Omicron: more virions, more contagions?

The Delta variant has lost the infection war, beaten by Omicron. However, the latter induces a viral load, the amount of virus present in an infected person, weaker than Delta. This is the conclusion ofa pre-published study conducted at several American universities, including the Harvard School of Public Health. What is the secret of Omicron’s contagiousness?

Contagiousness independent of viral load

During a screening test PCR, the amount of virus present in the sample is estimated by the Ct (cycle threshold or cycle threshold). The higher this value, the greater the amount ofRNA from coronavirus present in the sample is low. It therefore takes more amplification cycles to make it detectable. For a Ct > 25, it is considered that there is not much viral RNA in the sample, on the contrary for a Ct < 10, the amount of viral RNA present is very high.

To compare the viral load induced by Omicron and Delta, scientists took nasopharyngeal swabs from members of the National Basketball Association (NBA) between July 2021 and January 2022. Of the 10,324 samples taken, 97 were positive for Omicron and 107 for Delta. The infected participants took samples every day to follow the dynamics of the viral load over time. On average, Omicron infection lasts 9.87 days and with a Ct of 23.3. In comparison, Delta infection lasts 10.9 days on average, with an average Ct of 20.5.

How? ‘Or’ What the Omicron variant could it have become the majority at the expense of Delta, by causing a lower viral load and a shorter infection? First, the scientists did not control for the presence of pre-existing immunity in the volunteers, which could partly explain why the viral load is lower after infection with Omicron. Nor does the Ct reflect an infectivity of the viral particles present in the sample. Nevertheless, the two variants have comparable dynamics. These results suggest that Omicron’s high infectiousness is not related to its viral load. If the Omicron infection lasts an average of 9.87 days, the scientists observed that 50% of the samples were still positive five days after the initial infection. Ten days after initial infection, 50% of these same samples contained sufficient virions to be contagious.

These observations are worrying because the quarantine ends ten days after the screening test in France. No test is necessary to exit this period of isolation. Some people could still be contagious after this time and participate in the spread of the variant. Respect for barrier gestures after isolation remains essential. The exhaust at immune system of Omicron could also favor its contagiousness.