Variant watch: Why symptom link is key to Omicron

The researchers found that during the Omicron wave, close to 30% of the people who turned up had an asymptomatic infection.

Why does the Omicron variant of Sars-CoV-2 spread significantly more readily than other variants of the coronavirus?

Viral load is often seen as a proxy for how infectious someone is.

For weeks now, it was thought to be the intrinsic traits scientists discovered when they tested the virus on human (or human-like) tissues. They found the variant multiplied significantly faster in upper air passages, thriving in parts of the body where, it was assumed, the virus was better placed to be exhaled or coughed out. Once it’s in the air, and makes it way to another host, its superior ability to latch on to cells – also demonstrated in the lab – gives it greater advantage.

But two new studies now call into question whether these factors are indeed the reason for Omicron’s rapid spread.

The first, by researchers from Harvard’s TH Chan School of Public Health, analysed the results of routine testing on National Basketball Association (NBA) staff between July 5, 2021 and January 10, 2022 – a period when Delta and Omicron outbreaks affected these people. Using the cycle threshold (CT) values from the tests and contact tracing, they found that the peak viral load in people who contracted Omicron was lower than those who had a Delta variant infection and that the rate at which the virus was cleared out from the nose and throat was quicker in the case of the newer VOC (variant of concern).

Viral load is often seen as a proxy for how infectious someone is.

“…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 said in their pre-print study, which is yet to be peer-reviewed. But there are two caveats that apply here.

First, as the authors note, it is unclear what role immunity difference between these two periods may have played; it could be that the people tested simply had more immune defences, as a result of prior exposure or vaccination, or both.

Second, RT-PCR CT values are not a perfect measure of infectiousness.

A second study offers a new explanation for why Omicron might spread so rapidly: It is more likely to lead to asymptomatic infection, thereby many are spreading it without knowing, the study reasons. Conducted by a team led by researchers from University of KwaZulu-Natal in South Africa, the study found that there were far higher numbers of asymptomatic Covid-19 in case of Omicron infections than the other variants.

The study was based on the mandatory testing of all people who came to get their vaccine doses. None of these people showed symptoms. The researchers found that during the Omicron wave, close to 30% of the people who turned up had an asymptomatic infection. In contrast, during the Beta and Delta wave, only 2.6% of the asymptomatic people who were tested were found infected.

“These findings strongly suggest that Omicron has a much higher rate of asymptomatic carriage than other VOCs and this high prevalence of asymptomatic infection is likely a major factor in the widespread, rapid dissemination of the variant globally, even among populations with high prior rates of Sars-CoV-2 infection,” the authors added.