University of WI-Madison & Public Health Study Indicates Vaccinated People May Spread Delta-based SARS-CoV-2 as Much as the Unvaccinated

A team of University of Wisconsin-Madison (UWM) and county public health researchers recently uploaded the results of a study to the preprint server medXriv where they investigate the SARS-CoV-2 Delta variant viral load in vaccinated and unvaccinated people. Collecting and analyzing samples of PCR threshold cycle (Ct) data from one major contact laboratory in Wisconsin, the study revealed that both vaccinated and unvaccinated individuals have similar viral loads in nasal swabs during the Delta variant COVID-19 surge. While the White House, supported by chief medical advisor Dr. Anthony Fauci, declared that the current pandemic is of the “unvaccinated,” the data across multiple sources and studies depict a very different reality. TrialSite reported on outputs from a study of Medicare data (Project Salus) indicating mass infection occurring among vaccinated people. Vaccinated people experience less hospitalization and death; however, the eradication of COVID-19 via vaccination may need a rethink.

The findings from this large team of public health-focused scientists align with other studies indicating scenarios demonstrating that vaccinated people can still have high viral loads and transmit the disease to others. Studies referred to by the Wisconsin group include one in England and another in Singapore. 

The Study

Led by corresponding author and public health professional Katarina Grande, a study was conducted involving the analysis of 719 individual specimens between June 29th, 2021, and July 31st, 2021—at the onset of the Delta variant-based surge. These specimens were taken from the Wisconsin Immunization Registry and Wisconsin Electronic Disease Surveillance System. Delta was the dominant strain of COVID-19 at the time, representing 69% of all Wisconsin-based sequences in GISAID starting June 27th and 95% of all samples by July 24th. 

The team was able to secure, review, and analyze the viral genome-based sequences of 122 samples. A vast majority of these samples (110 out of 122 or 90%) belonged to the Delta variant.  

The study cohorts were segmented by vaccinated and unvaccinated for this study. The study defined a vaccinated person as one who received a final dose at least two weeks before testing positive. Of the 719 individuals involved, the authors were able to obtain 293 vaccinated and 29 unvaccinated statuses from the following sources:

∙       Wisconsin Immunization Registry 

∙       Wisconsin Electronic Disease Surveillance System 

The remaining data originated from self-reported vaccination information totaling 18 vaccinated and 397 unvaccinated. The researchers then analyzed both fully vaccinated and unvaccinated samples during the time of testing.

Findings

The authors “Detected no significant differences in Ct values by vaccination status.” For example, the authors wrote, “212 of 311 (68%) of individuals with infection despite full vaccination had extremely low Ct values <25, consistent with high viral loads.”

The authors point out that SARS-CoV-2 infection isn’t assured with any particular Ct value; however, a body of research does indicate that “infectious SARS-CoV-2 can frequently be recovered from specimens with Ct values of 25-30 or lower. This research team sought to better qualify whether high viral loads suggest infectious SARS-CoV-2 by culturing the virus from a subset of 55 specimens with Ct values <25. The authors managed to isolate infectious SARS-CoV-2 via this method from 14 of 16 specimens (88%) from unvaccinated people and 37 of 39 specimens (95%) from vaccinated people indicating “that Ct >25 is frequently associated with the capacity to shed infectious SARS-CoV-2 even in fully vaccinated persons.”

The researchers further analyzed the data and explored symptom status data in 516 of the 719 individuals investigated, allowing them to further evaluate Ct values in test-positive specimens based on categories including vaccination and symptom status. They found that “For symptomatic cases, there was no significant difference in the time elapsed between symptom onset and testing for vaccinated vs. unvaccinated individuals.” 

Moreover, “Full vaccination did not affect Ct values observed in infected individuals, either with or without symptoms” during the testing period. This hammered home the overall hypothesis that those individuals with known symptom status (252 of 276 of the individuals) who were not fully vaccinated (91%) still reported symptoms during the testing period. Meanwhile, 228 out of 240 people fully vaccinated (95%) reported symptoms. In other tests, the study team found that asymptomatic vaccinated individuals could also have high viral loads, thus becoming contagious as well.

Limitations

The study authors reported three primary limitations, including 1) only one specimen from most of the individuals limited the ability to “know the trajectory of viral loads at the time of testing; 2) possible differences between “vaccinated and unvaccinated persons seeking testing that bias our results, and 3) inherent variability in PCR Ct values because of specimen variability that’s subject to several factors from “collection technique and other variables outside of our control.”

Conclusion

TrialSite shares that this study has yet to be peer-reviewed and should not be cited with authority for evidence until that milestone is achieved. The data is of interest, however, and fits into an emerging observable pattern. 

Corresponding Author

Katarina M. Grande, MPH, Public Health Madison & Dane County, Madison, WI