Impact of Delta and Vaccination on SARS-CoV-2 transmission risk: Lessons for Emerging Breakthrough infections

Abstract

With the continuous emergence of SARS-CoV-2 variants of concern and implementation of mass-scale interventions like vaccination, understanding factors affecting disease transmission has critical implications for control efforts. Here we used a simple adapted N95 mask sampling method to demonstrate the impact of circulating SARS-CoV-2 variants and vaccination on 92 COVID-19 patients to expel virus into the air translating to a transmission risk. Between July and September 2021, when the Delta was the dominant circulating strain in Mumbai, we noted a two-fold increase in the proportion of people expelling virus (95%), about an eighty-fold increase in median viral load and a three-fold increase in high emitter type (41%; people expelling >1000 viral copy numbers in 30 minutes) compared to initial strains of 2020. Eight percent of these patients continued to be high emitters even after eight days of symptom onset, suggesting a probable increased transmission risk for Delta strain even at this stage. There was no significant difference in expelling pattern between partial, full and un-vaccinated individuals suggesting similar transmission risk. We noted significantly more infections among vaccinated study patients and their household members than unvaccinated, probably due to increased duration from vaccination and/or increased risk behaviour upon vaccination due to lower perceived threat. This study provides biological evidence for possible continued transmission of the Delta strain even with vaccination, emphasizing the need to continue COVID-19 appropriate behaviour. The study also indicates that the mask method may be useful for screening future vaccine candidates, therapeutics or interventions for their ability to block transmission.

Shedding of infectious SARS-CoV-2 despite vaccination

“…Combined with other studies [41,42] our data indicate that vaccinated as well as unvaccinated individuals infected with SARS-CoV-2 Delta variants can shed, and potentially transmit, infectious virus [43,44]. We find low Ct values in substantial proportions of both unvaccinated and vaccinated individuals who tested positive for SARS--CoV-2 during the time when Delta variants predominated, in agreement with other recent reports [41,[44][45][46][47]. The occurrence in our dataset of positive samples from multiple Wisconsin counties without a linking outbreak (more than 80% of samples were not associated with an outbreak known to public health) indicate that Delta-lineage SARS-CoV-2 can achieve low Ct values consistent with transmissibility in fully vaccinated individuals across a range of environments.…”

Shedding of Infectious SARS-CoV-2 Despite Vaccination

“…Combined with other studies [41,42] our data indicate that vaccinated as well as unvaccinated individuals infected with SARS-CoV-2 Delta variants can shed, and potentially transmit, infectious virus [43,44]. We find low Ct values in substantial proportions of both unvaccinated and vaccinated individuals who tested positive for SARS-CoV-2 during the time when Delta variants predominated, in agreement with other recent reports [41,[44][45][46][47]. The occurrence in our dataset of positive samples from multiple Wisconsin counties without a linking outbreak (more than 80% of samples were not associated with an outbreak known to public health) indicate that Delta-lineage SARS-CoV-2 can achieve low Ct values consistent with transmissibility in fully vaccinated individuals across a range of environments.…”

The effect of vaccination on transmission of SARS-CoV-2 (COVID-19): a rapid review

“…In this update, there were 6 observational studies that directly assessed the effectiveness of vaccines in reducing the risk of transmission of SARS-CoV-2 from people who had COVID-19 (index cases) to household members or close contacts (secondary cases). Four of the studies were pre-prints: Allen et al (2022a), Jalali et al (2022), Lyngse et al (2022a), Sriraman et al (2022). The six studies were appraised using the Quality Criteria Checklist (QCC) tool.…”

“…The six studies were appraised using the Quality Criteria Checklist (QCC) tool. Two studies were assessed as high quality (Andrejko et al 2021, Lyngse et al 2022a, three were of medium quality (Allen et al 2022a, Jalali et al 2022, Sriraman et al 2022) and one study was of low quality (Brandal et al 2021). Five were cohort studies (Allen et al 2022a, Brandal et al 2021, Jalali et al 2022, Lyngse et al 2022a, Sriraman et al 2022 and one was a case-control study (Andrejko et al 2021) One study provided data from the UK (Allen et al 2022a), three from Europe (Brandal et al 2021, Jalali et al 2022, Lyngse et al 2022a, one from Asia (Sriraman et al 2022) and one from USA (Andrejko et al 2021).…”

“…Two studies focussed on the Omicron variant (Brandal et al 2021, Lyngse et al 2022a), one study looked at the Delta variant (Sriraman et al 2022), two studies looked at both variants (Allen et al 2022a, Jalali et al 2022, and one study did not report the variant, but was likely it looked at Delta as the study dates went from February 2021 to November 2021 (Andrejko et al 2021).…”