Public health experts in Japan are urging people to take anti-infection measures ahead of winter in response to surging coronavirus infections in the northernmost prefecture of Hokkaido.
After the nationwide state of emergency was lifted in late May, the daily tally in Hokkaido hovered around 30 until early October. But the number rose to 51 on October 23, the highest since April.
The figure almost doubled in less than two weeks, and exceeded 100 for the first time on Thursday.
Outbreaks of flu and other respiratory infectious diseases often occur in winter. Some experts say coronavirus infections could become more serious when temperatures and humidity fall.
A team led by researchers at the University of Maryland in the United States analyzed a possible correlation between temperature and humidity and the coronavirus in 50 cities around the world through early March.
The team says the virus spread most rapidly in areas where average temperatures are between 5 and 11 degrees Celsius with relatively low humidity.
But a Brazilian university group says its analysis of 17 international studies found that cold and dry conditions apparently facilitate the spread of the coronavirus, but it doesn't fully explain outbreaks.
The head of the Japanese government's advisory panel, Omi Shigeru, indicates that the virus is spreading in Hokkaido mainly because people spend more time with others in indoor spaces with limited ventilation.
The coronavirus spread in Hokkaido earlier than elsewhere in Japan, prompting the prefecture to declare its own state of emergency in late February.
The head of the Japanese Association for Infectious Diseases, Tateda Kazuhiro, warns that the coronavirus situation across the country should be monitored as winter approaches.
Tateda urges people to avoid the "Three Cs" -- closed spaces, crowded places and close-contact settings -- as well as making sure they wash hands and wear masks.
He also stresses the need to open windows at intervals to take in fresh air.