Transmission and Infectious SARS-CoV-2 Shedding Kinetics in Vaccinated and Unvaccinated Individuals

Abstract

Importance: Data are limited on whether patients with breakthrough COVID-19 infection have the potential to significantly contribute to the spread of SARS-CoV-2.

Objective: To compare the secondary attack rate and infectious viral shedding kinetics of SARS-CoV-2 between fully vaccinated individuals (breakthrough infection group) and partially or unvaccinated individuals (nonbreakthrough infection group).

Design, setting, and participants: This cohort study assessed secondary transmission by analyzing the epidemiologic data of health care workers, inpatients, and caregivers diagnosed with COVID-19 during hospitalization or residence in a tertiary care hospital between March 1, 2020, and November 6, 2021. To evaluate viral shedding kinetics, the genomic RNA of SARS-CoV-2 was measured using polymerase chain reaction and performed virus culture from daily saliva samples of individuals with mild COVID-19 infected with the Delta variant who were isolated in a community facility in Seoul, South Korea, between July 20 and August 20, 2021.

Exposures: COVID-19 vaccination.

Main outcomes and measures: The secondary attack rate and infectious viral shedding kinetics according to COVID-19 vaccination status.

Results: A total of 173 individuals (median [IQR] age, 47 [32-59] years; 100 female [58%]) with COVID-19 were included in the secondary transmission study, of whom 50 (29%) had a breakthrough infection. Secondary transmission was significantly less common in the breakthrough infection group than in the nonbreakthrough infection group (3 of 43 [7%] vs 29 of 110 [26%]; P = .008). In the viral shedding kinetics study, 45 patients (median age, 37 years [IQR, 25-49 years]; 14 female [31%]) infected with the Delta variant were included, of whom 6 (13%) were fully vaccinated and 39 (87%) were partially or unvaccinated. Although the initial genomic viral load was comparable between the 2 groups, viable virus in cell culture was detected for a notably longer duration in partially vaccinated (8 days after symptom onset) or unvaccinated (10 days after symptom onset) individuals compared with fully vaccinated individuals (4 days after symptom onset).

Conclusions and relevance: In this cohort study, although the initial genomic viral load was similar between vaccinated and unvaccinated individuals, fully vaccinated individuals had a shorter duration of viable viral shedding and a lower secondary attack rate than partially vaccinated or unvaccinated individuals. Data from this study provide important evidence that despite the possibility of breakthrough infections, COVID-19 vaccinations remain critically useful for controlling the spread of SARS-CoV-2.

Figure 1.. Cycle Threshold Values at Diagnosis in Cases of Breakthrough and Nonbreakthrough Infection in Cohort 1

Long horizontal lines indicate the median, and short horizontal lines indicate the IQR. No significant differences were found in the median (IQR) cycle threshold value at diagnosis in cases of breakthrough infection and nonbreakthrough infection (19 [16–24] vs 20 [15–29]; P = .64).

Figure 2.. Association Between Cycle Threshold Value at Diagnosis and Weeks From Second Vaccination to Diagnosis

The line represents the simple linear regression line. No significant association was found between the cycle threshold value and weeks from vaccination to diagnosis (P = .94 by linear regression).

Figure 3.. Viral Load Kinetics in Mildly Symptomatic Patients With COVID-19 According to Vaccination Status

Kinetics of genomic RNA (mean values with the estimated SEs) in fully vaccinated and partially or nonvaccinated individuals are shown (A). Viral copy number and culture positivity according to the symptom onset date in fully vaccinated individuals (n = 6) (B), partially vaccinated individuals (n = 11) (C), and nonvaccinated individuals (n = 28) (D) are shown.