Investigating SARS-CoV-2 persistent contamination in different indoor environments

Abstract

Environmental contamination caused by COVID-19 patients could be a medium of transmission. Previous reports of SARS-CoV-2 in environmental surfaces were about short-term contamination. This study investigated SARS-CoV-2 RNA existence in room-temperature and low-temperature environments long after exposure (>28 days). A department store, where a COVID-19 outbreak was occurred in January 2020 (the epicenter of 43 COVID-19 patients), and a patient's apartment were included as room-temperature environments after being blocked for 57 days and 48 days, respectively. Seven cold storages and imported frozen foods inside were included as low-temperature environments (under -18 °C). Twenty food markets with potential contamination of imported frozen foods were also included to study the consecutive contamination. Information about temperature, relative humidity, and the number of days of environmental samples since the last exposure was collected and analyzed. In sum, 11,808 swab samples were collected before disinfection, of which 35 samples were positive. Persistent contamination of SARS-CoV-2 RNA was identified in the apartment (6/19), the department store (3/50), food packages in cold storages (23/1360), environmental surfaces of cold storages (2/345), and a package in the food market (1/10,034). Two positive samples were isolated from the bathroom of the apartment (66.7 %, 2/3), and doorknobs were proved with contamination in the apartment (40 %, 2/5) and cold storage (33.3 %, 1/3). The epidemiology information and environmental contamination results of an imported frozen food related COVID-19 case (138th COVID-19 patient in Tianjin) were analyzed. Based on the Ct values, the number of copies of two target genes was calculated by standard curves and linear regressions. In conclusion, SARS-CoV-2 RNA can be detected in room-temperature environments at least 57 days after the last exposure, much longer than previous reports. Based on the results of this study and previous studies, infectious SARS-CoV-2 could exist for at least 60 days on the surface of cold-chain food packages. Doorknobs and toilets (bathrooms) were important positions in COVID-19 control. High-risk populations of cold-chain-related logistic operations, such as porters, require strict prevention and high-level personal protection.

Keywords: COVID-19; Disinfection; Low-temperature; Persistent contamination; SARS-CoV-2.

Fig. 1

Environmental sampling sites and RT-PCR results in patient's apartment. An illustration shows the patient's apartment, which was blocked and sampled 48 days after exposure. The circles in the figure show the RT-PCR results of environmental samples collected before disinfection. The red circles indicate positivity for both two SARS-CoV-2 target genes (= positive). The blue circles indicate negativity. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2

Environmental sampling sites, RT-PCR results, and patients’ exposure range in Baodi Department Store. An illustration shows the structure and distribution of positive/negative environmental samples in the Baodi department. The circles in the figure show the RT-PCR results of environmental samples collected before disinfection. The red circles indicate positivity for both two SARS-CoV-2 target genes (= positive). The blue circles indicate negativity. The yellow background indicates patients' exposure during COVID transmission. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

A typical route of frozen food international transportation. An illustration shows the main process of frozen food international transportation. A. Meat plants in the exporter country; B. Foods are transported to containers in the refrigerated trucks; C. Containers transferred to the container ship; D. The ship arrived in the importing country; E. The container is unloaded to a refrigerated truck. F. Truck arrives at cold storage and has food unloaded in the anteroom; F. Foods are stored in the cold storage; G. Foods are distributed by small refrigerated vehicles. H. The packages are opened, and the meat is distributed by retailers.

Fig. 4

Temperature distribution and period (number of days) of environments with potential contamination by COVID-2019 patients before sample collection. An illustration shows temperature and period since contamination of all the environmental samples in this study. Environmental samples were categorized by locations or food classifications. The color blocks in the figure show the maximum ambient temperature of each environment every three days. Information about Day 94–174 was shown in one block. The number of total samples and positive samples of each classification was shown in the figure. A. Number of days since last exposure and peak air temperature of each three days of the department store and the apartment. B. Number of days since last exposure and average air temperature of each three days of the department store and the apartment. C. Number of days since last exposure and average air temperature of cold-chain-related food packages and environments. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5

Quantification curve of ORF1ab and N gene. An illustration shows the linear regression analyses of 6 serial dilutions (ranging from 5 to 5 × 10⁵ copies per reaction) using linearized plasmid with ORF1ab or N gene region, respectively, tested by RT-PCR for SARS-CoV-2. Ct means cycle threshold. A. Quantification curve and linear regression formulation of ORF1ab by RT-PCR. B. Quantification curve and linear regression formulation of ORF1ab by RT-PCR.

Fig. 6

Distributions of the number of copies of two target genes (ORF1ab and N) of 27 positive samples from different study sites. The number of copies of the samples was quantified using quantification curves. The X-axis shows sample names and the Y-axis shows different ranges of the number of copies (copies/mL or copies/cm²). Red lines in the illustration indicate the median value of samples collected from the department store, the patient's apartment and food packages in cold storage, respectively. A. Histogram of target gene ORF1ab (copies/mL); B. Histogram of target gene ORF1ab (copies/cm²); C. Histogram of target gene N (copies/mL); D. Histogram of target gene N (copies/cm²). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7

Main disinfection measures of imported frozen foods. An illustration shows three main disinfection measures of imported frozen food. A. The whole container is disinfected with frozen foods inside of it. A hydrogen peroxide disinfector is used for virus inactivation. In brief, 35 % food-grade hydrogen peroxide solution is vaporized by the disinfector and being released in the container. Then vaporized hydrogen peroxide concentration is maintained at 50–60 ppm in the sealed container for 1 h. B. Before being stocked in the cold storage, a sanitation tunnel machine in the anteroom is used to inactivate surface contamination of frozen food with 200 ppm chlorine dioxide solution. C. The food case is opened by a retailer, and small packages are processed by sprayer and wipes with alcohol-based disinfectant.