A 2-month field cohort study of SARS-CoV-2 in saliva of BNT162b2 vaccinated nursing home workers

Abstract

Background: Nursing home (NH) residents have been severely affected during the COVID-19 pandemic because of their age and underlying comorbidities. Infection and outbreaks in NHs are most likely triggered by infected workers. Screening for asymptomatic NH workers can prevent risky contact and viral transmission to the residents. This study examined 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: A 2-month cohort study was conducted among 99 NHs in the Walloon region (Belgium), at the start of February 2021. Three groups of workers, i.e., non-vaccinated (n = 1618), one-dosed vaccinated (n = 1454), and two-dosed vaccinated (n = 2379) of BNT162b2 mRNA COVID‑19 vaccine, were followed-up weekly. Their saliva samples were used to monitor the shedding of SARS-CoV-2. All positive samples were sequenced and genotyped to identify the circulating wild-type virus or variants of concern.

Results: The protection fraction against the excretion of the SARS-CoV-2 in the saliva samples of the workers after the second dose is estimated at 0.90 (95% CI: 0.18; 0.99) at 1 week and 0.83 (95% CI: 0.54; 0.95) at 8 weeks. We observe more circulating SARS-CoV-2 and a greater variability of viral loads in the unvaccinated group compared to those of the vaccinated group.

Conclusions: This field cohort study advances our knowledge of the efficacy of the mRNA BNT162b2 COVID-19 vaccine on the viral shedding in the saliva specimens of vaccinated NH workers, contributing to better decision-making in public health interventions and management.

Keywords: Epidemiology; RNA vaccines.

Fig. 1. Experimental field design of the cohort study.

a Study participants enroled. b Duration of the observation. ^$The number of nursing homes (NHs), residents and workers were estimated in a previous study. ^#FTE, full-time equivalent. In the 99 studied NHs, the number of residents was estimated at 7.651 individuals. Based on a ratio of 20.5 equivalent full-time workers for 30 residents, the estimated study population of NH workers was around 5228 adults. During the study, the maximum number of participating NH workers (i.e., physical persons; not all are employed full time) exceeded slightly the estimated FTE number by 4.38% (5457 versus 5228). *, points of comparison between each vaccinated group and the non-vaccinated group.

Fig. 2. Participating (N = 99) and non-participating (N = 472) nursing homes in the study; map of Belgium, with Wallonia located in the south.

Black circles, participating nursing homes; White circles, non-participating nursing homes.

Fig. 3. Sampling effort by week for each group of nursing home workers.

a Non-vaccinated group of workers (from 99 nursing homes). b On dose-vaccinated group of workers (from 80 nursing homes). c Two dose-vaccinated group of workers (form 94 nursing homes). The maximum numbers of workers for each group (1618 + 1454 + 2379 = 5451) were close to the 100% of eligible workers (5228). The slight difference is related to the fact that some workers are not fully-employed.

Fig. 4. Sensitivity analysis based on bootstraps estimating the cumulative incidence rate ratio of excretion of SARS-CoV-2 in saliva sample of fully vaccinated workers from nursing homes with COVID-19 mRNA BNT162b2 vaccine.

cIRR, cumulative incidence rate ratio; LL and UL are the lower and the upper limits of the 95% confidence interval (exact binomial approximation).

Fig. 5. Number of wild-type virus and variants of concern identified over time in function of the vaccine status.

a Non-vaccinated workers. b One-dose vaccinated workers. c Two-dose vaccinated workers. Legend: W, week.

Fig. 6. Logarithm of the number of genomic copies estimated based on ORF1ab gene.

a Non-vaccinated versus vaccinated groups. b Wild-type virus versus Alpha variant. Log, logarithm in base ten. The dashed line represents the median of the log of genome copies; the solid lines below and above each rectangle represent, respectively, the first and the third quartiles; adjacent lines to the whiskers represent the limits of the 95% confidence interval; small circles represent outside values.