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

Abstract

AbstractWith 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.