of those - grammar question

[svforumer]

The fact that only 22 persons became ill, and of those only 5 died, is a tribute to the rapid and effective response of the clinicians and public health workers involved.

My question: How to explain the grammar structure of "of those" in the above sentence?

[Mister Micawber]

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I have answered this for you HERE.
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[hlbert03]

It simply means that the "of those" is in reference to 22 people. Meaning, 5 died out of 22 who became ill.

[Casiopea]

Your comments are useful, hlbert03, and greatly welcomed. However, please take a look at the other posts in the thread before posting. The question has already been answered.

[hlbert03]

Can I use 'of which' instead of 'of those' in the sentence given? Is this correct?

The fact that only 22 people became ill, and of which only five died, ........

Please help