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Blood tests that verify coronavirus exposure are starting to appear online, and preliminary results suggest that many people have unknowingly been infected.
Even people who eventually experience the common symptoms of COVID-19 do not begin to cough and increase fever the moment they become infected.
William Petri is a professor of medicine and microbiology at the University of Virginia and specializes in infectious diseases. Here, review what is known and what is not about asymptomatic cases of COVID-19.
How common is it for people to contract and fight viruses without knowing it?
In general, having an infection without any symptoms is common. Perhaps the most infamous example was typhoid Mary, who transmitted typhoid fever to others without having any symptoms in the early 1900s.
My colleagues and I have discovered that the body fights many infections without the person knowing it. For example, when we carefully follow children for parasite infection Cryptosporidia, a leading cause of diarrhea, almost half of people with infections showed no symptoms at all.
For the flu, an estimated 5 to 25 percent of infections occur without symptoms.
For the most part, the symptoms are actually a side effect of fighting an infection. The immune system takes a while to regain that defense, so some cases are considered more presymptomatic than asymptomatic.
How can someone transmit the coronavirus if they are not coughing or sneezing?
Everyone is on guard against drops coming out of a cough or sneeze from a coronavirus patient. They are a great reason why public health officials have suggested that everyone should wear masks.
But the virus also spreads through normal exhalations that can carry tiny droplets that contain the virus. Regular breathing can spread the virus several feet or more.
The spread could also come from fomites: surfaces, such as a doorknob or the handle of a grocery cart, that are contaminated with the coronavirus by contact with an infected person.
What is known about how contagious an asymptomatic person can be?
Whatever happens, if you have been exposed to someone with COVID-19, you should be quarantined for the entire 14 day incubation period. Even if you feel good, you still risk passing the coronavirus on to others.
More recently, it has been shown that high levels of the virus are present in respiratory secretions during the “presymptomatic” period that can last for days or more than a week before the characteristic COVID-19 fever and cough.
This ability of the virus to be transmitted by people without symptoms is one of the main reasons for the pandemic.
After an asymptomatic infection, would anyone still have antibodies to SARS-CoV-2 in their blood?
Most people develop antibodies after COVID-19 recovery, probably even those without symptoms. It is a reasonable assumption, as far as scientists know about other coronaviruses, that these antibodies will offer some measure of protection against reinfection.
But still nothing is known for sure.
Recent serological surveys in New York City that test people’s blood for antibodies to SARS-CoV-2 indicate that up to one in five residents may have been previously infected with COVID-19.
Their immune systems had fought off the coronavirus, whether they knew they were infected or not, and many apparently not.
How widespread is asymptomatic COVID-19 infection?
No one knows for sure, and at the moment much of the evidence is anecdotal.
For a small example, consider the nursing home in Washington where many residents became infected. Twenty-three tested positive. Ten of them were already sick. Ten eventually more developed symptoms. But three people who tested positive never contracted the disease.
When doctors evaluated 397 people who were staying at a homeless shelter in Boston, 36 percent tested positive for COVID-19, and none of them had complained of any symptoms.
For Japanese citizens evacuated from Wuhan, China and tested for COVID-19, 30 percent of those infected were asymptomatic.
A preprinted Italian study that has not yet been peer-reviewed found that 43 percent of people who tested positive for COVID-19 showed no symptoms.
Of concern: The researchers found no difference in how potentially contagious people were with and without symptoms, based on the amount of virus the test found in the individuals’ samples.
Serological antibody surveys are launched in different Parts of the country add more evidence that a good number, possibly 10 to 40 percent, of those infected may not experience symptoms.
Asymptomatic SARS-CoV-2 infection appears to be common and will continue to complicate efforts to control the pandemic. 
William Petri, professor of medicine, University of Virginia.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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