Aerosoles como vector de transmisión del virus del SARS CoV-2: revisión y análisis de las tasas de positividad en interiores y exteriores

Abstract

The rapid appearance and transmission of the SARS-CoV-2 virus throughout the world has promoted the publication of studies investigating the possible routes of transmission of this virus. The first hypotheses were based on a possible transmission by macroscopic droplets and by fomites. The aerosol route of transmission gained force to be considered the main route of transmission as more studies were published. To expand scientific knowledge and try to enclose the information recorded since the start of the pandemic, a systematic bibliographic review of publications that carried out research on the presence of genetic material of SARS-CoV-2 in aerosol samples in both outdoor and indoor environments has been carried out, based mainly on an intensive search in leading academic databases (Scopus and Web of Science). Once all the information extracted from the publications of interest has been registered and structured, the results obtained were analyzed and compared, paying special attention to the positivity rates and the classification of the different samples according to the type and nature of the environment in which the samples were collected (indoor/outdoor and hospital/non-hospital). Overall, 47 publications regarding the detection of the SARS-CoV-2 virus in aerosols were included in this review, containing a total of 87 different datasets. The extracted information was structured and represented in 4 tables that collect, on the one hand, the different types of environments sampled and, on the other, the type of information shown in each article and dataset. An analysis of the most featured variables is carried out, starting with the geographical distribution of the publications, observing that the countries that have published the most articles on the subject are Iran, China, United States and Italy. The countries that recorded the highest positivity rates were Spain, India, Singapore, Israel and Mexico. Regarding the analysis of the type of environment sampled (indoor/outdoor), 80.5% (70/87) of datasets generated from indoor sampling have been reviewed and 19.5% (17/87) from outdoor environments. Regarding positivity rates, a higher value was obtained in indoor samples (16.7%) with respect to outdoor samples (12.6%). A total of 1928 samples were collected, obtaining a total positivity rate for the systematic review of 15.9%. Delving into this analysis, the data have been classified according to the nature of the environment sampled in relation to sampling in places such as hospitals or health care areas. Regarding indoor sampling, 54/70 (77.1%) studies carried out in hospital indoor environments were analyzed, and 16/70 (22.9%) studies in non-hospital indoor environments; and in terms of outdoor sampling, 6/17 (35.3%) studies were carried out in outdoor hospital environments, and 11/17 (64.7%) studies in outdoor non-hospital environments. Positivity rates were calculated in reference to the 4 sub-groups analyzed, obtaining the highest positivity rate (24.7%) for studies in non-hospital indoor settings, 16.7% for outside hospital settings, 15.5% for indoor hospital environments and, finally, 10.5% for outdoor non hospital environments. After analyzing the effect of the aerosol sampling technique used, it has been observed that the large majority of datasets have focused on the use of filtration devices (54/87, 62.1%). The remaining proportion was evenly distributed: impinger devices (13/87, 14.9%), impaction devices (12/87, 13.8%) and cyclone devices (11/87, 12.6%). The high frequency of use of filtration-based samplers has made it possible to study the types of substrates used; PTFE filters are the most used together with quartz fiber and gelatin filters. In contrast, the positivity rate calculated for filters made from other materials such as fiberglass or polyester was higher. The overall positivity rate for samples collected from the filtration technique was 21%, the highest obtained, followed by the rate obtained in samples collected from impactors (9.7%) and, lastly, impingers (7.7%) and cyclones (7.2%). An analysis has been carried out about the possible effect of the flow rate and/or the volume of air sampled on the positivity rate, not observing any indication of the effect of the volume of air sampled, except in the publications of Ang et al. (2021) and Dubey et al. (2021). The results regarding the attempted culture of SARS-CoV-2 from environmental aerosol samples that were positive are very limited, as is the publication of results on the RNA concentration of the virus (copies/m3 ). Despite the drawbacks found for the study, the reported results show great variability and suggest a large majority of cases in which the detected SARS-CoV-2 genetic material was probably not viable. As a conclusion, from the extraction of the information registered in the 47 publications included in the review carried out during this Master's Thesis (87 datasets) a positivity rate of 15.9% in the detection of genetic material of SARS-CoV-2 has been determined for aerosol samples. This value represents the high variability between the options within the sampling method used in each publication, which suggests a relatively low level of risk as long as the agglomeration of people who may have previously been infected with COVID-19 is avoided, especially in outdoor environments where the virus concentration is lower. It is necessary to continue investigating the possible presence of genetic material of SARS-CoV-2 in aerosol suspended in ambient air through environmental aerosol sampling using standardized techniques that allow expanding existing knowledge on the subject and being able to compare the results obtained in all and each of the studies carried out.