By late Sunday, Japan's total number of confirmed cases climbed from five to 78 - many of them being high school pupils who had not travelled abroad. Most of them were in the western prefectures of Hyogo and Osaka, AP reports.

Workers at stores, restaurants and train stations in these two regions immediately wore masks as a precaution, while the Japanese government has been prompted to shut down schools and cancel public events like Kobe's annual festival. Japan had set up strict quarantines at airports, but decided on Saturday to focus on the domestic outbreak.

Until Sunday, the swine flu virus had affected at least 8,480 people in 40 countries, killing 75 of them, mostly in Mexico. In Europe, Spain and Britain have reported the highest numbers of cases - 103 and 101 respectively.

The World Health Organization (WHO)'s experts have warned a pandemic could occur if the virus is transmitted between humans on a large scale outside the Americas.

WHO Director General Margaret Chan will reveal recommendations on the production of a swine flu vaccine at the WHO’s annual meeting which begins on Monday. Pharmaceutical companies are willing to begin production, but there need many decisions about such issues as the amount of vaccine to be made, its distribution and its recipients.