People can always find something as a point of comparison as if that automatically added weight to what they are implying.  No doubt this reporter thought stats from the Vietnam war added gravitas to her query.

In any case, her disrespectful and rude question didn't deserve to be dignified with an answer.  But Trump answered her anyway.  Unfortunately, he didn't use the opportunity to his advantage by giving her a little history lesson, as discussed at The Daily Wire:

She may have been unaware that at the height of the Vietnam war, there was something that killed more Americans than the war did, and it was a similar virus — and no one questioned whether a president should be elected or not because of the virus, known as the Hong Kong Flu.

In 1968–69, the Hong Kong flu ravaged the world; it wound up killing more than one million people worldwide, over 100,000 of them in the United States. No lockdowns were imposed and people still went to work, albeit lessening bus travel and implementing social distancing and more washing of their hands.