A two-month field cohort study of mRNA BNT162b2 COVID-19 vaccinated workers from nursing homes demonstrated significant and relevant reduction of saliva excretion of SARS-CoV-2

Background: Residents of nursing homes (NHs) have been severely affected by the COVID-19 pandemic because of their old age and underlying comorbidities. Infection and outbreaks in NHs are most likely triggered by infected workers. Therefore, screening for workers who are asymptomatic carriers in NHs can prevent risky contact and viral transmission to the residents. The aim of this study was to examine the effect of the BNT162b2 mRNA COVID‑19 (Comirnaty®; BioNTech and Pfizer) vaccination on the saliva excretion of SARS-CoV-2 among NH workers, through weekly saliva RT-qPCR testing. Methods and principal findings: A two-month cohort study was conducted among 99 NHs in the Walloon region (Belgium), starting from the beginning of February 2021. Three groups of NH workers, i.e., non-vaccinated (n=1618), vaccinated by either one dose (n=1457) or two doses (n=2382) of BNT162b2 mRNA COVID‑19 vaccine were followed-up weekly. Their saliva samples were used to monitor the shedding of SARS-CoV-2 using RT-qPCR. In addition, all positive samples were sequenced and genotyped to identify the circulating wild-type virus or variants of concern. The protection fraction against the excretion of the SARS-CoV-2 in the saliva samples of the workers after the second dose of vaccine was estimated between 0.90 (95% CI: 0.18; 0.99) and 0.83 (95% CI: 0.54; 0.95) after one and eight weeks, respectively. Conclusion and significance: Weekly saliva RT-qPCR testing for SARS-CoV-2 demonstrated a significant effect of the COVID-19 mRNA BNT162b2 vaccine on the viral shedding in the saliva specimens of vaccinated NH workers. More varied and numerous genotypes of SARS-CoV-2 and more important variability in genomic copies (load estimate) were observed in non-vaccinated workers in comparison to vaccinated workers. The vaccination contributes to disrupting the chain of SARS-CoV-2 transmission between NH workers and residents. Field follow-up of vaccinated cohorts concerning both viral excretion and identification of circulating SARS-CoV-2 strain are of prime importance to properly assess the effectiveness of the vaccination protocol on wild-type virus and its variants. This field cohort study permitted advance of our knowledge of the efficacy of the currently approved COVID-19 vaccines, hence contributing to better decision-making in public health interventions and management.