Can you shed infectious SARS-CoV-2 virions even if vaccinated against COVID?

Researchers in the United States published a study in the journal PLOS pathogen who investigated the excretion of viral viruses of the infectious severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) despite vaccination against it.

Study: Excretion 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 has been associated with an increase in the incidence of coronavirus disease 2019 (COVID-19) infection in North America during the summer of 2021. The delta line was the first notable increase of SARS-CoV-2 infection rates after COVID-19 vaccines became readily available in the US.

As of July 2021, SARS-CoV-2 infection rates in the United States were low, and national and local health organizations were relaxing regulations on the use of face masks and other non-pharmaceutical measures to stop the virus from spreading. Whether individuals infected with SARS-CoV-2 could transmit the infection to others despite vaccination was a key consideration in developing these guidelines.

About the study

In the present study, researchers evaluated the SARS-CoV-2 ribonucleic acid (RNA) load in nasal swabs obtained from vaccinated and unvaccinated individuals to determine whether individuals with vaccine breakthrough infections were infected with SARS-CoV-2 delta viruses could excrete in an amount compatible with possible transmission.

The anterior nasal swab specimens sent to a commercial reverse transcriptase polymerase chain reaction (RT-PCR) service for clinical testing between June 28, 2021 and December 1, 2021 were used. The study used sample-related metadata to estimate the viral RNA load in individuals who tested positive for SARS-CoV-2 during a period of high delta variant prevalence and its association with the individual’s immunization status. At numerous clinic sites throughout Wisconsin, standardized collection kits were used to collect samples from patients requiring a SARS-CoV-2 RT-PCR test. To assess nasal viral RNA load, the team analyzed RT-PCR cycle threshold (Ct) data from 20,431 samples from fully vaccinated or unvaccinated subjects.

Ct levels were determined using the Flu-SC2 multiplex assay. This RT-PCR method can simultaneously identify 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 specimens. A no-template control, a positive extraction control containing human RNase P and an internal RNase P control were used as controls.

If the vaccination register or self-reported data showed that a final vaccination dose was received at least 14 days before transmission of a SARS-CoV-2 positive, the person was considered fully vaccinated at the time of testing. The team assessed the presence of infectious virus and observed the presence of cytopathic effects over five days using a first batch of samples with Ct values ​​less than 25. Samples were analyzed using N1-Ct matching between unvaccinated and fully vaccinated individuals selected.

Results

Infected SARS-CoV-2 have previously been associated with SARS-CoV-2 RT-PCR Ct values ​​below 25. Ct values ​​below 25 were observed in 6,253 of 9,347 fully vaccinated subjects and 6,739 of 11,084 unvaccinated subjects. The researchers derived standardized differences, ie the mean differences between the groups divided by the pooled standard deviations, to determine the size of the differences between the groups.

Individuals infected with SARS-CoV-2 despite full vaccination have low Ct levels 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, responsible for over 10% of all PCR testing in Wisconsin during this period, using a qualitative diagnostic assay targeting the SARS-CoV-2 N gene (oligonucleotides , which are identical to CDC's N1 primer and probe set).  approved by the FDA for emergency use (https://www.fda.gov/media/138328/download)).  An effect size of d<0.2 is negligible.  The number of samples in each group is listed below the scatter plot.  B. N1Ct values ​​for SARS-CoV-2 positive specimens grouped by vaccination status for individuals who were symptomatic or either asymptomatic or lacking information at the time of testing.  Light yellow box shows Ct values <25 an.  C. Wir führten Plaque-Assays an Vero E6 TMPRSS2-Zellen an einer Untergruppe von Proben durch.  Die Proben wurden durch N1 Ct-Matching zwischen vollständig geimpften und ungeimpften Personen ausgewählt.  Proben von Personen mit unbekanntem Impfstatus wurden von der Analyse ausgeschlossen.  Infektiöse Titer werden als Plaque-bildende Einheiten (PFU) pro Milliliter Probe ausgedrückt.  Die Proben wurden vor der Virustitration einem Gefrier-Tau-Zyklus unterzogen.Individuals infected with SARS-CoV-2 despite full vaccination have low Ct levels and shed similar amounts of infectious virus as unvaccinated individuals.​​​​​​​A N1 Ct values ​​for SARS-CoV-2 positive samples were grouped by vaccination status. RT-PCR was performed by Exact Sciences Corporation, responsible for over 10% of all PCR testing in Wisconsin during this period, using a qualitative diagnostic assay targeting the SARS-CoV-2 N gene (oligonucleotides , which are identical to CDC’s N1 primer and probe set). approved by the FDA for emergency use (https://www.fda.gov/media/138328/download)). An effect size of dB. N1 Ct values ​​for SARS-CoV-2 positive specimens grouped by immunization status for individuals who were symptomatic or either asymptomatic or lacking information at the time of testing. Light yellow box indicates Ct values ​​C We performed plaque assays on Vero E6 TMPRSS2 cells on a subset of samples. Samples were selected by N1Ct matching between fully vaccinated and unvaccinated subjects. Samples from individuals with unknown vaccination status were excluded from the analysis. Infectious titers are expressed as plaque forming units (PFU) per milliliter of sample. Samples were subjected to freeze-thaw cycling prior to virus titration.

We found no discernible correlation between Ct values ​​in infected individuals and vaccination status. Regardless of whether they were symptomatic at the time of testing, vaccinated subjects had low Ct values, with Ct values ​​less than 25 found in 65% of symptomatic unvaccinated subjects and 70% of fully vaccinated symptomatic subjects.

Notably, the interval between symptom onset and testing in symptomatic patients was unaffected by vaccination status. In our cohort, both vaccinated and unvaccinated subjects reported a mean delay of 2.4 days between onset of symptoms and testing. In this sample, 92% of people sought a test within six days of symptom onset.

Conclusion

Study results showed that a significant proportion of those who developed SARS-Cov-2 delta virus infection after vaccinations had low Ct values ​​consistent with the possibility of shedding infectious virus. The results suggested that people who are infected despite vaccination could spread SARS-CoV-2. To stop transmission, infection prevention is essential. The researchers believe those who have and have not received the COVID-19 vaccine should continue to take non-pharmaceutical measures to limit transmission of COVID-19.

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