Earlier quoted context omitted.
"Trying to work across party lines" went out the window when the Republicans decided that "prevent absolutely anything the Democrats want to do, no matter how objectively beneficial it would be or whether it was fully supported by Republicans in the past" was a viable and acceptable method of getting re-elected.
“If the public-health professionals, if Dr. Fauci, if the doctors tell us that we should take it, I'll be the first in line to take it — absolutely," the California senator said during the first and only vice-presidential debate. "But if Donald Trump tells us we should take it, I'm not taking it." -Kamala Harris Let’s not act like both parties don’t do the exact same thing, eh?
Harris's answer was the right one, and the same one I would have given. An unqualified "yes" would be saying you'd take a vaccine approved by the administration even if the medical community wasn't confident in it, and the phrasing of the question was designed to elicit that clarification.
And let's not pretend it's surprising that her qualification was even needed; We're talking about the same president who used the presidential soap box to urge people to try curing and preventing COVID using Hydroxychloroquine despite no such recommendation from health officials or the medical community at large.