PONE-D-21-03787

COVID RADAR app: Description and Validation of Population surveillance of Symptoms and Behavior in relation to COVID-19.

PLOS ONE

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Additional Editor Comments:

Please take into consideration all the reviewers comments/issues in editing the new version. Especially pay attention in acknowledging any other potential limitations and biases present in a self-selecting app-based population, and add some more formal analysis to support the claims being made.

[Note: HTML markup is below. Please do not edit.]

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: No

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: This is interesting work. I've a number of commments and suggestions that I think could further improve the quality of this manuscript; including better presentation, more acknowledgement of the potential limitations and biases present in a self-selecting app-based population, and some more formal analysis to support the claims being made.

Abstract, line 63: should be have, not having. And I'm not entirely sure what you mean by "Symptom variables increased noticeably in the week" - do you mean reports of users experiencing symptoms increased?

Were there any limits on who could sign up to the app - e.g., were children allowed to? Were users encouraged to report on behalf of others on the multiple accounts, or were people asked to each fill out their own reports?

Line 126 - I'd prefer 'individual with Covid-19' to the more colloquial 'Corona patient'.

Giving user feedback in the app may alter their behaviour and make the app less representative of the population behaviours. Did you see any evidence of this? I think this merits a bit more dicussion that already present as it has the potentialy to significantly bias your app poulation, and undermine the ability of the Covid RADAR to detect wider population changes.

I think at times the authors have put too much into the supplementary material and it has prevented the manuscript from being self contained. For example, line 135 "Following the data cleaning process detailed in the online supplement..." It may be that the cleaning process is extensive and a full description of it best belongs in a supplement, but I think it would help readers if a brief overviewing of the process was included in the main text.

Many of the figures don't have labels for the y-axis, please add.

There are a huge number of figure. It's good to have included so much detail, but I think some of these could be condensed into single figures with multiple panels. I think this would make it easier for readers to engage with and understand your work; it is difficult to do so when frequently scrolling between the texts and figures. For example, figures 5 and 6 could be combined into one.

It is well documented that populations of participatory digital surveys tend to not be representative of the population - they tend to be more female, younger, more educated. It appears the Covid RADAR is no exception; it would be help if this was made explicitr by comparison of your demographic to the country's general population, and the implications of this discussed. There are also harder to quantify biases: signing up to the app suggests a greater interest in Covid-19 than the average person, which might mean e.g. they are more likely to take the rules seriously and stick to them. It is very difficult to quantify the effect of this but it should be acknowledged as a limitation.

It is suprising that the Covid positive population tended to be older than the negative population. Could you comment on this - does this finding agree with other data sources?

Figure 9: are you able to show number of tests conducted as well as number of positive tests; as it's difficult to see whehther the peaks are true rises in population Covid-19 or just due to increased testing. The comparison is qualitative, and should be supported by measuring the quantiative correleation between your app data and cases; this would also help you formally justify your claim that the correlation is higher in regions with greather app participation.

Your claims that "‘fever’, ‘pain in the chest’ and ‘loss of smell’ are associated with COVID-19 case count while variables ‘coughing’ and ‘sore throat’ correlated more

closely with Rhinovirus cultures" is again, completely based on qualitative comparison and would be strenghtened by quantitative measurements. These findings appear to me consistent with findings in the literature, which is encouraging, and I think this should be noted.

Reviewer #2: The submitted manuscript reports analysis from data obtained from COVID RADAR app available in The Netherlands during COVID-19 pandemic outbreak (2020-2021). Overall, the analysis makes sense and the results are coherent. I have only a minor issue to address. As the app is based on self-reporting by app users, it should be specified that a bias toward serious illness exists, as the hospitalized users into Intensive Care Unit are likely not reporting their symptoms. This is a limitation in the analysis because it would be have been interesting to infer which symptoms and which areas might be prone to serious illness, i.e. also try to assess if pollution is playing a role. Other minor comments regard the manuscript, where sometimes too colloquial words are used, e.g. Corona. Lastly, on plots, x and y axis labels should be added.

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Reviewer #1: Yes: Mark Graham

Reviewer #2: No

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COVID RADAR app: Description and Validation of Population surveillance of Symptoms and Behavior in relation to COVID-19.

PONE-D-21-03787R1

Dear Dr. Kiefte,

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,

Simone Lolli

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: (No Response)

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

Reviewer #1: Yes

Reviewer #2: Yes

**********

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: (No Response)

Reviewer #2: I am happy that the authors addressed all my previously raised issues. Now the manuscript is ready for publication after technical corrections, e.g. typos, low-res

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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