Interesting correspondence published by Pan and others in The Lancet Microbe May 12th 2001 tries to question and answer this.
They report that Cevik and colleagues found that no study was able to culture live virus from any respiratory sample taken after day eight or beyond day nine of symptoms despite a persistently high viral RNA load.
This is important because unlike SARS-CoV-2 and MERS-CoV for which symptom severity correlates to infectiousness, most patients with COVID-19 continue to have worsening symptoms beyond day nine but might become progressively less infectious.
The authors comment that none of the 11 studies have attempted to isolate live SARS-CoV-2 in the data included patients who were severely immunosuppressed.
They suggested that there is emerging evidence showing long term SARS-CoV-2 culture positivity in this specific cohort up to about 119 days following symptom onset, with emergence of mutations identical to those found in the South African, Brazilian and Kent variants.
Increased vigilance is needed to protect the immunosuppressed individuals as well as their close contacts from being infected.
Secondly, the authors mentioned in their discussion that repeat RNA PCR testing in clinical practice might not be indicated to classify patients as no longer infectious. The correspondence authors agree with this statement.
Current NICE guidelines suggest that patients with COVID-19 awaiting an urgent operation can have the procedure postponed if they have a recent positive PCR test.
Some countries also require passengers from the United Kingdom to have a negative PCR test before flying.
The correspondence authors recommended modifying such criteria to state that patients who have recovered from COVID-19 can have an operation or fly 10 days after their first positive swab, or 10 days after clear symptom onset.
However with the exceptions for those who had severe symptoms or are heavily immunosuppressed.
They go on to suggest that this guidance will require further updating as the evidence evolves.
They continue to suggest that future studies should focus on simpler and faster methods of quantifying the infectiousness of an individual with COVID-19 beyond viral cultures, which themselves are labour intensive and require laboratory facilities with high bio safety levels.
They suggest that one possible strategy could be to sample virus from exhaled breath rather than the nasopharyngeal tract using specialised matrices embedded within facemasks.
The rapid identification of infectious individuals in the community as well as in hospitals is crucial for the effective contact tracing of future respiratory virus outbreaks with a pandemic potential.
The London General Practice, the leading London doctors’ clinic undertakes all forms of currently available commercial testing.
This involves lateral flow antigen, COVID PCR, all testing for flights, fit to fly, day two and day eight, test for release, day five testing.
Dr Paul Ettlinger
BM, DRCOG, FRCGP, FRIPH, DOccMed