Viral cultures for COVID-19 infectivity assessment – a systematic review (Update 4)
Authors: Jefferson T , Spencer EA , Brassey J , Heneghan C 
Published: August 2020

Objective

to review the evidence from studies comparing SARS-CoV-2 culture, the best indicator of current infection and infectiousness with the results of reverse transcriptase polymerase chain reaction (RT-PCR).

Methods

We searched LitCovid, medRxiv, Google Scholar and the WHO Covid-19 database for Covid-19 using the terms ‘viral culture’ or ‘viral replication’ and associated synonyms up to 10 September 2020. We carried out citation matching and included studies reporting attempts to culture or observe SARS-CoV-2 matching with cutoffs for RT-PCR positivity. One reviewer extracted data for each study and a second reviewer checked end edited the extraction and summarised the narratively by sample: fecal, respiratory, environment or mixed.

Where necessary we wrote to corresponding authors of the included or background papers for additional information. We assessed quality using a modified QUADAS 2 risk of bias tool.

This review is part of an Open Evidence Review on Transmission Dynamics of COVID-19. Summaries of the included studies and the protocol (v1) are available at: https://www.cebm.net/evidence-synthesis/transmission-dynamics-of-covid-19/. Searches are updated every 2 weeks. This is the fourth version of this review that was first published on the 4th of August and updated on the 21t of August

Results

We included 29 studies reporting culturing or observing tissue invasion by SARS-CoV in sputum, naso or oropharyngeal, urine, stool, blood and environmental samples from patients diagnosed with Covid-19. The data are suggestive of a relation between the time from collection of a specimen to test, cycle threshold and symptom severity. The quality of the studies was moderate with lack of standardised reporting.

Twelve studies reported that Ct values were significantly lower and log copies higher in samples producing live virus culture. Five studies reported no growth in samples based on a Ct cut-off value. These values ranged from CT > 24 for no growth to Ct ≥ 34. Two studies report a strong relationship between Ct value and ability to recover infectious virus and that the odds of live virus culture reduced by 33% for every one unit increase in Ct. A cut-off RT-PCR Ct > 30 was associated with non-infectious samples. One study that analysed the NSP, N and E gene fragments of the PCR result reported different cut-off thresholds depending on the gene fragment analysed. The duration of RNA shedding detected by PCR was far longer compared to detection of live culture. Six out of eight studies reported RNA shedding for longer than 14 days. Yet, infectivity declines after day 8 even among cases with ongoing high viral loads. A very small proportion of people re-testing positive after hospital discharge or with high Ct are likely to be infectious.

Conclusion

Prospective routine testing of reference and culture specimens are necessary for each country involved in the pandemic to establish the usefulness and reliability of PCR for Covid-19 and its relation to patients’ factors. Infectivity is related to the date of onset of symptoms and cycle threshold level.

A binary Yes/No approach to the interpretation RT-PCR unvalidated against viral culture will result in false positives with possible segregation of large numbers of people who are no longer infectious and hence not a threat to public health.