In considering the spread dynamics of the new coronavirus — or SARS-CoV-2 — researchers, and health authorities have been pondering the importance of “silent” transmission.
This concept says that people who may have contracted the virus but who do not experience any symptoms could unwittingly contribute to the spread by not realizing they are carriers.
It remains unclear just what the likelihood is of asymptomatic transmission.
Recently, Dr. Maria Van Kerkhove, Head of Emerging Diseases and Zoonosis at the World Health Organization (WHO), said that this form of transmission was “rare,” though later, she and her colleagues revised that statement in a Q&A session.
Now, a new study from the Scripps Research Translational Institute in La Jolla, CA, emphasizes just how many cases of SARS-CoV-2 infection are asymptomatic.
Its authors, behavioral scientist Daniel Oran and Dr. Eric Topol, professor of Molecular Medicine at Scripps Research, warn that the high proportion of asymptomatic infections may contribute to the wide transmission of the virus.
Furthermore, they warn that even people who have not experienced any symptoms may still face long-term effects following exposure to the virus.
The two researchers present their work in a study paper now featuring in Annals of Internal Medicine.
Dr. Topol and Oran reviewed the data of SARS-CoV-2 studies that included clear information about testing methods for diagnosing infection with the virus.
They ended up assessing studies of 16 different cohorts, including groups of cruise ship passengers, prison inmates, and nursing home residents tested for COVID-19.
“What virtually all of them had in common was that a very large proportion of infected individuals had no symptoms,” notes Oran commenting on the findings.
“Among more than 3,000 prison inmates in four states who tested positive for the coronavirus, the figure was astronomical: 96% asymptomatic,” he emphasizes.
Looking at all the data together, the investigators estimated that around 40–45% of people contracting the new coronavirus are likely to be asymptomatic.
They also inferred that people who showed no COVID-19 symptoms were, nevertheless, liable to spread the virus over 14 days or more after infection.
“The silent spread of the virus makes it all the more challenging to control,” notes Dr. Topol.
“Our review really highlights the importance of testing. It’s clear that with such a high asymptomatic rate, we need to cast a very wide net; otherwise, the virus will continue to evade us,” he adds.
Still, Dr. Topol and Oran note that it remains difficult to tell just how likely it is for asymptomatic people to spread the virus further, even though both symptomatic and asymptomatic individuals present similar viral loads.
To find out, the researchers explain, we would need access to more longitudinal studies with representative cohorts of asymptomatic individuals. For the current review, the researchers were able to obtain longitudinal data on only five cohorts.
Another issue that the researchers draw attention to in their review is the impact of the new coronavirus on the health of asymptomatic individuals.
Looking at CT scan results for a cohort of 76 asymptomatic individuals who were present on the Diamond Princess cruise ship, they found that 54% of these individuals presented subclinical lung abnormalities.
In their paper, the authors qualify this finding as “disturbing,” as it suggests that even in those who experience no symptoms of infection, the new coronavirus may be causing harm, possibly affecting normal lung function.
Still, further research must confirm whether or not such lung abnormalities cause poorer respiratory health.
Another issue deriving from the lack of adequate longitudinal data is that it can be hard to tell between individuals who are presymptomatic — that is, yet to develop symptoms of infection with SARS-CoV-2 — and those who remain asymptomatic throughout the course of the infection.
Dr. Topol and Oran thus encourage further longitudinal testing to gain more clarity about SARS-CoV-2.
Even so, the researchers insist that the apparent high number of asymptomatic individuals is a good enough reason to urge everyone to wear face masks in public for the time being.
“Our estimate of 40–45% asymptomatic means that if you’re unlucky enough to get infected, the probability is almost a flip of a coin on whether you’re going to have symptoms. So to protect others, we think that wearing a mask makes a lot of sense.”
– Daniel Oran
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