Summary report on Hong Kong influenza in Japan.

In the 2 months following the onset of the influenza epidemic in Hong Kong in July 1968, there must have been many importations of the virus into Japan both from Hong Kong and from other countries subsequently involved in the epidemic. Over a dozen such incidents were reported in Japan and confirmed serologically or by direct virus isolation. Although they must have seeded the virus at least in and near the ports of importation, a true epidemic of Hong Kong influenza did not start until October. It spread gradually, first to the larger and more crowded cities and then to smaller urban areas, but in a sporadic manner and without massive involvement of the rural areas. This was in marked contrast to an influenza B epidemic that affected the whole country during the same period-September-October 1968 to March 1969-and also in contrast to the 1957-58 Asian influenza epidemic.