Researchers in the USA published a study within the journal PLOS Pathogens that examined the shedding of infectious severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virions despite vaccination against it.

Study: Shedding of infectious SARS-CoV-2 despite vaccination. Image Credit: MrSquid / Shutterstock

Background

The SARS-CoV-2 Delta variant was first identified in March 2021 and was linked to an upsurge within the incidence of coronavirus disease 2019 (COVID-19) infection in North America in the summertime of 2021. The surge of cases linked to viruses of the Delta lineage was the primary notable rise in SARS-CoV-2 infection rates after COVID-19 vaccines became readily accessible within the US.

By July 2021, SARS-CoV-2 infection rates were low in the USA, and national and native public health organizations were relaxing regulations on the usage of face masks and other non-pharmaceutical measures to stop the spread of the virus. Whether individuals infected with SARS-CoV-2, notwithstanding vaccination, could spread the infection to others was an important consideration in developing these policies.

In regards to the study

In the current study, researchers evaluated the SARS-CoV-2 ribonucleic acid (RNA) burden in nasal swabs obtained from vaccinated and unvaccinated people to determine if individuals with vaccine breakthrough infections might shed SARS-CoV-2 Delta viruses at levels compatible with the potential transmission.

The anterior nasal swab samples sent for clinical testing to a industrial reverse transcription-polymerase chain response (RT-PCR) testing service between 28 June 2021 and 1 December 2021 were employed. The study utilized sample-associated metadata to estimate viral RNA burden in individuals who tested positive for SARS-CoV-2 during a period of high Delta variant prevalence and its association with the person’s vaccination status. At quite a few clinic locations throughout Wisconsin, samples were obtained using standardized collection kits from patients who required SARS-CoV-2 RT-PCR testing. To evaluate the nasal viral RNA load, the team analyzed RT-PCR cycle threshold (Ct) data related to twenty,431 specimens from completely vaccinated or unvaccinated people.

The degrees of Ct were determined using the Flu-SC2 Multiplex Assay. This RT-PCR method can concurrently discover influenza A and B and SARS-CoV-2 nucleic acid in anterior nasal swabs. Reverse transcription into complementary deoxyribonucleic acid (cDNA) and amplification was performed on the RNA obtained from anterior nasal swab samples. A no-template control, a positive extraction control containing human RNAse P, and an internal RNAse P control were all used as controls.

If vaccine registry or self-reported data showed that a final vaccine dosage was received no less than 14 days before the submission of a SARS-CoV-2 positive, the person was deemed completely vaccinated on the time of testing. The team evaluated the presence of an infectious virus and observed the presence of cytopathic effects throughout the course of 5 days using an initial batch of specimens with Ct values lower than 25. Samples were chosen by employing N1 Ct-matching between unvaccinated and completely vaccinated individuals.

Results

Infected SARS-CoV-2 have previously been linked to SARS-CoV-2 RT-PCR Ct values below 25. Ct values lower than 25 were observed in 6,253 of 9,347 fully vaccinated individuals and 6,739 of 11,084 unvaccinated people. The researchers derived standardized differences, that are the typical differences between the groups divided by the pooled standard deviations, to find out the dimensions of the differences between groups.

Individuals infected with SARS-CoV-2 despite full vaccination have low Ct values and shed similar amounts of infectious virus as unvaccinated individuals. ​​​​​​​A. N1 Ct values for SARS-CoV-2-positive specimens were grouped by vaccination status. RT-PCR was performed by Exact Sciences Corporation, accountable for over 10% of all PCR tests in Wisconsin during this era, using a qualitative diagnostic assay targeting the SARS-CoV-2 N gene (oligonucleotides an identical to CDC’s N1 primer and probe set) that has been authorized for emergency use by FDA (https://www.fda.gov/media/138328/download)).  An effect size of d< 0.2 is negligible. The variety of samples in each group is listed under the dot plot. B. N1 Ct values for SARS-CoV-2-positive specimens grouped by vaccination status for people who were symptomatic or either asymptomatic or didn’t have any information, on the time of testing. Light yellow box indicates Ct values <25. C. We performed plaque assays on Vero E6 TMPRSS2 cells on a subset of specimens. Specimens were chosen by N1 Ct-matching between fully vaccinated and unvaccinated individuals. Specimens from individuals with unknown vaccination status were excluded from the evaluation. Infectious titers are expressed as plaque-forming units (PFU) per milliliter of specimen. Specimens underwent a freeze-thaw cycle prior to virus titration. 

We found no discernible correlation between Ct values in infected individuals and vaccination status. No matter whether or not they had symptoms on the time of testing, vaccinated individuals had low Ct values, with Ct values lower than 25 being present in 65% of symptomatic unvaccinated individuals and in 70% of completely vaccinated symptomatic patients.

Notably, the interval between the beginning of symptoms and testing was unaffected by vaccination status for patients with symptoms. In our cohort, each vaccinated and unvaccinated people reported a median delay of two.4 days between the start of symptoms and testing. On this study sample, 92% of individuals sought testing inside six days of the onset of symptoms.

Conclusion

The study findings showed that a big fraction of those that developed SARS-Cov-2 Delta virus infections after receiving vaccinations had low Ct values compatible with the opportunity of shedding infectious viruses. The outcomes indicated that people who find themselves infected despite receiving vaccinations could spread SARS-CoV-2. To be able to stop transmission, infection prevention is crucial. The researchers imagine that those that have and haven’t received the COVID-19 vaccine should proceed to follow non-pharmaceutical measures to limit the transmission of COVID-19.