According to her brother, as reported by Rolling Stone:
According to Jacobs, Dean was staying in New York with her fiancé, Luke Volker, when she started experiencing some “minor” symptoms that concerned her enough asked Volker to go with her to an emergency room. She then died in a matter of hours, Jacobs says, with the exact cause of her death still pending an autopsy that was underway Wednesday.
“This happened literally within the span of a day: getting a headache, going to sleep, waking up her boyfriend and saying please take me to the hospital,” Jacobs, 21, tells Rolling Stone.
“We still aren’t entirely sure (what happened). There’s an autopsy being done which we know might take quite a while. But what we have heard it that there was a viral infection in her lungs,” he says.
https://www.rollingstone.com/tv-movies/tv-movie-news/charlbi-dean-death-brother-denies-antivax-conspiracy-theories-1234585375/