Coronavirus: the vaccine greatly reduces the risk of being contagious


Researchers have found that the viral load depends on the variant and that vaccination reduces it but, for Omicron, only from the 3rd dose.

For the Delta variant, people who are doubly vaccinated have a much lower viral load than those who have not been vaccinated. For Omicron, this is only seen after the booster dose.

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When a person tests positive for coronavirus, following a PCR test, we do not know what their viral load is. These tests can only detect the presence of viral RNA, but do not indicate whether the virus is still intact and able to spread. Theoretically, the higher the viral load, the greater the risk of an infected person infecting others. A team from the University of Geneva (UNIGE) and the University Hospitals of Geneva (HUG) has just provided valuable information on the influence of the variant and of vaccination on this viral load.

It was able to analyze samples taken since the start of the pandemic in the HUG screening center and which are kept for research purposes, with the authorization of the persons concerned. “We thus measured the viral load of 3 cohorts of patients during the first 5 days following the appearance of their symptoms in order to compare the viral load generated by the original virus (118 people, spring 2020), the Delta variant (293 people , fall 2021) and the Omicron BA.1 variant (154 people, winter 2021-2022), explains Benjamin Meyer, researcher at the Center for Vaccinology in the Department of Pathology and Immunology of the Faculty of Medicine. For the last two cohorts, we also examined whether a significant difference could be detected between vaccinated and unvaccinated people”. The vaccine was indeed not available for the original virus, in the spring of 2020.

Omicron’s viral load is surprisingly low

Result: the infectious viral load caused by Delta was much higher than that due to the original variant. But people infected with Delta who had received two doses of the vaccine had a much lower viral load than unvaccinated people affected by this same variant.

For Omicron, there was a surprise: “Contrary to what one might assume given its rapid spread, the infectious viral load was overall lower than that of the Delta cohort”, points out Isabella Eckerle, professor in the Department of medicine from the UNIGE Faculty of Medicine and head of the HUG-UNIGE Center for Emerging Viral Diseases, who directed this work published in “Nature Medecine”. In addition, people infected with Omicron who received two doses of vaccine had no benefit in this respect compared to unvaccinated people. We had to wait for the third dose, the booster, to see a drop in their viral load.

“This is consistent from an immunological point of view: many vaccines require three doses spaced several months apart to induce a lasting immune response, for example that against hepatitis B.” says Isabella Eckerle.

But the researchers are still unable to explain why Omicron is more contagious with a lower viral load than the other variants. “Our data suggest that other infectious mechanisms are at play,” explains Pauline Vetter, head of clinic at the HUG-UNIGE Center for Emerging Diseases. “It is now clear that Omicron’s mutations strongly differentiate it from other variants, which allows it to partially escape the vaccine, and decreases the effectiveness of certain antiviral treatments used so far.”

Be careful with new variants

Still, vaccination has nevertheless proven to be useful in limiting the occurrence of serious symptoms as well as, most likely, the transmission of the virus. In countries where the population, especially the elderly, is poorly vaccinated, Omicron has proven to be just as deadly, the researchers point out.

This study shows that the nature of the variant has a considerable influence on the viral load and on the efficacy of treatments and that the consequences of the appearance of each new variant must therefore be carefully analyzed. “In view of our results, the greatest caution should be taken when faced with a virus whose evolutions are not fully understood and against which currently available treatments are losing their effectiveness,” conclude the authors.

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