The current pandemic is caused by a new strain of coronavirus called SARS-CoV-2. COVID-19 (coronavirus disease 2019) is the name of the disease caused by this virus. The outbreak started in Wuhan, China, in December 2019, after which it rapidly spread throughout the world. On March 11, 2020, the World Health Organization declared the SARS-CoV-2 outbreak a global pandemic.1
The response to this pandemic has been unprecedented. Governments of various levels, public health organizations, and research institutes are scrambling to mitigate the spread of SARS-CoV-2, develop treatments, and understand how the virus is transmitted.
Although new information is frequently released regarding the methods by which SARS-CoV-2 spreads, both the CDC and WHO have established a few ways people may contract the virus. Viral spread is primarily through people in close contact with one another. When an infected individual coughs, sneezes, or speaks, they produce respiratory droplets or aerosols that may contain the virus. These aerosols can then be inhaled by healthy individuals through either the nose or mouth. It is also possible for these aerosols to land on commonly touched surfaces such as tables, handrails, and doorknobs. Alternatively, an infected individual may cough or sneeze into their hands, then touch a surface. If a healthy person touches a contaminated surface and then touches their eyes, nose, or mouth, they may contract the virus.1,2 Recent studies indicate the virus may spread via airborne aerosols to a greater extent than previously thought, prompting the WHO to revise its guidelines to include the possibility for this route of transmission.3,4,5 According to these studies, the virus may reside in tiny aerosols which can travel longer distances and linger in the air for an extended period of time. To make matters more complicated, there is strong evidence that people who have contracted the virus but are either presymptomatic or asymptomatic may shed viable viral particles and infect others.6
It is possible to determine whether SARS-CoV-2 is present in aerosols by using a specially designed air sampler. Collecting virus-containing aerosols with an air sampler is not a novel concept. For decades, researchers have used air sampling systems equipped with special filters to capture a variety of viruses, including but not limited to: influenza, rhinovirus, adenovirus, poliovirus, and SARS-CoV-1 (which caused the 2002-2004 SARS outbreak).7,8,9,10,11 Aerosol sampling has been utilized extensively during the current pandemic to determine the transmission characteristics of the virus.4,12,13
The Sensfy SARS-CoV-2 aerosol sampling kit utilizes a sampling approach that is based on substantial precedent. Airborne aerosols are drawn through a sterile PTFE filter with a 0.1µm pore size (which is approximately the same size as SARS-CoV-2 virions14). Samples should be drawn through the filter for 1-8 hours in front of room air intake or in an area with high foot traffic and aerosol generation, such as a restroom, break room, or classroom. A longer sampling duration is preferred, as that will increase the total volume of air passed through the filter.
Surfaces can become contaminated after virus-containing aerosols deposit onto them, or after an infected person coughs/sneezes onto their hands and then touches a surface.1,2,5
The most common method to test whether a surface may be contaminated with an infectious virus is to use a sterile swab. A swab is wetted with viral transport medium (VTM), wiped across a surface in multiple directions while rotating, then placed in a vial with VTM for preservation and put in a self-sealing bag for shipping. The sampling kit includes more detailed instructions, which are based on WHO guidelines.15
Sensfy analyzes environmental samples via real-time reverse transcription polymerase chain reaction RT-PCR, which is a very common and reliable method (this is the same analytical method used for clinical testing).16 Targeted portions of the SARS-CoV-2 RNA sequence are copied as complementary DNA, which is then amplified and measured using fluorescence.
The specificity for this method is extremely high, meaning the likelihood of a false positive is low. The laboratory will only report a positive result if two separate and unique regions of the SARS-CoV-2 RNA sequence are present. If only one of these regions is detected, this will be indicated on the report with an “inconclusive” result.
The laboratory uses a standard solution known to contain two target regions of the SARS-CoV-2 gene sequence. These controls are analyzed with every analytical batch. Positive controls must yield a positive result for the analytical batch to be considered valid.
The laboratory uses specially prepared nuclease-free water as a negative control. These controls are included with every analytical batch. If a negative control yields a positive result, this is an indication that a contamination event has likely occurred during batch preparation, and the analytical batch is invalid.
The RT-PCR analyzer used in the Sensfy laboratory features a batch analysis chip with multiple “wells” for analyzing both samples and controls. In order for a sample in any given well to yield a positive result, at least five copies of the SARS-CoV-2 gene sequence must be present. The volume of sample solution dispensed in each well is a small fraction of the total volume of sample solution prepared from either an air filter or a surface swab. Using the known volumes for these two types of samples, it is possible to calculate the method detection limits for both surface swabs and air filters.
Surface swab MDL: 2550 copies
Air filter MDL: 850 copies
For context, the average concentration of SARS-CoV-2 in human respiratory droplets is 7 million copies per mL. Speaking for fifteen minutes generates tens of thousands of tiny droplets with a total volume up to 11.25 µL, containing approximately 75,000 copies of the virus17. Singing, sneezing, or coughing can generate significantly more droplets than normal speech.
- World Health Organization. Q&A on coronaviruses (COVID-19). April 2020. https://www.who.int/emergencies/diseases/novel-coronavirus-2019/question-and-answers-hub/q-a-detail/q-a-coronaviruses
- Centers for Disease Control and Prevention. How COVID-19 Spreads. June 2020. https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/how-covid-spreads.html?CDC_AA_refVal=https%3A%2F%2Fwww.cdc.gov%2Fcoronavirus%2F2019-ncov%2Fprepare%2Ftransmission.html
- Stadnytskyi, V. et al. The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission. Proceedings of the National Academy of Sciences Jun 2020, 117 (22) 11875-11877; DOI:1073/pnas.2006874117
- Liu, Y., Ning, Z., Chen, Y. et al.Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature 582, 557–560 (2020). https://doi.org/10.1038/s41586-020-2271-3
- World Health Organization. Transmission of SARS-CoV-2: implications for infection prevention precautions. July 2020. https://www.who.int/news-room/commentaries/detail/transmission-of-sars-cov-2-implications-for-infection-prevention-precautions
- Furukawa NW, Brooks JT, Sobel J. Evidence supporting transmission of severe acute respiratory syndrome coronavirus 2 while presymptomatic or asymptomatic. Emerg Infect Dis. July 2020. https://doi.org/10.3201/eid2607.201595
- Booth TF, Kournikakis B, Bastien N, et al. Detection of airborne severe acute respiratory syndrome (SARS) coronavirus and environmental contamination in SARS outbreak units. J Infect Dis. 2005;191(9):1472-1477. doi:10.1086/429634
- Wallis C, Melnick JL, Rao VC, Sox TE. Method for detecting viruses in aerosols. Appl Environ Microbiol. 1985;50(5):1181-1186.
- Myatt TA, Johnston SL, Rudnick S, Milton DK. Airborne rhinovirus detection and effect of ultraviolet irradiation on detection by a semi-nested RT-PCR assay. BMC Public Health. 2003;3:5. doi:10.1186/1471-2458-3-5
- Yadana S, Coleman KK, Nguyen TT, et al. Monitoring for airborne respiratory viruses in a general pediatric ward in Singapore. J Public Health Res. 2019;8(3):1407. Published 2019 Dec 4. doi:10.4081/jphr.2019.1407
- Nguyen, T. et al, Bioaerosol Sampling in Clinical Settings: A Promising, Noninvasive Approach for Detecting Respiratory Viruses, Open Forum Infectious Diseases, Volume 4, Issue 1, Winter 2017, ofw259, https://doi.org/10.1093/ofid/ofw259
- Santarpia, J. et al. Transmission Potential of SARS-CoV-2 in Viral Shedding Observed at the University of Nebraska Medical Center. (2020) doi:10.1101/2020.03.23.20039446.
- Chia, P.Y., Coleman, K.K., Tan, Y.K. et al.Detection of air and surface contamination by SARS-CoV-2 in hospital rooms of infected patients. Nat Commun 11, 2800 (2020). https://doi.org/10.1038/s41467-020-16670-2
- Bar-On YM, Flamholz A, Phillips R, Milo R. SARS-CoV-2 (COVID-19) by the numbers. Elife. 2020;9:e57309. Published 2020 Apr 2. doi:10.7554/eLife.57309
- World Health Organization. (2020).Surface sampling of coronavirus disease (COVID-19): a practical “how to” protocol for health care and public health professionals, 18 February 2020, version 1.1. World Health Organization. https://apps.who.int/iris/handle/10665/331058
- Watzinger F, Suda M, Preuner S, et al. Real-time quantitative PCR assays for detection and monitoring of pathogenic human viruses in immunosuppressed pediatric patients. J Clin Microbiol. 2004;42(11):5189-5198. doi:10.1128/JCM.42.11.5189-5198.2004
- The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission
Valentyn Stadnytskyi, Christina E. Bax, Adriaan Bax, Philip Anfinrud
Proceedings of the National Academy of Sciences Jun 2020, 117 (22) 11875-11877; DOI: 10.1073/pnas.2006874117