The third follow-up RT-PCR
According to the recommended CDC definitions [28], re-infection is a probable diagnosis for the three symptomatic patients who tested positive in the last RT-PCR test; but it is less likely in case of the one asymptomatic patient. The third RT-PCR screening was performed 120 days after the diagnosis of COVID-19 in 32 patients (who had tested negative on both of the previous tests). Although, the symptoms could have been caused by re-activation of dormant infection and release of viruses from body reservoirs, they could also be caused by re-infection. A similar process involving latent infection of cells followed by transcription of viral genome has also been suggested, which would result in reactivation of the virus from a latent to a lytic stage after a symptom-free period, causing a resurgence of COVID-19 symptoms [39]; but the long interval between the two positive RT-PCR results makes re-activation an unlikely diagnosis [21]. These four patients were not significantly different from those who tested negative in terms of past medical history or severity of the initial episode of COVID-19. Also, we did not find any risk factors that could help distinguish patients who are more susceptible to re-infection from those who are not.