Shedding of infectious SARS-CoV-2 despite vaccination

In the summer of 2021, a pivotal shift occurred in the COVID-19 pandemic: the emergence of the Delta variant. This highly contagious strain of the SARS-CoV-2 virus led to a significant surge in cases, even among fully vaccinated individuals. Researchers wanted to understand if vaccinated people who became infected could spread the virus, especially since vaccines were initially believed to greatly reduce transmission risks. Let’s dive into what was discovered, using simple terms and occasional scientific references for clarity.

​What Is the Delta Variant?

The Delta variant, a strain of the SARS-CoV-2 virus, was first identified in March 2021. By summer, it dominated infections in North America due to two key traits:

1. High Transmissibility: It spreads more easily from person to person.
2. Partial Immune Escape: It can sometimes bypass the immunity provided by vaccines or prior infections.

​Why Study Breakthrough Infections?

​A "breakthrough infection" happens when someone fully vaccinated against COVID-19 gets infected. Initially, it was thought these cases were rare and unlikely to contribute to the virus's spread. However, public health officials noticed more vaccinated individuals testing positive for COVID-19, prompting researchers to investigate further.

​Key Findings: Can Vaccinated People Spread Delta?

1. Virus Levels (Ct Values)
   ​Researchers measured the amount of virus in nasal swabs from over 20,000 people, comparing vaccinated and unvaccinated individuals. They used a method called RT-PCR to calculate something called the "cycle threshold" (Ct) value. A lower Ct value means a higher amount of virus is present. Surprisingly, both vaccinated and unvaccinated individuals showed similar Ct values when infected with Delta.
2. Infectious Virus Detection
   A subset of samples with low Ct values (indicating high virus levels) was tested to see if the virus was still infectious. Both groups—vaccinated and unvaccinated—were equally likely to shed live virus.
3. Implications for Transmission
   These findings indicate that vaccinated people infected with Delta can carry and potentially spread the virus just like unvaccinated individuals. This was a crucial discovery that informed public health policies, such as reintroducing mask mandates during surges.

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 tests in Wisconsin during this period, using a qualitative diagnostic assay targeting the SARS-CoV-2 N gene (oligonucleotides 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)). See also Table 1. An effect size of d< 0.2 is negligible. The number 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 individuals who were symptomatic or either asymptomatic or did not have any information, at 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 selected by N1 Ct-matching between fully vaccinated and unvaccinated persons. Specimens from individuals with unknown vaccination status were excluded from the analysis. Infectious titers are expressed as plaque-forming units (PFU) per milliliter of specimen. Specimens underwent a freeze-thaw cycle prior to virus titration.

​Does Vaccine Type or Timing Matter?

The study also explored if the type of vaccine or the time since vaccination affected virus levels. Results showed:

• No significant differences in virus levels based on the vaccine type (Pfizer, Moderna, or Johnson & Johnson).
• A small increase in virus levels as time passed after vaccination, particularly for Pfizer recipients.

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References

Riemersma, Kasen K., et al. “Shedding of Infectious SARS-CoV-2 despite Vaccination.” PLOS Pathogens, vol. 18, no. 9, 30 Sept. 2022, p. e1010876, https://doi.org/10.1371/journal.ppat.1010876.