A health ministry official had no immediate comment, but Nikkei said the ministry was checking the officer's contacts with colleagues and family members.
Mark Kortepeter, a professor of epidemiology at the University of Nebraska Medical Center, said that although it was hard to reach conclusions based on limited information, the ship infections pointed to an "agent that is highly transmissible," at least in enclosed environments.
"The infected quarantine officer probably indicates the high risk, even for individuals who are trying to protect themselves, because the virus is unforgiving and there is no room for error in use of personal protective equipment and hand hygiene," he said. "A small number of virus particles is likely needed to infect, making it an efficient spreader."
The government was considering allowing elderly and those with chronic illnesses to disembark before the Feb. 19 target date for the end of quarantine, some media reported, but added it would take time to determine where they could be sent.
About 80% of the passengers were aged 60 or over, with 215 in their 80s and 11 in the 90s, the English-language Japan Times newspaper reported.
An improvement in life for people on the boat came several days ago with the announcement they can receive packages sent to the address of the pier - a service that Rob Thomson, a New Zealander living in northern Japan, whose parents are on the boat, made prompt use of.
"The one thing that they were missing were hot drinks during the day - they were getting coffee or tea at breakfast, but then during the day it was cold water or Coca-Cola or soft drinks ... and Dad was saying he'd like fresh fruit other than apples or oranges," he said, adding that he placed an order Tuesday night.
"So I tried sending some grapes and strawberries and stuff. And apparently the fresh things have already been delivered to somebody on the dock."
The foreign ministry released a notice on Wednesday urging its nationals to consider delaying trips to China and for Japanese in China to return home.
Japan has sent four chartered flights to China's Hubei province, the epicentre of the outbreak, for its citizens there to return, and plans a fifth. The people who returned on the first flight will be released on Wednesday if they test negative for the virus, media said.
