Hokkaido appears to be leading that trend, with at least 160 cases on Tuesday and a record 200 new cases on Monday, surpassing the previous high of 187 new cases on Saturday. Across Japan’s largest prefecture, 4,198 people are being treated and experts warn that hospitals are already coming close to capacity.

“This is a really serious problem for Hokkaido as temperatures and humidity levels are dropping, meaning that people are keeping their windows closed to keep the warmth in and we know that it is easier for the virus to spread in a closed environment,” said Yoko Tsukamoto, a professor of infection control at the Health Sciences University of Hokkaido.

“Ideally, we need people to keep their windows open or at least ventilate shared spaces as much as possible, but that is not always possible because temperatures are falling now,” she said.

“Experts have been predicting that the problem was going to get worse in the winter months so this is not a surprise – but I do not think they knew that cases were going to increase this rapidly again,” she said.

“It’s faster than we expected, the experts are worried and if it can happen here in Hokkaido then it will definitely happen in Tokyo and elsewhere in Japan when temperatures fall there.”

The government’s panel of experts has expressed similar concerns, cautioning that case numbers will rapidly increase unless emergency countermeasures are introduced immediately.

The panel is proposing efforts to avoid clusters of cases emerging and then spreading to the wider population, as has been seen in several cities. Clusters have largely been traced back to the nightlife districts of major cities, such as Tokyo’s Kabukicho district and the Susukino area of Sapporo, the biggest city in Hokkaido.

Many of those cases were linked to host or hostess bars that did not fully implement social distancing recommendations.

Clusters have also been reported at schools and universities, where younger people may not display symptoms but can spread the virus.

The Hokkaido government raised its five-level coronavirus alert to three on Saturday, calling on restaurants and bars in Susukino to close at 10pm.

On Monday, the government indicated it will exclude Hokkaido from its Go To tourism promotion campaign, which will come as another blow to the embattled travel sector.

“In my view, the cases are just in a specific area of Hokkaido and they do not affect most people living or visiting here,” said Aya Sugimoto of the Sapporo Convention Bureau. “We need to take the matter seriously and take precautions, such as wearing a mask and washing our hands, but in general I think the city has done the best it can and it is quite safe.

“I do not think local people are scared at this point, no-one is really worried because the problem is mostly occurring in the bars and nightclubs of Susukino,” she said.

The city is also going ahead with its annual Snow Festival in February next year, although it will be on a far smaller scale than in previous years and the ban on international travellers will make it an all-domestic affair.

A major outbreak occurred at the festival earlier this year. The governor declared a state of emergency across the prefecture on February 29 after cases were traced back to a Chinese tourist from Wuhan who had contracted the virus before arriving in Hokkaido.

Tsukamoto said the government – both at national and local levels – needs to be more forceful with its messaging on the dangers associated with the virus and be firmer on shutting down businesses that pose a risk.

“They say that the bars and restaurants in Susukino have been shut down, but this virus can easily spread out of that small area and go anywhere in the city,” she said, pointing out that the entertainment district is directly alongside the area where the snow festival will take place. “Shutting down one area does not work.”

Tsukamoto also admitted that health authorities are “extremely concerned” about reports of mutations of the virus in other countries, including in animals, such as mink in Denmark.

“We have to be worried, although there is not very much that we can do at the moment. All we can do is to be vigilant,” she said.

“I have been hearing people say that the toxicity of the virus has been falling because there have been relatively fewer deaths recently and that people are now seeing it as little more than a bad case of the flu, but that’s not true,” Tsukamoto said. “The mortality rate may have dropped, but we do not know what is going to happen with any mutations that emerge, so we still have to be very careful.”

The strain on Hokkaido’s health facilities is also becoming apparent. People diagnosed with the virus are being told to quarantine in a number of segregated hotels around the prefecture, although more than 200 people are still waiting for a place at a hotel.

“If those people live alone, that’s fine, but if they are a member of a family there is obviously a high chance of them passing the virus on to those around them,” she said.

“I am very fearful that we are going to see numbers continue to go up very rapidly. And if that happens, I estimate that Hokkaido will be completely out of hospital beds in less than a month.”