A short rant, precipitated by a Letter to JAMA that I was reading today.
Detection of viral RNA DOES NOT EQUAL detection of infectious virus particles which DOES NOT EQUAL transmission risk. Are all these things related? Of course. Are they the same thing? NO.
This was prominent in the news recently with the finding that viral RNA was detected on the Disney Princess for 17 days (prior to disinfection). This was widely reported as “coronavirus survived for 17 days”. What was actually measured by the CDC in this case was viral RNA. “SARS-CoV-2 RNA was identified on a variety of surfaces in cabins of both symptomatic and asymptomatic infected passengers up to 17 days after cabins were vacated on the Diamond Princess but before disinfection procedures had been conducted”. Just because viral RNA was detected, does not mean there was any viable infectious virus left. And this is a distinction that really matters for healthcare settings, mitigation/containment regulations etc. You can completely kill a virus and still detect viral RNA.
But even when infectious virus particles are detected, we don’t know the transmission risk from objects and surfaces. Without more data it’s dangerous to make assumptions about the transmission risk in either direction. If we assume there is no transmission risk we could blindly ignore something that puts people, especially healthcare workers, in danger. If on the other hand we equate infectious virus on surfaces with transmission we have problems for a risk/benefit analysis. For example, a preprint just came out showing that infectious virus could be detected on a mask for 7 days after inoculation. That’s indeed worrisome. But what’s the tradeoff of wearing/handling that mask versus not having one at all? Obviously a fresh mask would be better, but that’s not always an option at this point.
I think authors should be very clear whether they are talking about the detection of viral RNA, actual viable virus, or an documented route of transmission. A simple “putative” or “possible” would clear up some of the confusion about coronaviruses on objects and surfaces until we know more.
4 thoughts on “Detection of viral RNA, detection of infectious virus, and transmission risk are not interchangeable #COVID19”
Hello! I am not in anyway related to the medical field, and I am certainly not a microbiologist. I’m just your average lay person trying to understand PCR testing and its limitations. It is my understanding that a PCR test detects viral RNA, not active virus. Is this correct? If so, how much viral RNA would need to be detected in order to make the assumption that there is active virus present?
Hello Jamie. So the PCR test does indeed just detect viral RNA and can’t tell us anything about whether the virus is active. But the correlation you are asking about doesn’t exist. There could be huge piles of RNA and no active virus… or most of the RNA could have come from active virus. There’s no way from a PCR test alone to get a sense of that information. That requires culturing the virus itself and figuring out how many active virions there were to start.
Hello i am a covid positive patient i think because of a test a swab test. It says VIRAL RNA DETECTIVE! does it means i am a carrier of a virus? And how does it risk to other? Do i have to stay away from people? Thank you.
Michael. We’re not qualified to give any kind of medical advice. You should ask for more information from your healthcare provider or whoever provided the test.