TOKYO (REUTERS) - A domestic tourism campaign promoted by Japan's Prime Minister Yoshihide Suga may have contributed to a sharp increase in coronavirus infection cases in the country, a prominent adviser to the government's pandemic response said.

While the government has said there is no evidence that its "Go To" travel campaign spread the coronavirus, Mr Suga suspended it in December to contain mounting Covid-19 cases, and as his approval ratings sank over the handling of the pandemic.

The campaign, which began in July, had offered various travel-related discounts and vouchers in a bid to prop up regional economies hammered by the pandemic, but critics said it risked spreading the virus from cities to the countryside.

"The present study is the first to demonstrate that the number of travel-associated Covid-19 cases that involved crossing of prefectural borders increased during the Go To travel campaign," Kyoto University researchers Asami Anzai and Hiroshi Nishiura wrote in their study. The study was published by the Journal of Clinical Medicine on Thursday (Jan 21).

Professor Nishiura has acted as an adviser to the Japanese government's pandemic response, usually advocating for stricter measures to control the spread. He became known in local media as "Uncle 80%" for his advice that people reduce their social interactions by that amount.

The paper said the incidence of travel-associated Covid-19 cases during the campaign was around three times greater than the control period of one month prior to its kick-off.

The study looked at 3,978 confirmed Covid-19 cases between May and August, of which some 20 per cent were travel-related. Another study published last month found a higher incidence of Covid-19 symptoms among people who had participated in the travel campaign.