Dr Basharat Hussain Shah MD, FCCP
In early April, South Korea announced that patients who were declared as Covid-19 recovered have once again tested positive, suggesting that it could be a case of reinfection or reactivation (where the virus is not completely cleared and once again begins to replicate). By the end of April, South Korea had recorded 263 such cases. China, too, reported such cases.
Cause of concern
This news caused concern as it highlighted the issue of low levels of immunity in people who have recovered and the short duration of protection offered by antibodies developed in response to the infection. If natural infection shows only a short duration of protection, it raises concerns about the duration of protection that vaccines can offer.
But on April 30, infectious disease experts in South Korea confirmed that dead virus fragments still present in recovered people had led people numbering over 260 to test positive again for the novel coronavirus even weeks after marking full recovery.
The same recovered persons can test positive because fragments of the disease remained in their body.
The RT-PCR used for testing samples amplifies the genetic material of the virus prior to testing. The molecular test cannot distinguish between dead and live genetic fragments and hence cannot make out whether the virus is alive or not.
The Korea Centers for Disease Control and Prevention (KCDC) found out that so far there is no indication that patients who retest positive are contagious, even though about 44% of them showed mild symptoms. At the moment, there is no danger of further secondary or tertiary transmission.
Family of cases
The KCDC had investigated three cases from the same family where patients tested positive after recovering. But scientists were unable to grow (culture) the virus. Culturing the virus is typically done for testing and producing vaccines. The inability to grow the virus in a cell culture confirmed that live virus was not present but dead fragment of the virus.
The respiratory epithelial cell has a half-life of up to three months, and RNA virus in the cell can be detected with PCR testing one to two months after the elimination of the cell.
Dr Shah is Senior Consultant Pediatrics at LD Hospital’s NICU