COVID-19: Vaccination greatly reduces infectious viral load, study finds


Date:
April 14, 2022
Source:
Universite' de Gene`ve
Summary:
By comparing the infectious viral load caused by ancestral
SARS-CoV-2 as well as by the Delta and Omicron variants, scientists
highlight the benefits of vaccination.



FULL STORY ========================================================================== Measuring the viral load of people infected with SARS-CoV-2 is one of the
main factors in evaluating the infectiousness of COVID-19 patients. Viral
load can be influenced by the infecting SARS-CoV-2 variant as well as
the vaccination status of the patient.


==========================================================================
A research team from the University of Geneva (UNIGE) and the University Hospitals of Geneva (HUG) measured the infectious viral load of nearly
600 symptomatic patients to detect possible differences between the
original virus, Delta and Omicron sublineage BA.1, as well as according
to vaccination status.

They discovered that Delta causes a higher viral load than the original
virus and the Omicron variant. For Delta and Omicron breakthrough
infections, vaccination drastically reduces the viral load. In the case
of Omicron, however, the decrease was only observed after three doses
of vaccine.

Furthermore, Omicron's very high infectiousness is seemingly related
to factors other than viral load alone. These results, to be read in
the journal Nature Medicine, highlight the benefit of vaccination for
public health in addition to individual protection against the severe
form of the disease, and remind us that variants of the virus must be
closely monitored to prevent further massive outbreaks.

The diagnosis of COVID-19 consists of a PCR test performed on a
nasopharyngeal or salivary swab. "This test is very effective in
identifying infected people, but does not indicate whether they are
infectious, that is, capable of transmitting the virus to other people,"
says Isabella Eckerle, professor in the Department of Medicine at UNIGE
Faculty of Medicine and head of the HUG- UNIGE Centre for Emerging Viral Diseases, who led this work. "However, the notion of contagiousness
is essential for deciding on collective prevention measures, such as
periods of isolation." PCR tests can only detect the presence of viral
RNA, but do not indicate whether the virus is still intact and able to
spread. The measurement of the infectious viral load necessarily involves culturing the virus for several days in a biosafety level 3 laboratory,
a procedure impossible to perform routinely.

Lower viral load due to vaccination Since the beginning of the pandemic, samples taken at the HUG screening centre have been kept for research
purposes, with the authorisation of the persons concerned. "We were
able to reanalyze samples from previous waves of the disease," explains Benjamin Meyer, a researcher at the Centre for Vaccinology in the
Department of Pathology and Immunology at UNIGE Faculty of Medicine. "We measured the infectious viral load of 3 cohorts of patients during the
first 5 symptomatic days to compare the viral load caused by the original
virus (118 samples, spring 2020), the Delta variant (293 samples, fall
2021) and the Omicron variant sublineage BA.1 (154 samples, winter 2022),
as well as, for the last two cohorts, whether a significant difference
could be detected in vaccinated and unvaccinated individuals." Overall,
the infectious viral load for the Delta cohort was significantly higher
than that of the cohort with the original virus. However, people infected
by Delta who received two doses of mRNA vaccine had a significantly
lower infectious viral load than unvaccinated people. "For the Omicron
cohort, contrary to what can be assumed given its rapid spread, the
infectious viral load was overall lower than that of the Delta cohort,"
says Isabella Eckerle.

In contrast, only people who were boosted (that is, having received three
doses of the vaccine) had their viral load decreased; people who received
two doses only had no benefit in this regard compared to unvaccinated
people. "This is immunologically consistent: many vaccines require 3
doses spaced several months apart to induce a sustained immune response,
such as that against Hepatitis B virus," explains Isabella Eckerle.

Omicron: a variant far away from the previous ones Why is the Omicron
variant so contagious, if the viral load it induces is lower than
its predecessors? "We still don't know, but our data suggest that
other infectious mechanisms are at play," explains Pauline Vetter,
clinic director at the HUG-UNIGE Center for Emerging Diseases. "It
is now clear that the mutations of Omicron strongly differentiate
it from other variants, allowing it to partially escape the vaccine,
and diminish the effectiveness of some antiviral treatments used so
far." However, vaccination has been shown to be useful in limiting the occurrence of severe symptoms and most likely also the transmission of
the virus. Indeed, in countries where the population, especially the
elderly, is poorly vaccinated, Omicron has proven to be just as deadly.

The Geneva study also shows that the knowledge acquired for previous
variants must be updated every time a new variant emerges to be able
to adapt the means of combating COVID-19. "In view of our results,
the greatest caution should be exercised in the face of a virus whose
evolution is not fully understood, and against which currently existing treatments lose some of their effectiveness," conclude the authors.

This work was carried out thanks to grants from the Swiss National Science Foundation (SNSF), the Pictet Group's Fondation Ancrage bienfaisance
and the HUG Private Foundation.


========================================================================== Story Source: Materials provided by Universite'_de_Gene`ve. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Olha Puhach, Kenneth Adea, Nicolas Hulo, Pascale Sattonnet, Camille
Genecand, Anne Iten, Fre'de'rique Jacque'rioz Bausch, Laurent
Kaiser, Pauline Vetter, Isabella Eckerle, Benjamin Meyer. Infectious
viral load in unvaccinated and vaccinated individuals infected
with ancestral, Delta or Omicron SARS-CoV-2. Nature Medicine,
2022; DOI: 10.1038/s41591-022- 01816-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220414110836.htm

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