Viral Shedding in Vaccinated Individuals
Viral shedding refers to the expulsion of live virus from an organism in which the virus has successfully replicated.1 Shedding can occur via several different mechanisms, such as budding, whereby viral particles exit the host cell membrane by surrounding themselves with molecules from the membrane, and apoptosis, whereby viral particles force cells to undergo cell death and can then be released into the extracellular space.2 The amount of viral shedding by infected individuals depends on the virus in question, the stage of infection, and the host’s immunity against the virus, including if they have been vaccinated.
Vaccination can protect individuals from viral infection such that even upon exposure to a virus, the individual’s vaccine-induced immunity kills the infectious agent before it can replicate and cause illness. In reality, breakthrough infections can also occur, in which vaccinated individuals contract a viral illness. Vaccinated individuals who contract breakthrough infections may nevertheless have better outcomes than unvaccinated individuals who become infected. Variants of SARS-CoV-2, such as the Delta variant, have caused breakthrough infections, but it has been found that people vaccinated against COVID-19 are much less likely than unvaccinated individuals to develop severe symptoms.3
Does this mean that fully vaccinated individuals are also less likely to transmit the virus to others – do they shed less virus? Dr. Jiwon Jung and other South Korean researchers recently published their findings on this very question. In a recent Journal of the American Medical Association paper, Jung et al. reported on their study of the transmissibility of COVID-19 according to vaccination status.4 The study involved two cohorts of vaccinated and unvaccinated individuals. The first cohort was subject to a secondary transmission study: the contacts of the cohort members were tracked, and their COVID-19 infections were recorded. The second cohort underwent a viral kinetics shedding study: patients submitted saliva samples each day of the study, and the viral load in the samples was measured using polymerase chain reaction (PCR).
The results from both cohorts indicate that vaccinated individuals with breakthrough infections shed less virus than unvaccinated infected patients. The risk of secondary transmission was significantly lower in the breakthrough group than in the non-breakthrough group. Additionally, while patients in the breakthrough and non-breakthrough had a similar initial viral load after infection, fully vaccinated patients had a shorter duration of viral shedding compared to partially vaccinated and unvaccinated individuals.
Though convincing, the study, as its authors acknowledge, has several limitations. Perhaps most significantly, viral shedding was only measured by the amount of virus in saliva samples. Due to “logistical challenges,” nasopharyngeal swab samples were not used, though they could have enabled more accurate data on viral shedding, especially given the incomplete data regarding the amount of viral shedding at different body sites over the course of infection.5 Furthermore, the study only tracked nosocomial secondary transmission – that is, infections originating in a hospital – which may bias the results. Finally, the cohort 1 study began before the emergence of the infectious Delta variant but concluded after it was widespread. The study did not control for SARS-CoV-2 variant, and the conclusions drawn from it are therefore limited. Nevertheless, this important study hopefully paves the way for future research into viral shedding in vaccinated individuals.
References
1. Yan, D. et al. Characteristics of Viral Shedding Time in SARS-CoV-2 Infections: A Systematic Review and Meta-Analysis. Front. Public Health 9, (2021), DOI: 10.3389/fpubh.2021.652842
2. Badu, K. et al. SARS-CoV-2 Viral Shedding and Transmission Dynamics: Implications of WHO COVID-19 Discharge Guidelines. Front. Med. 8, (2021), DOI: 10.3389/fmed.2021.648660
3. CDC. COVID-19 Vaccination. Centers for Disease Control and Prevention https://www.cdc.gov/coronavirus/2019-ncov/vaccines/effectiveness/why-measure-effectiveness/breakthrough-cases.html (2020).
4. Jung, J. et al. Transmission and Infectious SARS-CoV-2 Shedding Kinetics in Vaccinated and Unvaccinated Individuals. JAMA Netw. Open 5, e2213606 (2022), DOI:10.1001/jamanetworkopen.2022.13606
5. Congrave-Wilson, Z. et al. Change in Saliva RT-PCR Sensitivity Over the Course of SARS-CoV-2 Infection. JAMA 326, 1065–1067 (2021), DOI: 10.1001/jama.2021.13967