Not very, it spreads human to human but it’s much harder to spread than flu or covid.

“We do believe that there may be some human-to-human transmission that’s happening among the really close contacts, the husband and wife, people who’ve shared cabins,” said Maria Van Kerkhove, the director of epidemic and pandemic management at the World Health Organization, speaking at a press conference on Tuesday.

…

However, Debbink says, if this is indeed human-to-human transmission, the virus does not seem to be highly transmissible because then “you would have a lot more cases on the cruise ship, just from people being around each other in pretty close proximity.”

Per NPR