Why Some Patients Test Positive Even After Recovering From COVID 19 ?
Tags: Covid 19 Testing, Corona, Covid 19, Health, Coronavirus

COVID-19 Sample collection.

Not just India, many other countries have had cases where patients tested positive for COVID 19, after being fully recovered from the disease. In such cases, some of these patients have been quarantined again. But why does it happen? Science has an explanation.

According to the guidelines from the World Health Organisation, clinically recovered patients should test negative for virus twice.

A person’s exposure to the novel coronavirus can be detected by two methods — by testing the presence of the virus in the body or by the presence of anti-bodies released by our bodies to fight the virus.

The second test is also called a fast or serological test. It is used only a week after a person is infected by the virus. This is the time body is required to produce anti-bodies which then show up in our blood samples.

So, if a person is tested for the antibody test anytime before the gestation period, there are chances that the test results will be negative.

The serological tests are useful to conduct tests on a large scale and evaluate the spread of disease in a cluster or community. The results come handy in devising models to evaluate the trajectory of proliferation.

Why some patients test positive after recovering from the disease

This can happen if the sample which is collected has dead virus. The pathogen cannot be cultured inside the laboratories but the test can show the presence in the swab collected for examination.

This is why a patient might test negative in the second week of disease and then again test positive later.

The RT-PCR test

On the other hand, WHO recommends the Reverse Transcriptase- Polymerase Chain Reaction(RT-PCR), which is the most commonly used confirmatory test to identify COVID-19 cases. This test reads the presence of RNA in the sputum or swab taken from the nose, throat or lower respiratory organ to detect if a patient is a carrier.

The RT-PCR is the more sensitive test because while detecting the virus, the nucleic acid is amplified many times over to detect the presence of the microbe. The detection during the first week is better done using the RT-PCR test. But sometimes, this test also might fail to detect the presence of the virus.

The RT-PCR test might give false results for three out of 10 patients.

So what’s going on?

One explanation could be that people can be reinfected with SARS-CoV-2 — that is, mount a new immune response to the virus even after their body has beaten back an initial infection. If that is the case, as countries around the world begin lifting social distancing restrictions and revving up their economies, they may need to reinstate mitigation efforts and prepare to support newly-overwhelmed healthcare systems.

But experts say it seems unlikely that reinfection is the cause of these new positive results.

One explanation could simply be that the tests administered to these patients weren’t sensitive enough to distinguish between an active infection and one from which a patient has mostly recovered. “What many people don’t understand is that PCR tests simply for the virus’ genetic material and it is not an assay for active virus,” says Richard Condit, a molecular biologist and professor emeritus at the University of Florida College of Medicine.

As the body is recovering from an infection, it clears what Condit calls “virus litter” — inactive debris of viral cells — from the lungs. That litter can be coughed up into the throat, where clinicians take a sample, and a sensitive diagnostic test might mistake that debris for an active infection. It’s also possible — though rare — for a swab to pick up virus in the throat but for that virus to never lead to an active infection.

There could be other testing issues, too. False negatives are surprisingly common, notes Purvi Parikh, an immunologist at NYU Langone Health. Sometimes healthcare workers don’t collect enough material from a patient to get a clear answer; sometimes the test themselves vary so widely that a negative in one could be a positive in another.

Another possibility is that the amount of virus in the patients’ bodies dipped, then spiked again. “They could have had low levels [of the virus] that the test wasn’t picking up and started to replicate again,” says Brianne Barker, an associate biology professor at Drew University. Viruses can reactivate or resurge after a period of latency in the body, though it’s more common in other kinds of viruses — herpes, for example, and varicella, the virus that causes chickenpox and shingles. If SARS-CoV-2 could act similarly, the body may have some immunity when it resurges, meaning that a person may not be able to transmit the virus or that symptoms could be much more mild the second time around.

Yet another option could be that a patient is infected with a different strain of SARS-CoV-2 (preliminary studies show that there are three) the second time they test positive, which could elicit a different immune response, Parikh suggests.

Thank you.