Air filter and floor sampling for detection of SARS-CoV-2 RNA in schools


A major concern at the start of the 2019 coronavirus disease pandemic (COVID-19) was the possibility that SARS-CoV-2, the pathogen responsible for the disease, could spread across surfaces.

Many previous studies have shown that SARS-CoV-2 can remain active on surfaces for several days; however, there are no studies to prove the transmission of infection from the surface to humans. These studies suggest monitoring the environment with surface swabs, which would help to analyze the transmission of SARS-CoV-2. Environmental monitoring acts as an additional tool for reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) in identifying the number of infected people cost-effectively.

A recently published study of medRxiv* The preprint server monitors the presence of SARS-CoV-2 RNA in five schools (96 classrooms) in Davis, California (USA), collecting weekly samples of surface swabs from classroom floors and / or portable high-performance air (HEPA) devices.

Study: The challenge of environmental monitoring SARS-CoV-2 in schools using floor and portable HEPA filter devices: fresh or relic RNA ?.  Image credit: LBeddoe / Shutterstock

Evaluation of environmental sampling strategies

This SARS-CoV-2 environmental monitoring study, conducted from January to August 2021, collected samples of floor swabs and / or HEPA filters in five primary schools.

To study the effectiveness of floor samples and HEPA filters, the researchers compared SARS-CoV-2 RNA and the cases of COVID-19 in two schools.

Oral swabs were used for sampling and RT-qPCR was used to quantify SARS-CoV-2 RNA. Prior to sampling, the detection of SARS-CoV-2 on surfaces was validated using opportunistic sampling at two sites within six days.

One of the two key sampling schemes was applied in each school: (1) floor only or (2) floor and HEPA filter. Nylon fiber pads with an acrylonitrile butadiene styrene (ABS) handle pre-moistened in a DNA / RNA shield were used to collect environmental samples. Samples were processed for RNA extraction on the same day of collection.

The researchers hypothesized that SARS-CoV-2 virions would accumulate on the outer surface of the filters due to air circulation, making HEPA filter sampling more effective for identifying infected individuals than floor sampling.

Positive and negative SARS-CoV-2 rooms based on floor samples collected in schools (A) A, (B) B and (C) C during the pilot environmental monitoring study.  Episodes with a positive floor sample are marked in red, negative episodes in green, and episodes without a sample are marked in white.  Sampling of the air filter in schools A and B began on May 5, 2021 and is indicated by the orange line.  No positive air filter samples were found.

Positive and negative SARS-CoV-2 rooms based on floor samples collected in schools (A) A, (B) B and (C) C during the pilot environmental monitoring study. Episodes with a positive floor sample are marked in red, negative episodes in green, and episodes without a sample are marked in white. Sampling of the air filter in schools A and B began on May 5, 2021 and is indicated by the orange line. No positive air filter samples were found.

Survey results

The researchers compared the two strategies by testing samples from one floor and two HEPA filter samples in 57 classrooms in schools D and E. During the study, clinical testing of COVID-19 was offered free of charge to all schools.

The results show that HEPA filter sampling has 3.02% and 0.41% levels of filter sample positivity collected for schools D and E, respectively; however, only 0.48% and 0% sample positivity levels compared to 0.48% and 0% for floor sampling.

The results show that tampons with HEPA filters are more sensitive than floor tampons when SARS-CoV-2 RNA is detected indoors.

In addition, a confirmed positive environmental sample was obtained during on-site clinical trials in schools D and E, but interestingly, there were no cases of COVID-19-positive students in classrooms with positive environmental samples. Therefore, positive samples may be due to the presence of relic viral RNA from people infected before the start of the monitoring program and / or RNA transported by the fomites in the classrooms.

Although no infectious viruses were detected in samples with cycle threshold values ​​(Ct) above 30, additional tests are needed to assess the viability of the virus on different surfaces.

“The portable HEPA filter and floor sampling are environmental monitoring tools that can successfully detect SARS-CoV-2 RNA.”

Conclusions

The results of the study show that environmental monitoring tools such as a HEPA filter and floor sampling can successfully identify SARS-CoV-2 RNA.

However, the study found that sampling from the HEPA filter was more effective than floor swabs. HEPA filter testing is effective in detecting SARS-CoV-2 RNA, which helps to investigate the transmission of SARS-CoV-2 in schools with limited participation in clinical trials.

One challenge posed by both the HEPA filter and floor sampling methods is the differentiation of fresh, relict, and outsider viral RNA. Therefore, further studies are needed to differentiate fresh and relict SARS-CoV-2 RNA in environmental samples and to determine Ct thresholds for HEPA filter monitoring that suggest close active infections.

*Important message

medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered convincing, guiding clinical practice / health-related behavior, or treated as established information.

Reference in the magazine:

  • The challenge of environmental monitoring of SARS-CoV-2 in schools using flooring and portable HEPA filter devices: fresh or relict RNA? Rogelio Zuniga-Montanez, David A Coil, Jonathan A Eisen, Randi Pechacek, Roque G Guerrero, Minji Kim, Karen Shapiro, Heather N Bischel, medRxiv, 2021. DOI: https://doi.org/10.1101/2021.11.12.12 https http://www.medrxiv.org/content/10.1101/2021.11.12.21266178v1



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