![]() ![]() In contrast, a study using longitudinal samples did not find a difference among peak RNA VL of pre-VOC, Alpha and Delta 17. Similarly, Delta also showed 10–15-fold-higher RNA levels compared to pre-VOC strains in some studies 15, 16. ![]() For Alpha, an approximately ten-fold-higher RNA VL was described compared to pre-VOC viral strains, which was correlated with increased isolation success 14, 15. These VOCs exhibit various mutations 11 that lead to immune evasion and/or higher transmissibility, to which increased viral shedding (among other factors, such as environmental stability) may significantly contribute 12, 13. Although most variants vanished quickly, others, such as D614G and the VOCs Alpha, Beta, Gamma, Delta and Omicron, harbor an apparent selection advantage and outcompeted other variants locally or even globally. Since the start of the pandemic, SARS-CoV-2 has constantly evolved, leading to the emergence of new variants. Virus isolation success from respiratory tract samples can give information only about the presence or absence of infectious virus but is not able to quantify the infectious viral titer in samples of the URT 10. Instead, isolation success in cell culture-that is, the ability to replicate the virus in cell culture-was found to correlate with the ability to shed and transmit fully competent viral particles 9. ![]() Moreover, viral RNA detection did not correlate with infectiousness in an animal model 8. In most studies, infectious virus was not detected in respiratory samples collected from non-hospitalized immunocompetent individuals later than 8 days post onset of symptoms (DPOS) 5, 6, 7. Infectious SARS-CoV-2 is shed in the URT, starting, on average, from 2 days before symptom onset. In several epidemiological studies, higher VL measured by viral RNA was associated with increased secondary transmission in household settings 3, 4. Although the transmission process is complex, higher VL can serve as a proxy for greater risk of transmission. The two key measurements of VL are RNA levels, often expressed in cycle threshold (Ct) values, and infectious virus that is assessed by virus isolation in cell culture. SARS-CoV-2, the causative agent of COVID-19, primarily infects the cells of the upper respiratory tract (URT) where VL increases during the course of infection 2. ![]() Similar content being viewed by othersĪs of 6 March 2022, the Coronavirus Disease 2019 (COVID-19) pandemic has caused more than 443 million cases and just over 5.9 million deaths globally 1. Our findings indicate that vaccines may lower transmission risk and, therefore, have a public health benefit beyond the individual protection from severe disease. In addition, infectious VL was lower in fully vaccinated Omicron BA.1-infected individuals compared to fully vaccinated Delta-infected individuals, suggesting that mechanisms other than increased infectious VL contribute to the high infectiousness of SARS-CoV-2 Omicron BA.1. For Omicron BA.1 breakthrough cases, reduced infectious VL was observed only in boosted but not in fully vaccinated individuals compared to unvaccinated individuals. Full vaccination (defined as >2 weeks after receipt of the second dose during the primary vaccination series) significantly reduced infectious VL for Delta breakthrough cases compared to unvaccinated individuals. Unvaccinated individuals infected with pre-VOC SARS-CoV-2 had lower infectious VL than Delta-infected unvaccinated individuals. In this study, we quantified infectious VL in individuals infected with SARS-CoV-2 during the first five symptomatic days by in vitro culturability assay in unvaccinated or vaccinated individuals infected with pre-variant of concern (pre-VOC) SARS-CoV-2, Delta or Omicron BA.1. Studies on the kinetics of infectious VL are important to understand the mechanisms behind the different transmissibility of SARS-CoV-2 variants and the effect of vaccination on transmission, which allows guidance of public health measures. RNA VL measured by qRT–PCR is only a weak proxy for infectiousness. Infectious viral load (VL) expelled as droplets and aerosols by infected individuals partly determines transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). ![]()
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