "because": "as she was an infant" means "because she was an infant". So you are changing the meaning. "As an infant" is complete, there is no ellipsis. Sure, you could create a different sentence with different meaning by adding more words, but that's not what an "ellipsis" means.
It is incorrect to use an ellipses. An ellipses is used when information is not said. However, I found an error in your version of the sentence.
What you should use instead of "as she was" is the word when.
She was seriously ill as she was an infant
'When' is the proper word because she is not coming from sickness. Sickness is not a place. This sentence would be more correctly written as
She was seriously ill when she was infant.
She very well may have once been sick, but this sentence needs the word 'when' to show that she was sick at the same time she was an infant. Additionally, using 'as she was' in that context is unconventional English. The version I provided is more standard.