Disturbing findings in study on the silent transmission of COVID-19

A new study on COVID-19 in Singapore and China has found that socalled silent transmission by asymptomatic and especially pre-symptomatic individuals can fuel the spread or outbreaks of the virus, findings that amplify the importance of both contact tracing and the wearing of masks. The study concluded that simply isolating the symptomatic is not enough to prevent COVID-19 outbreaks.

In an interview with CTV News Network, Dr. Marla Shapiro explained:

“It’s actually quite worrisome. When you look at the cases, you assume slightly less than 20 per cent (about 17 to 18 per cent) are people who are considered pre-symptomatic or asymptomatic. If you did the contact tracing, about 50 per cent of future transmissions came from these individuals, with most of it coming from pre-symptomatic individuals (about 48 per cent) and another 3 to 4 per cent coming from asymptomatic. So it’s pre-symptomatic people, meaning the day or two before they develop symptoms, where they are viral-shedding and spreading the infection. It tells us that if we just quarantine those who are symptomatic, we can still have more than 50 per cent of transmission from pre-symptomatic and our small number of asymptomatic.

“The CDC has revised its guidelines to make it clear that if we simply isolate our symptomatic without doing aggressive contact tracing to find those who’ve been exposed to pre-symptomatic or asymptomatic, we are missing all the silent transmission that will subsequently ensue. And it’s really important because if you are pre-symptomatic or asymptomatic, you are less likely to socially distance. You may not be as good about putting on a mask. This is why I think the masking universally that we now have in Ontario is so critical, because it hopes to address some of that. But as the flu season now approaches in October, that puts on additional layer of major concern.

“What this means is that we have to be more aggressive regarding contact tracing. When we do identify someone who’s been symptomatic, you have to know everybody they’ve been in touch with so that they understand that they may have been exposed and not developed symptoms yet; they have to be tested and isolated to make sure that they don’t have symptoms and there’s no silent transmission. It’s the people who have been exposed, don’t have symptoms yet, are doing viral shedding and then become ill; in that window of time before they are aware that they are ill, they have exposure to people and are putting them at risk. The layer between them, the mask, becomes very important. We are opening up (businesses) now, going on to phase 2, phase 3, phase 4 — we have to be very, very aware of the role of pre-symptomatic shedding.” ■

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