Objectives: The purpose of this study was to detect COVID-19 cases with persistent positive RT-PCR results for SARS-CoV-2, for which viable virus can be inferred, due to the presence of subgenomic (SG) viral RNA, which is expressed only in replicating viruses. Methods: RNA remnants, purified from diagnostic nasopharyngeal specimens, were used as templates for RT-PCR specific detection of SG E gene RNA. As controls, we also detected viral genomic RNA for the E gene and/or a human housekeeping gene (RNase P). Results: We assessed the samples of 60 RT-PCR-positive cases with a prolonged viral SARS-CoV-2 shedding (24-101 days) since the first diagnostic RT-PCR. SG viral RNA was detected in 12/60 (20%) of the persistent cases, 28-79 days after the onset of symptoms. The age range of the cases with prolonged viral shedding and presence of SG RNA was quite wide (40-100 years), and they were equally distributed between males (42%) and females (58%). None was HIV positive, although seven were immunosuppressed. According to the severity of the COVID-19 episodes they were mild (40%), intermediate (20%), and severe (40%) Conclusion: In a percentage of persistent positive SARS-CoV-2 PCR positive cases the presence of actively replicating virus may be inferred, far beyond diagnosis. We should not assume a universal lack of infectiousness for COVID-19 cases with prolonged viral shedding.

Estimates of the burden of severe acute respiratory syndrome coronavirus 2 reinfections are limited by the scarcity of population-level studies incorporating genomic support. We conducted a systematic study of reinfections in Madrid, Spain, supported by genomic viral analysis and host genetic analysis, to cleanse laboratory errors and to discriminate between reinfections and recurrences involving the same strain. Among the 41,195 cases diagnosed (March 2020–March 2021), 93 (0.23%) had 2 positive reverse transcription PCR tests (55–346 days apart). After eliminating cases with specimens not stored, of suboptimal sequence quality, or belonging to different persons, we obtained valid data from 22 cases. Of those, 4 (0.01%) cases were recurrences involving the same strain; case-patients were 39–93 years of age, and 3 were immunosuppressed. Eighteen (0.04%) cases were reinfections; patients were 19–84 years of age, and most had no relevant clinical history. The second episode was more severe in 8 cases.

SARS‐CoV‐2 RT‐PCR cycle threshold values from 18,803 cases (2 March–4 October) in Madrid define three stages: (i) initial ten weeks with sustained reduction in viral load (Ct: 23.4–32.3), (ii) stability with low viral loads (Ct: 31.9–35.5) in the next nine weeks and (iii) sudden increase with progressive higher viral loads until reaching stability at high levels in the next twelve weeks, coinciding with an increased percentage of positive cases and reduced median age. These data indicate differential virological/epidemiological patterns between the first and second COVID‐19 waves in Madrid.

196 travellers with negative-COVID-19-tests prior to departure tested positive on arrival at Madrid (April/June-2021), from a total of 45.211 travellers tested (0.43%). Viral loads (Ct 20.3), were higher compared to the general population (Ct 27.09). Our data reveal weaknesses in pre-departure testing and alert about high-viral-load-SARS-CoV-2 carriers on intercontinental flights.

Our data indicate that the dynamics of SARS-CoV-2 VOCs turnover in our setting differ from those proposed for other countries. A systematic genomic analysis, updated on a weekly basis, of representative randomly selected samples of SARS-CoV-2 circulating variants allowed us to define a lapse of 7 weeks between the start of VOC Alpha decline and the final emergence of VOC Delta.