The present study addressed the question whether false-positive results for SARS-CoV-2 could result from droplet or airborne contamination originating from individuals that have been sampled previously in the same room. Our experimental series encompassed various exposure times, two different settings regarding ventilation and two PCR assays, including a commonly used assay for SARS-CoV-2 diagnostics and a highly sensitive high input PCR, did not detect any contamination of open sampling tubes with SARS-CoV-2.
Prior studies reported contamination of several surfaces with SARS-CoV-2 next to infectious individuals . Surroundings of infected individuals can either get contaminated by SARS-CoV-2 containing droplets, which is considered as by far the most relevant transmission mode, or exhaled aerosols or via contact transmission, e.g. if devices are touched after hands have been contaminated during coughing. The estimated median half-life of viable SARS-CoV-2 on different materials in an experimental setting was about 6.8 h and 5.6 h on plastics and stainless steel, respectively . The high proportion of COVID-19 affected individuals in dedicated test centers promotes environmental contamination with SARS-CoV-2, and the level of contamination presumably increases with proximity to infected persons. As detailed above, the main transmission mode of SARS-CoV-2 occurs via droplets expelled during talking, coughing or sneezing . For smaller droplets or aerosols, which can be generated during coughing by some individuals, ventilation becomes relevant for clearance [3, 7]. Our experimental set-up included a room without any enhanced air exchange and a room with continuous air exchange via tilted windows. Independent of the set-up all samples tested negative, even in a high input PCR with increased sensitivity.
Our study has some limitations. We can only provide information on the number of individuals that tested SARS-CoV-2 positive during the experimental series in both rooms. We did not register which patients were assigned to either room A or B. Nevertheless, as all samples tested negative, the assignment to the individual room does not seem crucial for the interpretation of the results. In addition, we did not swab surfaces to test for environmental contamination. However, considering the strict application of personal protective equipment, hand hygiene and surface disinfection, contamination around infected individuals is likely very limited. Strict application of hand hygiene and glove change of HCWs were likely crucial to prevent contamination of virus sample tubes.
In conclusion, we could not detect SARS-CoV-2 contamination of open collection tubes during NPS collection, even if these specimens remained unsealed for more than 3 h. Stringent use of personal protective equipment, including wearing of a surgical mask by the individual to be tested as a component of source control, hand hygiene and regular surface disinfection prevented successfully contamination. Furthermore, adequate specimen handling includes immediate closure after insertion of the NPS, thus minimizing the risk of contamination.