-->PONE-D-25-62493-->-->Potential Airborne Transmission of SARS-CoV-2 Through Bathroom Ventilation Ducts Associated with an Outbreak in a Residential Building in Santander, Spain, 2020-->-->PLOS One

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Academic Editor

PLOS One

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Reviewers' comments:

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1. Is the manuscript technically sound, and do the data support the conclusions?

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Reviewer #1: Yes

Reviewer #2: Yes

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-->2. Has the statistical analysis been performed appropriately and rigorously? -->

Reviewer #1: I Don't Know

Reviewer #2: N/A

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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-->5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)-->

Reviewer #1: The manuscript presents a compelling multidisciplinary investigation into potential airborne SARS-CoV-2 transmission via shared bathroom ventilation ducts in a residential building. The integration of epidemiological data, genetic sequencing, field measurements, CFD simulations, and multi-zone modeling provides robust evidence for the proposed transmission route. Strengths include rigorous model validation (e.g., r² = 0.83 for airflow comparisons) and practical recommendations for mitigation, such as installing exhaust fans with non-return flaps.

Minor suggestions:

Expand on the limitations of accessing only one apartment for measurements; discuss how this might affect generalizability to other floors or buildings.

In the discussion of quanta concentrations (e.g., Figs. 12-13), consider adding quantitative summaries (e.g., mean or range values) for key zones to complement the visual patterns.

Overall, the work is technically sound and contributes valuable insights to indoor aerosol transmission in multifamily housing. I recommend acceptance after minor revisions.

Reviewer #2: 1. Executive Summary

This manuscript investigates a vertically clustered outbreak of SARS-CoV-2 in a multi-family residential building in Santander, Spain. The authors present a compelling case that a shared vertical bathroom ventilation duct (shunt system) served as the conduit for airborne transmission, infecting 15 residents across four apartments. The study employs a robust multi-disciplinary methodology, combining epidemiological tracing, whole-genome sequencing, field environmental measurements, CFD simulations, and CONTAM multi-zone modeling. The findings highlight the critical role of building ventilation defects in disease transmission, particularly in older housing stock.

2. General Assessment

The manuscript is well-written, timely, and addresses a significant intersection between public health and building engineering. The integration of genetic sequencing with engineering airflow analysis provides a high level of credibility to the conclusions.

Strengths:

• Genetic Confirmation: The use of genomic epidemiology effectively rules out community transmission. The authors note:

"The sequences derived from the building were similar, with one or two nucleotide differences among them and at least 11 differences from the closest control sequence." This strongly supports the hypothesis of a single internal source.

• "Negative" Evidence: The observation that apartments with modified ventilation systems were spared provides excellent real-world validation of the proposed transmission mechanism.

"Occupants of three homes... in which the bathroom ventilation had been modified (by installing an exhaust fan with a no-return flap) did not get infected."

However, there are specific areas regarding the generalization of results and the discussion of "control" areas within the building that require clarification before publication.

3. Specific Comments & Critique

A. The "Control Group" and Building-Wide Generalization

Critique: The study focuses heavily on the infected vertical stack (Home A/B stack). While this is logical for forensic analysis, the manuscript lacks physical measurements from the other three courtyards (patios) in the building that did not experience outbreaks.

The authors state:

"No positive PCR tests were detected among the residents of the homes surrounding the three other patios of the building."

However, regarding physical measurements, it is noted:

"Additional measurements in other homes were not possible due to a lack of access."

Recommendation: The authors must address this limitation more explicitly in the Discussion section. A critical reader might ask: "Did the other courtyards not have outbreaks because their ventilation worked better, or simply because there was no virus present?" Based on the architectural symmetry implied in the text ("The building... comprises seven floors with eight homes per level, grouped around four patios"), it is scientifically sound to assume the airflow physics are identical. The authors should explicitly state that the lack of infection in other courtyards is likely due to the absence of an Index Case in those stacks, rather than superior ventilation performance. This distinction is crucial for generalizing the risk to the entire building type, not just the specific duct investigated.

B. The Exacerbating Role of Kitchen Hoods

Critique: One of the most significant findings of this study—which has profound public health implications—is the counter-intuitive impact of kitchen exhaust fans. The data suggests that operating these fans can be more detrimental than passive ventilation issues.

The manuscript states:

"airflow into the upper bathroom reaches approximately 2 m/s, and CO2 levels are higher. These results indicate that operating a kitchen hood may enhance the risk of aerosol transmission... more than opening the patio window."

Furthermore, the mechanism is clearly identified:

"When exhaust rates are high and makeup air is insufficient, the resulting depressurization may draw air and potentially contaminants from adjacent areas."

Recommendation: This finding deserves greater prominence in the Abstract and Conclusions. The authors should formulate a specific recommendation for residents in buildings with shared ducts: utilizing powerful kitchen extraction without ensuring adequate make-up air (e.g., opening a street-facing window) can actively suck pathogens from neighbors' apartments.

C. Generalization from Single-Floor Measurements

Critique: The environmental boundary conditions for the models were derived from a single apartment on the 4th floor.

"Environmental measurements were collected in the 4th-floor bathroom... of Home B."

Recommendation: While the CFD and CONTAM models extrapolate this to other floors, the authors should briefly acknowledge in the Limitations section that "stack effect" pressure differentials vary by height. Therefore, the infiltration rates on the 1st or 2nd floor might differ slightly in magnitude from the measured 4th floor, though the direction of flow (reverse flow) likely remains consistent under the observed conditions.

D. CFD Model Simplification

Critique: The authors mention simplifying the geometry for the simulation.

"The model focused on airflow and CO2 distribution... with some non-essential features simplified to reduce computation time."

Recommendation: Please verify or briefly confirm in the text that these "non-essential features" do not include roughness elements or duct junctions that might significantly induce turbulence or pressure drops, which could affect the vertical propagation speed of the aerosols.

4. Conclusion

This paper provides valuable evidence supporting airborne transmission via shared infrastructure. The methodology is sound, and the conclusions are supported by the data. I recommend Minor Revisions to address the discussion points regarding the unmeasured courtyards and to strengthen the warnings regarding kitchen exhaust operation.

Status: Accept with Minor Revisions.

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Reviewer #1: No

Reviewer #2: No

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Attachments

Attachment

Submitted filename: Peer Review Report PONE-D-25-62493.pdf

Potential Airborne Transmission of SARS-CoV-2 Through Bathroom Ventilation Ducts Associated with an Outbreak in a Residential Building in Santander, Spain, 2020

PONE-D-25-62493R1

Dear Dr. Miller,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Baohua Wen

Academic Editor

PLOS One

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

-->Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.-->

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

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-->2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented. -->

Reviewer #1: (No Response)

Reviewer #2: Yes

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-->3. Has the statistical analysis been performed appropriately and rigorously? -->

Reviewer #1: (No Response)

Reviewer #2: Yes

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-->4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.-->

Reviewer #1: (No Response)

Reviewer #2: Yes

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-->5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.-->

Reviewer #1: (No Response)

Reviewer #2: Yes

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-->6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)-->

Reviewer #1: (No Response)

Reviewer #2: (No Response)

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Reviewer #1: Yes: Pari Alavi

Reviewer #2: Yes: Abdullah Al_Sarraf

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