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Exploring the latest spike in COVID‐19 cases: Researchers find cell‐free viruses present in high percentages in saliva of infected individuals


A study led by Professor Kenichi Imai from the Nihon University School of Dentistry's Department of Microbiology, Division of Immunology and Pathobiology and Dr. Norio Takada, Director of Takada's Medical Office of Internal Medicine (Nagoya City), has revealed that the saliva of individuals infected with the Omicron strain of SARS‐CoV‐2 contains a higher percentage of cell‐free viruses ‐ viruses unattached to the inside or outside of host cells ‐ compared to conventional or delta strains. This finding is a world first.

The Omicron strain of SARS‐CoV‐2 is known to be spread through aerosol and airborne transmission, in addition to droplet infection, which is believed to be a significant contributor to the recent surge in infections. However, the underlying mechanism behind this change remains poorly understood. The research group speculated that there might be a high viral load in saliva and that there may be changes to the virus contained within.

Their results showed that the conventional strain had a median of approximately 1.23 million viruses per milliliter of total saliva. Among these, about 180,000 were cell‐free viruses, making up about 5.9% of all viruses in the saliva. In comparison, the Delta strain had approximately 18.6 million viruses, which is 15 times more than the conventional strain. About 1.17 million were cell‐free viruses, accounting for about 4.8% of total saliva, a value similar to conventional strains.

In contrast to the conventional and Delta strains, the Omicron strain had a median of approximately 9.52 million viruses per milliliter of total saliva, slightly more than half of the Delta strain. Additionally, the number of cell‐free viruses in the Omicron strain was about 3.21 million, which is a significant increase: 2.7 times that of the Delta strain and 17.8 times that of the conventional strain. The percentage of cell‐free viruses in total saliva was about 21.3%, a four‐fold increase in the percentage of cell‐free viruses compared to the conventional and Delta strains.

Imai commented, 'We were the first in the world to advocate the importance of cell‐free viruses in aerosol and airborne transmission. In the future, we would like to clarify the changes over time, whether there are differences between whole saliva and saliva supernatant, whether there are differences in the dynamics of cell‐free virus and cell‐associated virus, the infectivity and strength of cell‐free virus, whether the genetic mutation was involved in the increase of cell‐free virus, and the dynamics of cell‐free virus in other viral infections.'

Journal Information
Publication: JAMA Network Open
Title: SARS‐CoV‐2 Omicron Variant in Human Saliva Samples in Cell‐Free Form
DOI: 10.1001/jamanetworkopen.2022.50207

This article has been translated by JST with permission from The Science News Ltd. ( Unauthorized reproduction of the article and photographs is prohibited.

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