Impact of cobas PCR media freezing on SARS-CoV-2 viral RNA integrity and whole genome sequencing analyses

SARS-CoV-2 whole genome sequencing is an important molecular biology tool performed to support many aspects of the response to the pandemic. Freezing of primary clinical nasopharyngeal swab samples and shipment to reference laboratories is usually required since RNA sequencing is rarely available in routine clinical microbiology laboratories where initial diagnosis and support to outbreak investigations occur. The cobas PCR Media transport medium developed by Roche facilitates high throughput analyses on cobas multianalyzer PCR platforms. There is no data on the stability of SARS-CoV-2 RNA after freezing and thawing of clinical samples in this transport medium, but potential denaturing of the molecular template could impair test results.

Our objective was to compare the quality and results of SARS-CoV-2 genomic sequencing when performed on fresh or frozen samples in cobas PCR Media. Viral whole genome sequencing was performed using Oxford Nanopore Technologies MinION platform. Genomic coverage and sequencing depth did not significantly differ between fresh and frozen samples (n=10). For samples with lower viral inoculum and PCR cycle threshold above 30, sequencing quality scores and detection of single nucleotide polymorphisms did not differ either. Freezing of cobas PCR Media does not negatively affect the quality of SARS-CoV-2 RNA sequencing results and it is therefore a suitable transport medium for outsourcing sequencing analyses to reference laboratories.

Those results support secondary use of diagnostic nasopharyngeal swab material for viral sequencing without requirement for additional clinical samples.