underscoring

Hi, Navi,

@navi posted:

1) Delta is now the variant of the coronavirus identified third most often in California, according to new data — underscoring that the variant is highly contagious, a danger to people who have not been vaccinated against COVID-19.

Is '1' grammatical?

What is the subject of 'underscoring'?

I think the intended meaning is:

Delta is now the variant of the coronavirus identified third most often in California, according to new data. That fact underscores that the variant is highly contagious, a danger to people who have not been vaccinated against COVID-19.

I agree with your interpretation. "Underscoring" seems to me to be a sentential participle:

1.a. Delta is now the variant of the coronavirus identified third most often in California, according to new data, which underscores that the variant is highly contagious, a danger to people who have not been vaccinated against COVID-19.