• People who aren’t showing symptoms of coronavirus but have been infected are still contagious, a new study from China says.
  • Asymptomatic people experience a much milder version of COVID-19 that could cause subtle effects on the body.
  • The study included patients from the original coronavirus epicenter in Wuhan and concluded asymptomatic patients are infectious for an average of eight days.

Combine a long incubation period with an absence of distinct signs, and you end up with a pathogen that can spread with ease and infect plenty of people without alerting them to its presence. That’s why the novel coronavirus is so dangerous. Left unchecked, the virus moves from person to person faster than we could have imagined several months ago. A definitive COVID-19 diagnosis can’t be established without testing because of the lack of specific signs. The coronavirus causes symptoms shared with other pathologies, and the unique COVID-19 signs do not appear in all infected people.

And herein lies another big problem: Many people are asymptomatic, but they’re still contagious. A brand new study from China says that carriers without symptoms can be contagious anywhere from three to 12 days, for an average of around eight days. That’s why social distancing measures are so important even after opening up the economy, combined with proper access to testing, the use of face masks, and increased handwashing.

The study comparing symptomatic patients to asymptomatic COVID-19 patients comes from Wuhan, China, and was published a few days ago in The Journal of the American Medical Association (JAMA).

The researchers looked at 78 patients from 26 cluster cases of exposure to the Hunan seafood market or close contact with other COVID-19 patients. All of them were confirmed with RT-PCR tests, but 33 were asymptomatic — 42.3% of the total. That’s a high percentage that’s in line with recent findings from the CDC. Similarly, a study that analyzed the passengers of a cruise ship that left Argentina in mid-March with no confirmed infections on board found that 60% of the people on the boat were infected. Of those, 81% showed no symptoms.

The Wuhan study offered some conclusions about the asymptomatic patients that were observed during the study. Compared to symptomatic patients, they were younger and were more frequently female. Also, they had a lower proportion of liver injuries and less consumption of CD4 T lymphocytes. They also showed faster lung recovery in CT scans and a shorter duration of viral shedding from nasopharynx swabs.

What the study tells us is that asymptomatic patients may not feel anything, but their bodies still fight the virus. The coronavirus does affect the lungs, and lymphocyte counts can drop. Doctors from the UK think T-cell count is a marker that can predict severe COVID-19 cases, and boosting T-cells could improve the conditions of patients.

Viral shedding, or contagiousness, is around eight days on average, compared to up to 19 days for the symptomatic group. A separate set of studies say COVID-19 patients aren’t infectious after 11 days.

The study makes it clear, if there was ever any doubt, that asymptomatic people are still contagious, even if viral shedding can be as low as three days for some people:

Our finding of less consumption of CD4+T lymphocyte in asymptomatic infections suggests that damage to the immune system in asymptomatic infections was milder compared with symptomatic infections. Although patients who were asymptomatic experienced less harm to themselves, they may have been unaware of their disease and therefore not isolated themselves or sought treatment, or they may have been overlooked by health care workers and thus unknowingly transmitted the virus to others.

The research also explains that asymptomatic cases need to be identified and isolated as fast as possible:

Since patients with asymptomatic COVID-19 were relatively concealed, the fact of viral shedding detected via nasopharyngeal swabs must not be ignored. Therefore, identifying and isolating patients with asymptomatic COVID-19 as early as possible is critical to control the transmission of COVID-19. Close contacts of patients with COVID-19 should be closely monitored to avoid secondary transmission.

As with other COVID-19 studies, more research might be required to confirm these findings. But, as I’ve noted earlier, this Wuhan study couldn’t have come at a worse time. China just made a big deal of testing nearly 10 million out of Wuhan’s population of 11 million people and finding just 300 asymptomatic cases who were labeled not infectious. That seems highly unlikely. What’s worse is that China isn’t including these 300 people in its official COVID-19 statistics despite what studies like this one say. The simplest conclusion is that China keeps lying about its coronavirus cases.


Chris Smith started writing about gadgets as a hobby, and before he knew it he was sharing his views on tech stuff with readers around the world. Whenever he's not writing about gadgets he miserably fails to stay away from them, although he desperately tries. But that's not necessarily a bad thing.