People Without Symptoms May Account for Over Half of COVID-19 Spread

As our lives have been upended by the COVID-19 pandemic[1] in recent months, numerous measures have been put in place to try to slow the spread of the viral infection. Some of these steps — like taking temperature scans of people’s foreheads — are aimed at detecting people who may be infected. Other steps, like mask mandates, are more general and make no distinction between sick and healthy people.

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While it’s likely that multiple approaches to limit the spread of COVID-19 will be necessary for some time, knowing as much as we can about how the virus spreads can help us decide what measures are most important to stop its spread. And a new study suggests that while screening for sick people may be helpful in some contexts, it’s probably people without symptoms who are driving most of the virus’s spread.

Model looked at different scenarios for COVID-19 spread

The study, published in the journal JAMA Network Open[3], involved building a mathematical model for how COVID-19 spreads based on a range of respected studies. In each scenario for the model, the researchers changed certain numbers to reflect different findings on key aspects of how COVID-19 spreads. For example, peak infectiousness — the point in time when you’re most likely to infect someone else — was varied between three and seven days after developing the infection, since this trait may vary from person to person. The researchers also varied the proportion of people infected with the virus from 0% to 70%, since this number also varies throughout the population in different areas. In each version, people were assumed to be infectious for a 10-day period.


The researchers also assumed, based on previous study findings, that 30% of people infected with the COVID-19 virus never develop symptoms, and that these people are 75% as infectious as people who do develop symptoms. Under a broad range of scenarios — with different times of peak infectiousness, and different proportions of the population infected — the researchers found that over half of infections were likely to be spread from people who weren’t symptomatic at the time of infection.

Specifically, they found that people who never develop symptoms of COVID-19 account for 24% of transmission of the virus under the model. On top of that, 35% of transmission occurs from people before they develop symptoms of the infection. This makes a combined 59% of new infections coming from people who don’t display symptoms.

Results highlight importance of broad measures to stop virus

If the model that the study used is correct, then about six in 10 new cases of COVID-19 are caused by spread of the virus from people without any symptoms at the time. Since another four in 10 new infections are spread by people with COVID-19 symptoms, measures to identify these people — like temperature checks — are likely to be important tools, but won’t have an impact on the most common scenarios in which the virus spreads.

Absent extremely frequent and widespread testing for COVID-19 — something that’s nowhere close to being reality in the United States — the study highlights the need to continue with protective measures that affect sick and healthy people alike. Since most people who are actively spreading the virus won’t feel sick or even show any symptoms, measures like wearing masks and practicing physical distancing are likely to be important ways to help stop the spread of COVID-19 throughout the population.

Of course, telling people who feel sick to stay home is also very important, since a significant number of new infections come from these people spreading the virus. But the pandemic would most likely rage on even if these people isolated themselves perfectly.

“Measures such as wearing masks, hand hygiene, social distancing and strategic testing of people who are not ill will be foundational to slowing the spread of COVID-19 until safe and effective vaccines are available and widely used,” the researchers conclude.

Want to learn more about coronavirus and diabetes? Read “Coronavirus and Diabetes: A COVID-19 Update,”[4] “Healthy Eating During Hard Times”[5] and “COVID-19: Staying Safe at Work.”[6]

  1. COVID-19 pandemic:
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  3. JAMA Network Open:
  4. “Coronavirus and Diabetes: A COVID-19 Update,”:
  5. “Healthy Eating During Hard Times”:
  6. “COVID-19: Staying Safe at Work.”:

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