It is not uncommon for viruses to persist at low levels in the body even after someone recovers from an illness, said Ebenezer Tumban, a virologist at Michigan Tech University. For example, Zika virus and Ebola virus are known to stick around for months after patients recover, Johnson noted.
The test that the four patients from Wuhan, China, underwent looks for genetic fragments of the virus in the body, Tumban said. The Tamiflu they were taking could have pushed the number of viral copies in their bodies down to just a few, he said. At that point, the test would not have been sensitive enough to detect the virus.
After the antiviral treatment ended, the viruses may have begun replicating again at a low level, Tumban said. There would not have been enough of the virus to cause tissue damage, so the patients felt no symptoms. But the number of viral copies would have gotten high enough for the test to catch them again.
At that point, the individuals were likely not very contagious, Johnson said. Coughing and sneezing spews viral particles around, but these individuals were not coughing or sneezing. Their viral loads were also low. It would take more intimate contact to spread the virus.
“They should be careful in the household setting not to share drinks and make sure they’re washing their hands frequently,” she said. “But if they’re just a carrier, they shouldn’t be able to transmit outside of that close contact of shared beverage and food.”
None of the study patients’ family members tested positive for coronavirus at the time of the paper’s publication. However, the authors noted that the patients were all medical professionals who took very careful precautions to avoid spreading the disease while at home.
Virus that persists in the body may elicit enough of an immune response to provide some protection against new infection, Johnson said. There are many questions about how long immunity would last, though, Tumban said. For example, the body maintains immunity against the coronaviruses that cause the common cold for only a year or two, he said. And there is always the possibility that the new coronavirus would mutate as it moves through populations, changing into a version that already-exposed immune systems can’t recognize.
“The challenge is, how fast does this mutate?” Johnson said.
More follow-up studies are needed to understand recovery from COVID-19, Johnson said. The individuals in the study from Wuhan were all of similar age and health status, and none experienced severe illness from COVID-19.
Future research should also look at viral loads within the lungs, Tumban said. A throat swab captures the virus only from the upper reaches of the respiratory tract, but the virus makes its home deep in the lungs. Sampling from the lungs is a more invasive procedure, involving washing fluid through the alveoli (small air sacs in the lungs) and testing that fluid for viral particles, Tumban said. Still, the study suggests that long-term monitoring of recovered patients and their contacts is important.
“One week or two weeks after, is the amount of virus in the blood or lungs going to come up to a higher concentration so that the person can transmit it to other people?” Tumban said. “That’s something that we still don’t know.”
Originally published on Live Science.