Photo/Illutration An electron microscopic image of a novel coronavirus of the Omicron variant successfully isolated at the National Institute of Infectious Diseases (Provided by the National Institute of Infectious Diseases)

The saliva of patients infected with the Omicron variant of COVID-19 virus contains a larger number of novel coronavirus particles in a state that is more prone to transmission, a team of Japanese scientists discovered.

This means saliva droplets containing larger amounts of viruses travel farther and last longer in the air than the droplets from patients infected with the original virus strain, they said.

“The state of the viruses in saliva may be a major factor behind the spread of the Omicron variant through aerosol transmission,” said Kenichi Imai, a professor of infectious diseases and immunology with the Nihon University School of Dentistry, who took part in the research.

The researchers said the discovery likely accounts for why the waves of Omicron infections have been dragging on, adding that it will remain essential for people to ventilate spaces and wear masks in the days and months to come.

The research results were published on Jan. 10 in a scientific journal of the American Medical Association.

The Omicron variant, which caused the sixth wave of COVID-19 cases in Japan last winter, has remained the mainstay virus strain in outbreaks, including the current eighth wave.

It had been learned that the Omicron variant replicates faster in bronchial cells than the Delta variant, which caused the fifth wave in the summer of 2021.

However, the state in which virus particles of the Omicron variant are contained in saliva, which is responsible for aerosol transmission through fine droplets, remained a mystery.

Imai and his coworkers took saliva samples from 90 COVID-19 patients who visited a Nagoya clinic between November 2020 and February 2022.

The researchers used centrifugation and genetic testing to analyze the state of the novel coronaviruses of the Omicron variant, the Delta variant and the original non-mutant strain in the patients’ saliva.

Their study showed that virus particles exist in two states in saliva.

In one, the particles are associated with cells detached from the interior of the mouth, being either contained in those cells or attached to the periphery thereof. In the other, the particles are in a cell-free state, unassociated with the cells.

The scientists on the team focused on virus particles in the cell-free state. Their count was 3.21 million per cubic centimeter of saliva for the Omicron variant, or about three times the count of 1.17 million per cc for the Delta variant and 18 times the count of 180,000 per cc for the original virus strain.

Cells derived from the mouth’s interior measured at least 0.01 millimeter, the researchers said, and large droplets containing similar cells fell after traveling 1 to 2 meters.

Cell-free viruses, by contrast, are contained in fine droplets that measure 0.005 mm or less, which travel farther and stay in the air longer.

“It will be crucial in the future to study the ratio of cell-free viruses in saliva during the early phase of the emergence of a new virus variant,” Imai said.