Ughh... The study that they referenced used PCR to measure viral loads but PCR tests can’t differentiate between antibody-neutralised virions (or broken down viral debris) vs infectious particles. That was actually a common complaint from other virologists and immunologists who have looked it over but nevertheless that paper made it's rounds in the news for a week.
Thanks to that we have "vaccinated are spreaders too" argument.
Bringing this back up because I was thinking about the mmwr provincetown report that insinuated that vaccinated and unvaccinated people carry the same viral loads which spiraled out of control into "vaxxed spread it just as much the unvaxxed".
Again I hated that report because it was meaningless without measuring the amount of infectious virions between the two cohorts (vaxxed and unvaxxed) and PCRs don't distinguish between viral RNA that has already been destroyed by t cells, virions neutralized by antibodies, infectious virions, and retarded genome copies. Ok, I am speaking a little loose with the retarded genome copies thing but it's actually important to know that RNA viruses produce non infectious copies 90% of the time due to RNA polymerases not having mechanisms for proofreading (ie ways to correct mutations) like DNA polymerases.
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Anyways there is a new paper that answers the question if the infectious viral loads are the same between the unvaccinated and vaccinated by creating infectious viral titers (IVTs) for PCR positive individuals.
Infectious viral load in unvaccinated and vaccinated patients infected with SARS-CoV-2 WT, Delta and Omicron
Background
Viral load (VL) is one determinant of secondary transmission of SARS-CoV-2. Emergence of variants of concerns (VOC) Alpha and Delta was ascribed, at least partly, to higher VL. Furthermore, with parts of the population vaccinated, knowledge on VL in vaccine breakthrough infections is crucial. As RNA VL is only a weak proxy for infectiousness, studies on infectious virus presence by cell culture isolation are of importance.
Methods
We assessed nasopharyngeal swabs of COVID-19 patients for quantitative infectious viral titres (IVT) by focus-forming assay and compared to overall virus isolation success and RNA genome copies. We assessed infectious viral titres during the first 5 symptomatic days in a total of 384 patients: unvaccinated individuals infected with pre-VOC SARS-CoV-2 (n= 118) or Delta (n= 127) and vaccine breakthrough infections with Delta (n= 121) or Omicron (n=18).
Findings
Correlation between RNA copy number and IVT was low for all groups. No correlation between IVTs and age or sex was seen. We observed higher RNA genome copies in pre-VOC SARS-CoV-2 compared to Delta, but significantly higher IVTs in Delta infected individuals.
In vaccinated vs. unvaccinated Delta infected individuals, RNA genome copies were comparable but vaccinated individuals have significantly lower IVTs, and cleared virus faster. Vaccinated individuals with Omicron infection had comparable IVTs to Delta breakthrough infections.
Interpretation
Quantitative IVTs can give detailed insights into virus shedding kinetics.
Vaccination was associated with lower infectious titres and faster clearance for Delta, showing that vaccination would also lower transmission risk. Omicron vaccine breakthrough infections did not show elevated IVTs compared to Delta, suggesting that other mechanisms than increase VL contribute to the high infectiousness of Omicron.
And here's one of the authors talking about their study.
