This year, I’ve written more than a handful of COVID-19 diaries, and in one of them someone asked if it had been settled, once and for all, if the Covid vaccines (presumably the mRNA-based ones) protected the person from spreading the disease. Was it a barrier, or merely a dampener? It was clear by this year that the vaccines were effective at reducing severity of the disease, but could it stop transmission?
It appears the answer to the last is no. A study in PLos Pathogens explains that vaccines permit shedding of virus, even by those who have been vaccinated.
(That is, the virus can spread independent of vaccine status. That does not mean, of course, that the vaccine itself is responsible for this spread, much as some conservatives may believe.)
The analysis compared Ct values of fully vaccinated individuals to those who were unvaccinated. The Ct threshold is a measure of how detectable the virus is when subjected to the polymerase chain reaction (PCR) test. Many of us are familiar with the PCR test by now. Science explains:
The CT value is the number of cycles necessary to spot the virus; PCR machines stop running at that point. If a positive signal isn't seen after 37 to 40 cycles, the test is negative. But samples that turn out positive can start out with vastly different amounts of virus, for which the CT value provides an inverse measure.
According to the study itself, “samples were collected using standardized collection kits from individuals seeking Covid PCR testing at multiple clinic locations across the state of Wisconsin” from June 28, 2021 until December 4, 2021. This cutoff date was chosen so as to avoid confounding the results with Omicron data.
This was a sizable study. 20,431 specimens were examined in total, from fully vaccinated persons (n = 9,347) and unvaccinated individuals (n = 11,084).
Researchers found that the virus thresholds of vaccinated individuals were similar to those who had not been vaccinated, levels at which the virus easily spreads. “We observed no significant effect of vaccination status on Ct values in infected persons,” they said.
Low Ct values were detected in vaccinated people whether or not they reported symptoms at the time of testing, with Ct values <25 detected in 65% (95% CI: 63–66%) of unvaccinated symptomatic individuals and in 70% (95% CI: 69–71%) of fully vaccinated symptomatic individuals (p<0.0001).
This is important because, as the researchers point out, Ct values lower than 25 had previously been associated with the shedding of SARS-CoV-2.
The make of vaccine did not make any significant difference. “Vaccination had neglible effects on mean Cts in vaccinated as compared with unvaccinated individuals, regardless of the manufacturer.” Products by Janssen, Moderna, and Pfizer all had similar efficacy, in terms of Ct values.
Combined with other studies our data indicate that vaccinated as well as unvaccinated individuals infected with SARS-CoV-2 Delta variants can shed, and potentially transmit, infectious virus. We find low Ct values in substantial proportions of both unvaccinated and vaccinated individuals who tested positive for SARS-CoV-2 during the time when Delta variants predominated, in agreement with other recent reports. […]
Importantly, we also show that infectious SARS-CoV-2 is found at similar titers in vaccinated and unvaccinated persons.
The authors note that the time period was a confusing and discrepant one. Recall that directly following the first launch of vaccines, the U.S. government relaxed public restrictions on certain social measures, such as facemasking. There really was no overlap bridging the vaccination campaign and a reduction in these safety measures: they were consecutive in time.
At the time, prevailing public health recommendations were that vaccinated persons need not use face coverings in indoor settings. These recommendations were based in part on the fact that vaccines demonstrated remarkable effectiveness against test-positive SARS-CoV-2 infection in initial clinical trials conducted in 2020, suggesting that vaccinated persons might play negligible roles in SARS-CoV-2 transmission. However, the initial vaccine effectiveness studies were conducted when ancestral variants predominated, prior to the emergence of variants of concern.
From their explanation, it was sheer bad timing that caused a convergence of Delta. It’s possible, with a long enough lag window where vaccinated people also were masking and implementing the full complement of social safety measures, that the virus could have been held under more control.
The authors conclude:
Our findings support the notion that persons infected despite vaccination can transmit SARS-CoV-2. Therefore, preventing infection is critical to preventing transmission. … Continued adherence to nonpharmaceutical interventions during periods of high community transmission to mitigate spread of COVID-19 remains important for both vaccinated and unvaccinated individuals.