PONE-D-22-02504College openings in the United States increase mobility and COVID-19 incidencePLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. 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: Yes

Reviewer #2: Yes

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

Reviewer #1: Yes

Reviewer #2: Yes

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3. 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: Yes

Reviewer #2: Yes

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4. 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: 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 authors have an innovative paper on college reopening in Fall 2020. They show significant effects on mobility -- almost by definition mechanical effects since college campus are nearly vacant in summer -- using SafeGraph data. They also show significant increases in cases -- somewhat invariant to teaching modality -- at the county level relative to non-college counties. The paper does important diagnostics with generalized difference-in-differences / event studies. The paper is well written.

There are several important avenues to explore in a revision.

First, and most importantly, the most noteworthy and credible results are that college reopenings increased COVID-19 cases in the community (county). Yet the parameterization of "reopening" is challenging for two key reasons. One reason is that colleges vary tremendously in size (some large public universities have 50,000+ students while small liberal arts colleges might have 1,000 or so students), so the dosage within a community varies tremendously. Related, colleges size within a community matters a lot too -- colleges are a dominant shift in population in many "college towns" but may be insignificant in large urban centers. The second reason is that the authors focus on the first college reopening within the community. This is likely uncontroversial for small rural communities where there is one college, but is certainly problematic for analyzing COVID-19 spread within large urban centers (which can have dozens of colleges). There are some clear suggestions that can address these specific concerns. We'd expect the models of COVID cases, hospitalizations, etc., to have stronger effects in communities where student populations are an important part of county populations (e.g., I'm guess more rural settings). In addition, it appears that in "college counties" there are on average 2 colleges per county. But that almost certainly masks many rural counties with just 1 college, and some highly urban counties with 30+ colleges. By focusing on counties with a small number of colleges, the "first opening" measure would be more convincing.

Second, there was little mention of K-12 reopenings for Fall 2020. This must be addressed empirically as well; some states (e.g., Florida, Texas, etc.) were very aggressive about reopening public schools, while other states stayed online in Fall 2020. In principle, college reopenings in states (or counties) where K-12 remained online would provide the most convincing evidence on colleges; otherwise, there are natural concerns about similar timing of these two potential spreaders of COVID. It might be the case that the "no college" counties serve as a control for public school reopenings, but the case would need to be convincingly made.

Third, in the introduction the authors say "As we approach Fall 2021". They should update. The conclusion seems to be updated to 2022.

Overall, this is a terrific paper on a controversial topic that generates extreme reactions. The authors' findings make a lot of sense (although some of the extreme outcomes like mortality seem to increase relatively quickly compared to the lag we might expect from cases -- especially if spread starts among the young). However, the exploration I suggest isn't just "robustness" or footnote suggestions (in my view). Trying to characterize college reopenings in large urban areas is really hard given the multiplicity of colleges and differences in start dates. And even if colleges are areas of extreme transmission, some colleges are really small.

Reviewer #2: This study contributes to an important debate on the role of college reopening policies in mitigating the transmission of COVID-19 infections. The authors use data from a variety of sources to evaluate the association between college re-openings and mobility and COVID-19 related outcomes. The study is well done. I have some minor suggestions:

• In the introduction (paragraph 3), the authors write “As we approach Fall 2021, with expected mass movement events in the US …” Given that we are now in Spring 2022, this discussion should be updated to reflect the current situation. Many of these arguments remain relevant today.

• Is the natural log of county population the only variable that is included in the difference-in-differences model? Clarify whether other covariates are included in the propensity score.

• The discussion and/or conclusion sections should acknowledge the presence of pre-trends in mobility, which limits the causal interpretation of the estimates. Clarify that a causal interpretation is only valid under the assumption that any pre-trends are fully captured by linear trends.

• Given that the sample includes only 70% of four-year colleges, are counties with colleges not included in the C2i data dropped from the sample? In other words, does the control group include counties with colleges for which you do not have opening dates? This should be clarified.

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Reviewer #1: No

Reviewer #2: No

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College openings in the United States increase mobility and COVID-19 incidence

PONE-D-22-02504R1

Dear Dr. Andersen,

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,

Gabriel A. Picone

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: Yes

Reviewer #2: Yes

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

Reviewer #1: Yes

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: Yes

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: Yes

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: The authors have addressed all my concerns. I appreciate the care taken in this study and will use it in my classes.

Reviewer #2: (No Response)

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If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

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