5th June 2020
Researchers have been studying SARS-CoV-2 not only to find a way to keep it from making people sick but also to understand how it functions and interacts with the human body.
How long a virus stays in the body is known as viral persistence which can differ from the length of time a COVID-19 survivor can shed viral fragments which often lead to positive tests post-recovery.
COVID-19 persistence is how long someone remains contagious, how long of a hospital stay is necessary, and whether re-infection is possible.
Researchers have found that persistence for the COVID-19 causing virus differs per individual and even per organ.
Mary Kearney, a senior scientist who studies HIV drug resistance at the National Cancer Institute’s Center for Cancer Research explains that the virus is made up of RNA much like Hepatitis C, whereby persistent infections can lead to liver disease or cancer long after the original infection.
"Where there’s long-term persistence, there can be long-term consequences," she said.
A complication when determining the persistence of COVID-19 is its dependence on PCR reactions during testing to determine viral load.
Whilst PCR testing can tell one whether or not there was a recent infection, the test can not distinguish between the living and replicating fragments of the virus and non-infectious viral debris as it searches specifically for genetic fragments of a virus from, the given sample.
Andrew Karaba, an infectious disease fellow at Johns Hopkins University explains: "Even when the virus is no longer infectious, there’s a period when you can still detect its RNA."
The most accurate way to test for live viruses is by growing it in a cell culture petri dish, which is difficult to do since nasal swabs do not always gather enough infected cells.
The closest answers to COVID-19's viral persistence is a study in Germany that looked at mild cases and found that they could not grow live viruses from throat swabs or sputum samples after 8 days of symptoms manifesting.
They also found that people's production of viral DNA is highest in the first few days of infection.
Nature conducted a similar study on SARS-CoV-2 and found that the virus could still be cultured after 9 days from samples however RNA fragments were found in several samples after 31 days.
With that knowledge, questions arise on how reinfections happen.
In most cases where patients experience long-term symptoms, the reason was not due to reinfection.
South Korea's Centers for Disease Control and Prevention recently traced the contacts of 285 patients who had re-tested positive after a negative PCR result.
The patients could not transmit the virus to others nor had they been reinfected from one of their contacts.
Diane Griffin, a virologist at the John Hopkins Bloomberg School of Public Health explains: "Usually, when people recover from acute viral infections, their immune response kills the cells affected to eliminate the virus, but when viruses infect long-lived cells, such as neurons, the immune system can’t afford to destroy them. That means you don’t actually get rid of all the virus genome, instead, the virus might hide in parts of the body for long periods."
This means that even if viral spread is no longer happening, some virus proteins are being produced in a small number of cells and these fragments are what cause the body to maintain a perpetual immunity response.
"Some aspects of the immune system exist as they are because we are chronically infected," says Skip Virgin, executive vice president, and chief scientific officer of the biotechnology company Vir.
Avindra Nath, the clinical director of the National Institute of Neurological Disorders and Stroke at the National Institutes of Health says that this perpetual immunity function can cause "cytokine storms" in COVID-19 patients which is the damaging overdrive response of the immune system to infections, which could explain why patients relapse and experience long-term symptoms.
Santosh Vardhana, a medical oncologist at Memorial Sloan Kettering Cancer Center says that since viral persistence and immunity differ at an individual level, creating a vaccine that provides universal immunity may be more difficult than if viral particles predictably affected individuals.
The World Health Organisation's official website contains plenty of useful information about SARS-CoV-2, also known as the new coronavirus, including common Q&A as well as updates of vaccine research from all over the world.