The New England Journal of Medicine (NEJM) reported a letter to the editor today (April 6, 2020) from scientists in South Korea describing the challenges of dealing with early infection in patients who do not show symptoms.
Remember that South Korea started testing patients very early and tracking patients they tested. The first confirmed case of Corona Covid-19 was on January 20, 2020. They started testing people who had traveled and by Feb 10 there were 28 cases of laboratory confirmed Covid Virus ranging in age from 21 to 73. Only twenty of the patients developed early symptoms and only 8 of the 20 started with a fever. Some patients reported severe sore throat and some had only a mild cough.
They were able to track all of these patients who had recently traveled and see how they transmitted the virus. Patient 3, who had no symptoms, transmitted the virus infection to Patient 6, who in turn transmitted it to patients, 10,11 and 21. Patient 12 contracted the virus during a stay in Japan after he was exposed to a person with confirmed Covid-19. (He was not quarantined because he had no symptoms and they did not know about the exposure at that time) Patient 12 appears to have subsequently infected Patient 14.
Three of the 28 patients they followed remained asymptomatic and still were able to pass on the virus.
The scientists say, "Early detection of Covid-19 is difficult because of its apparent subclinical (no symptoms) nature in some persons."
There have been other reports and speculation of asymptomatic transmission but this is the first I've seen of the contact tracing and proven contagion in patients who never developed symptoms.
The real game changer will be when we have an immunoglobulin test that shows us who has been exposed and has antibodies, (IgM and IgG). These tests are being developed now and are different than the diagnostic tests currently being used to detect the virus.
We know that people are most infectious early and when they start with symptoms. But some people are asymptomatic and can still be infected and pass the virus.