The second follow-up RT-PCR
Among the 44 patients who tested negative in the first test and continued their cooperation with the study, 3 patients (6.81%) tested positive 60 days after the initial infection (one month after the first negative RT-PCR test). These patients again were asymptomatic and the interval between the initial infection and this positive test is not significant enough to rule out viral shedding. Although, in case of viral shedding and/or presence of remnant viral particles, we would have expected to achieve a positive result in the previous test as well as this one; but this dissonance can be explained in absence of re-infection. Given the limited accuracy of RT-PCR, in case of a false negative in the first follow-up test or a false positive in the second test, continued shedding or complete recovery (respectively) can be misdiagnosed as re-infection [35, 36]. On the other hand, studies have shown that COVID-19 patients have a lower concentration of ACE2 monocyte expression—the endogenous entry receptor of SARS-CoV-2—and researchers hypothesize that the virus can remain dormant in peripheral blood mononuclear cells and cause a relapse after the respiratory system has been cleared of the virus and patient has tested negative [37-39].
Positive RT-PCR tests in absence of significant symptoms in a recovering patient has also been reported in many studies [18, 40]. In some cases, symptoms were present, although less severe than the first episode [41]. Our findings include asymptomatic and mildly symptomatic patients who re-tested positive for SARS-CoV-2. None of our cases had symptoms as or more severe than the first episode of infection. Contrasting our results, the majority of other reports have described more severe symptoms in patients who re-tested positive [15-17, 42] and researchers have hypothesized that a selection bias towards testing and confirming re-infection in symptomatic patients [43] and/or a primed/heightened immune response upon the second episode of infection can be the reason why most cases of re-infection pertain to patients with intense symptoms upon second exposure and infection [15, 42].
An episode of re-infection can be proved by a positive viral culture [21, 44] and/or genetic sequencing of the infective virus in both episodes of infection to confirm that the second episode is caused by a different clade of the virus. Tillett et al, reported a case of re-infection in a 25-year-old male, who recovered from a RT-PCR-confirmed episode of COVID-19, only to become symptomatic again after a 30-day symptom-free period. Genetic sequencing showed distinct genetic differences between the two specimen, indicating two separate instances of infection with genetically different variants of the virus [15]. Similar cases of re-infection with a genetically different clade of the virus within 6 months of an original episode of COVID-19 have been reported [16-18].
Unfortunately, genetic sequencing is not readily available and based on CDC recommendations in absence of genetic proof of infection with a different clade of the virus, a positive RT-PCR test that has been obtained after the first 90 days of the onset of the initial infection can be considered indicative of re-infection. Although, a positive RT-PCR test after two consecutive negative results, especially if accompanied by typical symptoms, can be defined as re-infection even within the first 90 days of the first episode of infection [21].