Whole-town study reveals more than 40% of COVID-19 infections had no
symptoms

Date:
June 30, 2020
Source:
Imperial College London
Summary:
A study of COVID-19 in the quarantined Italian town of Vo`,
where most of the population was tested, reveals the importance
of asymptomatic cases.



FULL STORY ==========================================================================
A study of COVID-19 in the quarantined Italian town of Vo`, where most of
the population was tested, reveals the importance of asymptomatic cases.


==========================================================================
The authors of the new research, from the University of Padova and at
Imperial College London, published today in Nature, suggest asymptomatic
or pre- symptomatic people are an important factor in the transmission
of COVID-19.

They also argue that widespread testing, isolating infected people,
and a community lockdown effectively stopped the outbreak in its tracks.

The town of Vo`, with a population of nearly 3,200 people, experienced
Italy's first COVID-19 death on 21 February 2020. The town was put into immediate quarantine for 14 days. During this time, researchers tested
most of the population for infection of SARS-CoV-2, the virus that
causes COVID-19, both at the start of the lockdown (86 percent tested)
and after two weeks (72 percent tested).

The testing revealed that at the start of the lockdown, 2.6 percent
of the population (73 people) were positive for SARS-CoV-2, while
after a couple of weeks only 1.2 percent (29 people) were positive. At
both times, around 40 percent of the positive cases showed no symptoms (asymptomatic). The results also show it took on average 9.3 days (range
of 8-14 days) for the virus to be cleared from someone's body.

None of the children under ten years old in the study tested positive for COVID-19, despite several living with infected family members. This is
in contrast to adults living with infected people, who were very likely
to test positive.

As a result of the mass testing any positive cases, symptomatic or not,
were quarantined, slowing the spread of the disease and effectively
suppressing it in only a few short weeks.



========================================================================== Co-lead researcher Professor Andrea Crisanti, from the Department of
Molecular Medicine of the University of Padua and the Department of Life Sciences at Imperial, said: "Our research shows that testing of all
citizens, whether or not they have symptoms, provides a way to manage
the spread of disease and prevent outbreaks getting out of hand. Despite 'silent' and widespread transmission, the disease can be controlled."
The results of the mass testing programme in Vo` informed policy in the
wider Veneto Region, where all contacts of positive cases were offered
testing. "This testing and tracing approach has had a tremendous impact on
the course of the epidemic in Veneto compared to other Italian regions,
and serves as a model for suppressing transmission and limiting the
virus' substantial public health, economic and societal burden," added Professor Crisanti.

As well as identifying the proportion of asymptomatic cases, the team
also found that asymptomatic people had a similar 'viral load' -- the
total amount of virus a person has inside them -- as symptomatic patients.

Viral load also appeared to decrease in people who had no symptoms to
begin with but later developed symptoms, suggesting that asymptomatic
and pre- symptomatic transmission could contribute significantly to the
spread of disease, making testing and isolating even more important in controlling outbreaks.

Co-lead researcher Dr Ilaria Dorigatti, from the MRC Centre for
Global Infectious Disease Analysis, Jameel Institute (J-IDEA), at
Imperial College London, said: "The Vo` study demonstrates that the
early identification of infection clusters and the timely isolation of symptomatic as well as asymptomatic infections can suppress transmission
and curb an epidemic in its early phase. This is particularly relevant
today, given the current risk of new infection clusters and of a second
wave of transmission.



========================================================================== "There are still many open questions about the transmission of the
SARS-CoV- 2 virus, such as the role of children and the contribution of asymptomatic carriers to transmission. Finding answers to these questions
is crucial to identifying targeted and sustainable control strategies
to combat the spread of SARS-CoV-2 in Italy and around the world."
Professor Enrico Lavezzo, from the Department of Molecular Medicine
at the University of Padua said: "The result concerning asymptomatic
carriers is key.

We took a picture of the Vo` population and found that about half of
the population testing positive had no symptoms at the time of testing
and some of them developed symptoms in the following days. This tells us
that if we find a certain number of symptomatic people testing positive,
we expect the same number of asymptomatic carriers that are much more
difficult to identify and isolate.

"The fact that the viral load is comparable between symptomatic and asymptomatic carriers means even asymptomatic infections have the
potential to contribute to transmission, as some of the reconstructed
chain of transmission obtained from the detailed contact tracing conducted
in Vo` confirmed.

"On the one hand, it is likely that a symptomatic infection transmits
large quantities of virus, for example via coughing, but it is also
reasonable to think that symptoms may induce a person with a symptomatic infection to stay at home, limiting the number of contacts and hence the transmission potential. On the other hand, someone with an asymptomatic infection is entirely unconscious of carrying the virus and, according
to their lifestyle and occupation, could meet a large number of people
without modifying their behaviour." Co-first author Dr Elisa Franchin,
from the Department of Molecular Medicine of the University of Padua,
said: "This work highlights the efficacy of the containment strategies implemented since the finding of the first positive patient in the town
of Vo`. From a technical perspective, this work has been possible thanks
to the most advanced diagnostic technologies that we had available and
to the work of a large number of people with different skills: from
nurses to clerks, technicians, biologists and medical doctors. The en
mass participation of the Vo' population to this study has given us the opportunity to better understand the transmission of this virus and how
to avoid future infections." This research was funded by the Veneto
Region, Wellcome Trust, Royal Society, the European Union's Horizon
2020 research and innovation programme, the UK Medical Research Council
(MRC) and the UK Department for International Development (DFID) under
the MRC/DFID Concordat agreement, the EDCTP2 programme supported by the European Union and the Abdul Latif Jameel Foundation.


========================================================================== Story Source: Materials provided by Imperial_College_London. Original
written by Hayley Dunning. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Enrico Lavezzo, Elisa Franchin, Constanze Ciavarella, Gina Cuomo-
Dannenburg, Luisa Barzon, Claudia Del Vecchio, Lucia Rossi,
Riccardo Manganelli, Arianna Loregian, Nicolo` Navarin, Davide
Abate, Manuela Sciro, Stefano Merigliano, Ettore De Canale, Maria
Cristina Vanuzzo, Valeria Besutti, Francesca Saluzzo, Francesco
Onelia, Monia Pacenti, Saverio Parisi, Giovanni Carretta, Daniele
Donato, Luciano Flor, Silvia Cocchio, Giulia Masi, Alessandro
Sperduti, Lorenzo Cattarino, Renato Salvador, Michele Nicoletti,
Federico Caldart, Gioele Castelli, Eleonora Nieddu, Beatrice
Labella, Ludovico Fava, Matteo Drigo, Katy A. M.

Gaythorpe, Alessandra R. Brazzale, Stefano Toppo, Marta
Trevisan, Vincenzo Baldo, Christl A. Donnelly, Neil M. Ferguson,
Ilaria Dorigatti, Andrea Crisanti. Suppression of a SARS-CoV-2
outbreak in the Italian municipality of Vo`. Nature, 2020; DOI:
10.1038/s41586-020-2488-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200630103557.htm

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