Amid a jump in coronavirus cases over the past few days, a new study is offering a fresh warning that infections continue to be prevalent in closed environments without adequate ventilation, including entertainment venues, restaurants and gyms.

The study examined 61 clusters of COVID-19, the respiratory illness caused by the novel coronavirus, based on 3,184 cases between January and April this year. A cluster is defined as five or more cases reported in the same place, with the largest so far involving over 100 cases at a hospital.

The study, compiled by Japanese researchers, was published this month in a journal by the United States Centers for Disease Control and Prevention.