NHK has learned that people infected with the Delta variant of the coronavirus are estimated to carry a minimum of four times more pathogens than those carrying other variants.
Japanese laboratory testing service company BML compiled the data. The company conducts up to 20,000 PCR coronavirus tests per day.
As part of the process of carrying out such tests, the virus genes in a sample are multiplied. When the pathogen can be detected with fewer cycles of amplification, it means the sample contains a larger amount of the virus.
BML says the virus was detected after fewer than 20 cycles in 38 percent of the samples tested in January this year.
The percentage rose to 41.4 percent in April when the Alpha variant was spreading.
Once the Delta variant became dominant in July, the figure jumped to almost 66 percent and stayed at roughly 64 percent in August.
The number of cycles is called the Ct Value. When it's 40 or lower, the test is positive.
Based on the most common Ct Value in each month, BML estimates that the amount of virus in samples taken from Delta variant carriers is four to 64 times that seen with the original strain and the Alpha variant.
BML official Yamaguchi Toshikazu says the company started to see more samples with smaller Ct Values containing larger viral loads from July.
He says there is a marked difference in the amount of virus in Delta samples and those containing other variants.
Toho University Professor and member of the Japanese government coronavirus panel Tateda Kazuhiro says infection with the Delta variant results in a large viral load.
He says there is no evidence of airborne transmission, but a significant amount of the virus is thought to be contained in micro-droplets, which are released when people speak even at normal conversational volume.
He calls on people to wear masks made of nonwoven fabric and to ensure that indoor spaces are thoroughly ventilated.