Follow Us:
Sunday, May 22, 2022

Asymptomatic coronavirus carriers: How contagious are they?

More important than determining who has gotten the disease is determining who is giving it to others. Are people without symptoms transmitting the virus?

By: Bloomberg |
April 9, 2020 3:41:29 pm
Asymptomatic coronavirus carriers: How contagious are they? Vendors wait to enter a wholesale market to buy vegetables during the nationwide lockdown to curb the spread of coronavirus, in Jalandhar, Thursday, April 9, 2020. (PTI Photo/File)

To have any idea whether our current social distancing efforts in the United States are helping slow the spread of coronavirus, and to gauge how long we should go on this way, scientists need to know how many people have mild or asymptomatic cases of Covid-19. There could be many such people. Since their symptoms are vague and possibly non-existent, the case numbers that climb by the day aren’t what they seem. The true numbers may be much higher — or only a little higher. Understanding the mild cases can help researches get a handle on the spread of the disease — how it’s spreading and how widespread it’s already become.

As several experts have explained to me, it’s not enough to know the R0 number — the average number of people each positive case infects. An R0 of two might mean that 20 people spread the disease to two people each, or that 19 people give the disease to nobody and one person gives it to 40 others.

Jeffrey Shaman, a professor at Columbia University who studies how the environment affects infectious diseases, has gathered evidence that in China, the epidemic was driven by a lot of not-very-sick people. He estimates that about 86% of infections early in that outbreak were transmitted by people who never got sick enough to go to the doctor.

The paper he co-authored, published in Science, was popularized as a warning against “silent spreaders” — people who have no idea they are infected. Another study showed that in China, people within one family had symptoms that ranged from undetectable to severe. Random testing done in Iceland showed that 50% of people who tested positive had no symptoms.

Best of Express Premium

FY22: As Covid curbs ease, outward remittances up 55% to all-time highPremium
A letter from Mathura: ‘Radha ki chunari bhi Salma silti hai’Premium
A Letter From Varanasi: ‘It’s our waqt… Ayodhya will happen in Kash...Premium
Tavleen Singh writes: Evil under the sunPremium

📢 Express Explained is now on Telegram. Click here to join our channel (@ieexplained) and stay updated with the latest

But more important than determining who has gotten the disease is determining who is giving it to others. Are people without symptoms transmitting the virus? Shaman’s study suggested that about 86% of documented infections were picked up from people with undocumented ones. That’s why this issue is worth a closer look, and why I called him.

Asymptomatic coronavirus carriers: How contagious are they? Vegetable and fruit traders voluntarily close the market for a day to avoid the spread of coronavirus and to clean the market-yard, in Pune. (Express Photo: Pavan Khengre)

“You’re getting stuck on this symptomatic/asymptomatic divide,” he told me. Reality is not that clear cut. Symptoms are self-reported and subjective. The symptoms of mild coronavirus are pretty vague except for the fever, which people might not measure if they feel normal. New evidence suggests that those with very mild cases might experience a loss of the sense of smell. The prevalence of this symptom is not yet known – people who can still smell should not assume they are uninfected, but those who suddenly lose that sense should consider themselves sick.

Don’t miss from Explained | Tracking the coronavirus curve in India 

Long before this new coronavirus emerged, Shaman learned a lot about the sorts of symptoms typical of Covid-19 by studying the connections between symptoms and infection with flu and colds. By finding 2,500 people healthy enough to be out and about in New York City, testing them for cold and flu viruses, and asking them about symptoms, he found lots of variability – people with symptoms who had no infection, and people with infection who had no symptoms.

In another study, he and colleagues tracked 200 people by cell phone and noted daily symptom reports, while testing them for cold and flu viruses weekly. What they found was that only one in four people with the flu seek medical care, and only one in 25 people with a cold will do so.

And, if we’re honest, back in the pre-Covid-19 days, many of us had mild coughs, sore throats and other common-cold symptoms but went to work, shopped, ate in restaurants and even sometimes got on planes. That’s the reason, Shaman says, that these respiratory viruses circulate so successfully.

Asymptomatic coronavirus carriers: How contagious are they? Locals rush to purchase items after the Uttar Pradesh government seals certain areas on Wednesday. (Express Photo: Gajendra Yadav)

The range of severity is making this pandemic hard to fight — if it were severe in most people, as SARS was, it would be easier to stop the spread. If it were mild in most people, we’d treat it like the flu. But instead it’s extremely severe and deadly in some people and extremely mild — and possibly even silent — in others.

Even the line between severe and mild is blurry. In the United States, more younger people with the virus are landing in the hospital, while in China serious cases were concentrated among those over 60. It’s unlikely that this difference can be attributed to a mutation in the virus — scientists are tracking that. But there are differences in our underlying health conditions, and differences in awareness. The Chinese, early on, thought they were getting colds or flu.

Also read | Why children are less affected by COVID-19, but more susceptible

It’s been established that after people become infected, they can walk around for an average of five days and up to around two weeks before they start to feel sick, and it’s not yet clear how many of those days their bodies shed enough of the virus to infect others.

For their new research on Covid-19, Shaman and his colleagues tallied the reported symptoms of people who got the disease in China during the period between January 10 and January 23, right before the Chinese government started imposing travel restrictions.

The conclusion was that people without documented symptoms were about half as contagious as those with them, and yet they constituted the majority of people who got and spread the disease during that period. Whether they had no symptoms, were pre-symptomatic or had mild symptoms isn’t important. What is, he said, is the fact that many people with the virus felt pretty good, and were moving about society and creating new chains of transmission.

Asymptomatic coronavirus carriers: How contagious are they? The fire safety department of Ahmedabad Municipal Corporation conducted a disinfectant exercise in the city on Wednesday. (Express Photo: Javed Raja)

That’s why telling people to stay home if they feel sick isn’t enough — not just because everyone defines “sick” differently, but because people are contagious before they feel sick, and some may not feel sick at all.

Countries that have successfully avoided being overwhelmed by Covid-19 have used extensive testing and contact tracing, so that healthy-feeling people who had been in contact with someone who tested positive would stay home for two weeks just in case. The United States has so far failed on that front — but that’s the next vital step in curtailing this pandemic. In the meantime, everyone has to focus on keeping a safe distance from others in order to avoid getting or giving the virus.

Here’s a quick Coronavirus guide from Express Explained to keep you updated: What can cause a COVID-19 patient to relapse after recovery? | COVID-19 lockdown has cleaned up the air, but this may not be good news. Here’s why | Can alternative medicine work against the coronavirus? | A five-minute test for COVID-19 has been readied, India may get it too | How India is building up defence during lockdown | Why only a fraction of those with coronavirus suffer acutely | How do healthcare workers protect themselves from getting infected? | What does it take to set up isolation wards?

For all the latest Explained News, download Indian Express App.

  • Newsguard
  • The Indian Express website has been rated GREEN for its credibility and trustworthiness by Newsguard, a global service that rates news sources for their journalistic standards.
  • Newsguard
0 Comment(s) *
* The moderation of comments is automated and not cleared manually by