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  • A new study finds that the majority of COVID-19 cases are caused by people without symptoms.
  • Researchers say the findings show how identifying and quarantining symptomatic patients isn’t enough to control the rapid transmission of COVID-19.  
  • Evidence shows that the virus can incubate for 14 days. During that time the virus may replicate enough that a person may start to transmit it before any symptoms develop.

New research has found that over half of COVID-19 cases are likely caused by people without symptoms. 

Approximately 59 percent of all transmission is caused by presymptomatic individuals who hadn’t yet developed symptoms and asymptomatic people who never developed symptoms, according to the study published last week in JAMA Network. 

The researchers say the findings show how identifying and quarantining symptomatic patients isn’t enough to control the rapid transmission of COVID-19.  

Strict public health measures — including mask wearing and physical distancing — and widespread testing of people without symptoms is necessary to identify people who have COVID-19 and who may be silently transmitting the coronavirus. 

“Bottom line is that this SARS-CoV-2 virus has combined the best characteristics/traits of the coronavirus family and acquired the best ‘fit’ characteristics of a virus to go undetected and spread like wildfire,” said Dr. Daniel Fagbuyi, an emergency room physician who served as a biodefense expert in the Obama administration.

How much symptomless transmission is out there?

This new study suggests that over half of all transmission — 59 percent — can be traced back to asymptomatic and presymptomatic cases.

About 35 percent of transmission is thought to have come from presymptomatic individuals, or those who haven’t yet developed symptoms but will soon. 

Nearly 24 percent of transmission is thought to be caused by asymptomatic people who never experience symptoms.

Evidence shows that the virus can incubate for 14 days. During that time the virus may replicate enough that a person may start to transmit it before any symptoms develop. So while it may take time before a person develops noticeable symptoms, they could still carry and pass the virus to others.

“In essence, by the time you figure out your SARS-CoV-2 infection, sometimes it’s too late,” Fagbuyi said.

It’s long been thought that people with COVID-19 who don’t present symptoms play a significant role in community spread. 

Estimates have varied though, with some predicting asymptomatic individuals make up just 17 percent of cases, and others claiming that number is closer to 81 percent

According to this new study, recent estimates suggest around 30 percent of people with COVID-19 never develop symptoms and may be 75 percent as capable of transmitting the virus as those with symptoms. 

“SARS-CoV-2 has leveraged the exceptional infectivity rate of an airborne infectious disease and its ‘flying under the radar’ tactics by not having people show symptoms or only show symptoms by the time it is too late in up to 59 percent of the individuals,” Fagbuyi said. 

The role of asymptomatic transmission

When people are knowingly sick and have symptoms, they’re more likely to isolate because they don’t want to pass the illness to others and also may feel too sick to carry on with daily activities. 

People without symptoms may not realize they have the infection, giving them a false sense of security that increases the chance they may go out in the community and pass the virus to others. 

“This is problematic because many of these infected people who feel completely ‘well’ go to gatherings, stores, restaurants, and sporting events,” said Dr. Karen Jubanyik, a Yale Medicine emergency medicine physician and co-author of the book “Beat the Coronavirus: Strategies for Staying Safe and Coping with the New Normal During the COVID-19 Pandemic.”

What we’ve learned from other viruses

SARS-CoV-2 isn’t the first virus known to spread asymptomatically and presymptomatically.

Chickenpox can spread easily a couple of days before symptoms appear. 

The flu also has a short window in which there’s viral shedding, so while presymptomatic spread tends to be short-lived, it does occur, Jubanyik said.

Even with common colds, “by the time you developed symptoms of runny nose and cough, you have already spread it to other people,” Fagbuyi said. 

Historically, the poliovirus has been transmitted through significant asymptomatic spread. 

“We learned that polio vaccination is critical to reducing spread of this disease,” Jubanyik said. 

HIV can also spread presymptomatically as it’s often years before a person carrying the virus experiences symptoms of immunosuppression. 

Though there’s no effective vaccine available against HIV, “widespread testing of the population, including even those with minimal risk factors, is seen as the major initiative to prevent the spread of this virus,” Jubanyik said. 

The main way to manage HIV is to identify asymptomatic individuals via widespread testing along with safer sex practices and safer drug injection programs. 

“These are analogous to the mask mandates and social distancing recommendations for COVID-19,” Jubanyik said. 

What can be done about it?

Infectious disease experts say strict safety measures — mask wearing, physical distancing, hand washing — are necessary to control transmission from people who don’t have symptoms. 

Public health experts have also urged health officials to ramp up testing on asymptomatic and presymptomatic people. 

Doing so could help identify and isolate more people who have the virus and who are spreading it in their communities. 

“There should be prioritization of testing of asymptomatic people who are at risk of spreading it to others, which includes those coming back from necessary domestic and international travel, those coming back to school, those showing up for basic training, those needing a medical procedure/surgery, essential workers, including those working in stores and warehouses,” Jubanyik said. 

One issue with asymptomatic testing that has yet to be solved is the high rate of false negatives that could give some people who test negative a false sense of security. 

“Even a good negative test is just a freeze-frame at that moment,” Jubanyik said. There’s a 14-day incubation period, so while a person may test negative one day, they could test positive the next day.

They may just be “in the window between infection and virus shedding,” Jubanyik said. 

Widespread vaccination will also help manage asymptomatic and presymptomatic transmission, Jubanyik said. 

But until we achieve herd immunity in which most of the population is immune to the disease, everyone will need to remain vigilant. 

Mask up, wash your hands, keep a distance, and when in doubt, get tested.

The bottom line

New research has found that over half of COVID-19 cases are likely caused by people without symptoms.

Researchers say the findings show how identifying and quarantining symptomatic patients alone isn’t enough to control the rapid transmission of COVID-19.

Widespread testing for asymptomatic and presymptomatic individuals is also necessary to help identify and isolate silent spreaders who may account for a large percentage of community outbreaks.