Faster spread of Omicron is not due to higher viral load

Omicron has replaced the Delta variant, which was considered highly transmissible, in almost every country across the world

January 29, 2022 09:20 pm | Updated January 31, 2022 07:54 am IST

The Omicron variant became the dominant variant infecting both vaccinated and the unvaccinated persons in many countries just one month after the World Health Organization designated it a variant of concern on November 26, 2021. Two months since, Omicron has become the dominant variant in every country that is witnessing a new wave. In short, Omicron has replaced the Delta variant, which was considered highly transmissive in almost every country across the world.

One of the defining features of a new variant is the higher transmissibility than the existing variant. If the Delta variant was found to be highly transmissible when compared with the Alpha variant, the Omicron variant has been found to be extremely transmissive when compared with the Delta variant.

Early studies

Earlier studies suggested that the extremely high transmissibility of the Omicron variant was probably due to higher viral load in an infected person. The higher the viral load in a person, the greater are the chances that the infected person can successfully spread it to others. This is because the infected person tends to release larger amounts of the virus. While the Delta variant require a relatively longer period of exposure before a person gets infected, the Omicron variant has been found to spread within a few minutes of exposure.

But the results of a study posted in medRxiv preprint server (and is yet to be peer-reviewed) has found that the viral load is nearly the same with both variants — Delta and Omicron.

Backed by mutations

With the Omicron variant possessing many mutations that allows it to escape the immune system better even in previously infected or fully vaccinated people, the extremely high rate at which the Omicron variant is spreading might be due to inherent immune escape capabilities rather than the high viral load as it was previously thought.

A team led by researchers from the Harvard T. H. Chan School of Public Health in Boston, Massachusetts, obtained longitudinal, quantitative RT-PCR test results of swabs taken from the nose and throat of over 10,300 players from the National Basketball Association, the organisation responsible for professional basketball in North America. The period of study was restricted between July 5, 2021 and January 10, this year. From the beginning of the pandemic the players and other people associated with the Association have been undergoing regular testing. The league conducts frequent COVID-19 testing of its players and personnel.

The researchers used RT-PCR cycle threshold (Ct) values of less than 30 as a proxy for viral load. The PCR test amplifies the genetic material from coronavirus through multiple cycles. Since the coronavirus has RNA, it is first converted into DNA, and each cycle of amplification doubles the amount of DNA.

If there is just one DNA molecule to start with, the amount of DNA after 30 cycles of amplification will be one billion molecules. If there is more genetic material to begin with then fewer cycles of amplification would be sufficient to detect the DNA.

Surprising results

While the researchers were expecting that people infected with the Omicron variant would show higher viral load, the results were the just opposite. “The peak viral RNA based on Ct values was lower for Omicron infections than for Delta infections,” they write. For Omicron, the Cycle threshold (Ct) was 23.3, while for Delta the cycle threshold was 20.5. “These results suggest that Omicron’s infectiousness may not be explained by higher viral load measured in the nose and mouth by RT-PCR,” they write.

The clearance phase was also shorter for Omicron infections — 5.35 days — while for Delta it was 6.23 days.

Omicron and Delta

Another team led by researchers from the University of Geneva, Switzerland, went a step further to measure the number of infectious virus particles present on the swabs collected from 150 infected people. The results are posted on preprint server medRxiv, and the paper is yet to be peer-reviewed. They found that breakthrough infections caused by the Delta variant had lower number of virus particles than unvaccinated people with Delta infection. Surprisingly, the number of virus particles in vaccinated individuals with Omicron infection was comparable with individuals with Delta breakthrough infections.

Contrary to the previous notion that people infected with Omicron have higher viral loads than those infected with the Delta variant, the study by the University of Geneva found that vaccinated people infected with Delta or Omicron had nearly similar viral loads.

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