How common are high-risk coronavirus contacts? A video-observational analysis of outdoor public place behavior during the COVID-19 pandemic.

Epidemiological evidence and recommendations from the World Health Organization suggest that close face-to-face interactions pose a particular coronavirus transmission risk. The real-life prevalence and nature of such high-risk contacts are understudied, however. Here, we video-observed high-risk contacts in outdoor public places in Amsterdam, the Netherlands, during the COVID-19 pandemic. We found that high-risk contacts were relatively uncommon: Of the 7,813 individuals observed, only 20 (0.26%) displayed high-risk contacts. Further, we qualitatively examined the 20 high-risk contacts identified and found that they occurred disproportionally between affiliated persons engaged in affiliative behaviors. We discuss the potential public health implications of the relatively low incident rate of high-risk contacts.

Introduction

Social distancing in indoor and outdoor settings has been a critical non-pharmaceutical measurement to curb the global spread of the coronavirus. For example, in the Netherlands, public health agencies have advised maintaining 1.5 meters distance from people with whom one does not share a household [1]. However, not every behavioral violation of these social distancing directives is equally risky for coronavirus transmission. Evidence suggests that coronavirus primarily spreads via respiratory droplets during close face-to-face contact [2]. In line with this, the World Health Organization (WHO) [3] defines high-risk situations as contacts with a probable or confirmed COVID-19 case via physical contact or via face-to-face contact within one meter for at least 15 minutes.

Studies examining social distancing compliance do often not adhere to these insights and WHO’s definition of high-risk contacts. This is linked to the methodological reliance on self-reported or geo-tracking measures of social distancing compliance, which are too coarse-grained to capture whether people violate the specific behavioral criteria of high-risk contacts [4, 5]. Relatedly, there is a mismatch between people’s high self-reported willingness to distance [6] and direct observational evidence showing that people often cannot distance in practice [7-9]. With re-openings of societies around the globe, it is valuable for policymakers to have precise knowledge on high-risk contacts in outdoor settings: how common are these encounters and in what situations do they occur—and should potentially be targeted?

The current study addresses these pressing public health questions using a methodological approach that offers uniquely fine-grained insight into human interpersonal behavior: video-assisted naturalistic observation [10, 11]. In doing so, we provide—to our best knowledge—the first systematic examination of the real-life prevalence and nature of high-risk contacts as defined by the WHO.

Method

Data were footage of public places in Amsterdam, the Netherlands, recorded by three municipal surveillance cameras during the COVID-19 pandemic and provided by the Amsterdam Police Department (note that data were sourced from a wider pool of footage, which has been analyzed for different study purposes or using other methodologies [7–9, 12]). The project was approved by the Netherlands Public Prosecution Service (PaG/BJZ/49986), the Danish Data Protection Agency (514-0011/18-2000), and the Ethics Review Board of the Faculty of Social and Behavioral Science at the University of Amsterdam (2021-AISSR-14225).

The included cameras were located in relatively busy settings (i.e., shopping streets, public transportation), and the obtained footage was recorded on Thursdays and Saturdays between 9 a.m. and 8 p.m., from March 2020 to March 2021. Due to technical issues, the footage was often missing after 2 p.m.—therefore, and because video data coding is very labor-intensive, we only included footage from a short period between 1 p.m. and 1.30 p.m. This time of day was also chosen because it captured a relatively average level of pedestrian movement in public places [13]. Finally, we note that across the included months of footage, the COVID-19 infection rate fluctuated following European patterns, and a range of mitigation measures was implemented [12].

The coding was conducted by four trained research assistants and began by randomly selecting persons present on the footage—specifically, every third person crossing the streets for each camera throughout the observation period. In total, we video-observed 7,813 individuals. For each selected person, we recorded for a maximum of 120 seconds (M = 27.1, SD = 15.0, min. = 4, max. = 120) whether the person was involved in any social distancing violations. Here, we both applied a broad—or low-risk—definition of social distancing violations involving any interpersonal proximity within 1.5 meters [8], and a more narrow definition in line with the WHO’s high-risk definition: face-to-face encounters between two or more people who did not arrive together, and who were within less than one meters proximity for longer than 15 minutes, or had direct physical contact. Note that we excluded people arriving together from the definition of social distancing violations, given that these persons were likely to be affiliated [14] and belong to the same household and thus are falling outside the definition of a risk encounter.

Note that the interrater reliability of the high-risk contact measure was not tested, given that we implemented this measure at a late stage in the coding and research process. However, we did evaluate this for two related social distancing measures, which both reached acceptable AC1 agreement scores [15]—i.e., the broadly defined social distancing measure had an AC1 of 0.97, and a version of this measure restricted to interpersonal proximities lasting a minimum of 10 seconds had an AC1 of 0.84. Given that the high-risk contact measure is a subset of these measures, these acceptable scores demonstrate indirectly that the current high-risk contact measure most plausibly has reasonable reliability.

For each person involved in a high-risk contact, we furthermore conducted a detailed qualitative description of the event (e.g., type of contact and activity) and the persons involved in the encounter (e.g., their age and whether they were affiliated [14]), with the ambition of attaining a more fine-grained understanding of the behavioral sequences that led to the contacts [16]. As part of this analysis, we also categorized the types of high-risk situations according to five categories: 1) incidental touching, 2) asking questions, 3) catching-up (i.e., people meet, talk, and leave separately), 4) meeting (i.e., people meet, talk, and leave together), and 5) group reassembling (e.g., persons are waiting on each other, reassemble, and leave together).

Note that the regression and interrater reliability analyses were run with Stata 16. The coding of the video clips was done in SPSS 26.0. Replication data, scripts, and materials are available at osf.io/7ek9d.

Results

Of the 7,813 persons observed, 6,108 or 78% (CI 95% [77%, 79%]) were within 1.5 meters of another person. For the vast majority of these common violations, the encounters remained low-risk, see Fig 1. Only 20 persons or 0.26% (CI 95% [0.14%, 0.37%]) were involved in a more narrowly defined high-risk encounter.

Fig 1

Descriptive breakdown of the observed person’s involvement in risk behaviors.

Note that these figures represent the rate of contacts during the average observation time of 27 seconds. To assess these rates on a more standard time unit, we regressed social distancing on observation time. Observation time (in seconds) was found to be positively associated with social distancing measured on three ordinal levels (OR = 1.05, 95% CI [1.04, 1.05], p < .001), i.e., no violation, 1.5 meter violation, and high-risk contact.

This ordinal regression result is graphed as predicted probabilities in Fig 2 [17]. As we see, a large proportion of persons was involved in 1.5-meter distancing violations within seconds of observation, with the predicted probability approximating 1.0 after around 1.5 minutes. This pattern is consistent with the interpretation that the 1.5-meter violation risk is high because public place crowding offers many “situational opportunities” for brief encounters [7], which, in turn, accumulates mechanically as people move through space and pass by additional people [18]. By comparison, only a small proportion of persons was involved in high-risk contact within seconds of observation, with the predicted probability only increasing noticeably after one minute. A possible explanation of this slope profile was examined as part of our qualitative assessment of the high-risk encounters.

Fig 2

1.5-meter and high-risk violations regressed on observation time.

Note. The ordinal regression model was estimated with three outcome levels, but for brevity, we do not graph the no-violation outcome level.

In 14 of the 20 (70%) high-risk encounters, the individuals appeared to be affiliated rather than strangers—and within the subset of these affiliation cases, the persons were equally likely to be involved in catching up (36%), meeting (36%), and group reassembling (29%). None of these face-to-face activities lasted longer than fifteen minutes, but all of them involved social touching. Note that the two remaining activities (i.e., incidental touching and asking questions) did not occur between affiliated persons.

To illustrate how the interaction between affiliated persons may lead to high-risk contacts, we provide a video transcript of an event involving two persons catching up: A man walked down a shopping street with a wide walking area, shops on both sides, and a few scattered benches. He passed another man in the middle of the street and turned his head. The two men walked toward each other, shook hands, and bumped fists. They talked for a moment, bumped fists again, and continued walking in separate directions.

In the remaining six of the 20 (30%) high-risk encounters, the involved persons appeared to be strangers. Within this subset, the contacts were linked to asking questions (67%) or incidental touching (33%). Cases of asking questions involved, for example, borrowing a lighter, begging for money, or persons handing out flyers to others on the street while chatting with them. The following video transcript illustrates such a case: A man is walking along the same shopping street as in the previous example. He slows down and enters a queue in front of a pharmaceutical shop—while keeping a proper distance from the person in front of him. A young man approaches the queue, stops in front of the man, and stays within one-meter distance while saying something to him. The man reaches in his pocket and hands an object to the young man, apparently a lighter. During this interaction, the hands of both man touch. The young man lights a cigarette while remaining within a one-meter radius and returns the lighter. Then, the young man walks away from the man, who remains in the queue, and lights a cigarette himself.

The most noteworthy pattern in the qualitative analysis is that high-risk contacts appeared disproportionally associated with encounters involving affiliated persons engaged in affiliative behaviors. Adding to this interpretation, a binominal probability test offered some evidence that person affiliation is a necessary condition for high-risk contacts [19]: The proportion of affiliated (14/20 = 70%) was significantly greater than a benchmark proportion of 50% (i.e., a null-hypothesis assuming an equal ratio of affiliated and strangers), evaluated with an alpha threshold of 0.10 (which is sensible to use given the small sample size [20]). Relatedly, the qualitative analysis shed light on the mechanism underpinning high-risk contacts. Rather than incidental encounters accumulating with movement through space (they occur, but rarely), these are mostly cases of focused interactions with some duration [21]. People meet, halt, and interact, and therefore they tend to be observed for a disproportionally longer time, as captured in Fig 1.

Discussion

This study examined the frequency and qualitative nature of social distancing violations in public places—with a particular focus on high-risk contacts, to which prior research has shown limited attention. Our analysis demonstrated that the incident rate varied dramatically across social distancing definitions. Evaluated with a broad (and often legally sanctioned) definition that includes any interpersonal proximities within 1.5 meters, the violations were very common—i.e., similar to what is reported in prior observational studies [7-9]. However, the incident rate was comparably much lower when assessed with the WHO’s [3] more narrow definition, indicating that it is possible to be present in public space with limited exposure to high-risk contacts. In a further qualitative assessment, we found that the high-risk cases were disproportionally associated with encounters between affiliated persons engaging in friendly affiliative interactions involving social touching.

The low high-risk incident rate found in the current data adds an important nuance to the view that social distancing violations are often practically impossible to avoid in crowded urban settings [7-9]. Our result is understandable from the perspective of social-behavioral research, showing that everyday encounters among strangers in urban public places are typically brief and follow a norm of noninvolvement [21, 22]. By comparison, related research demonstrates that close interactions and social touches are more common and preferred among affiliated persons than strangers [21, 23], as we also observed.

The current results have several potential implications. To mitigate high-risk encounters in public, public health agencies could focus less on stranger encounters and more on coincidental interactions between affiliated. Further, public health agencies should perhaps focus less on implementing social distancing directives in outdoor public places than has been the case throughout the COVID-19 pandemic. Not only are high-risk contacts—as defined by the WHO—relatively uncommon in this setting, but the transmission risk is also lower outdoors than indoors [24]. On the other hand, recent coronavirus mutations have enhanced the infectivity and thus increased the transmission risk in outdoor settings [25, 26]. As such, these changing virological circumstances complicate what should be behaviorally defined as a high-risk contact, and we leave it to others—scholars and public health agencies alike—to determine whether or not the low high-risk incident rate should be considered epidemiologically trivial.

We acknowledged that the current study might have limited generalizability to other national urban contexts, times of day (e.g., rush hours), and public settings (e.g., semi-public indoor places such as bars and grocery shops). For example, national cultures have varying preferences for interpersonal distance and gesturing [27, 28], and cities have rhythms of rush hour and off-peak pedestrian movement [12, 13], and such circumstances may, in turn, influence the frequency of high-risk encounters. Another study limitation concerns our strictly behavioral focus, without data on the actual epidemiological risks associated with the contacts we denote as “high-risk.” We recommend that future research attempts to tie the behavioral and epidemiological dimensions together in studies examining how coronavirus transmission is not merely influenced by social distancing per se but by how this behavior is displayed in situ (e.g., among affiliated, with touches). Finally, we acknowledge that the exclusion of people arriving together from the definition of social distancing may have underestimated the high-risk incident rate. As such, it should be kept in mind that the reported incident rate concerns new encounters rather than any interpersonal proximities within 1.5 meters.

Full results of regression analysis.

(TXT)

Click here for additional data file.

10 Dec 2021

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Reviewer #1: This very research article entitled " How common are high-risk coronavirus contacts? A video-observational analysis of outdoor public place behavior during the COVID-19 pandemic" by Appelman et al., video-observed high-risk contacts in outdoor public places in Amsterdam, the Netherlands, during the COVID-19 pandemic. Authors found that high-risk contacts were relatively uncommon as out of 7,814 individuals observed, only 20 (0.26%) displayed such contact. While the topic is of increasing relevance, still, this reviewer has certain suggestions that would help produce a more comprehensive study of the topic:

Specific comments that the authors should consider

1, Authors have performed this study at outdoor public places in Amsterdam, it would be interesting to analyze other available data from other geographical location in the Netherlands.

2, Authors should provide their data graphical representable format.

3, Authors should discuss their study by putting/citing some current research.

4, At least one supplementary Figure as illustration may be afforded as to highlight the summary or prospect of this study.

5, What was the infection rate in Netherlands when this study was conducted and what might be the proposed reason for that?

Reviewer #2: This paper uses video-observational analysis to investigate the high-risk contact behaviour at outdoor public place during the COVID-19 pandemic. According to WHO, high-risk situation is defined as contacts with a COVID-19 patient via physical contact or via face-to-face contact within one meter, for at least 15 minutes. And the authors find that such high-risk contacts were relatively uncommon. The authors thus conclude that their results alleviate the public health concern regarding coronavirus transmission in outdoor public places.

This conclusion may mislead. From the several waves of SARS-CoV-2 virus infection outbreak in the past two years in almost all major countries in the world, we can see that although the rate of outdoor person-to-person viral transmission is much lower than that of the indoor transmission, outdoor transmission is still a considerable form of transmission. The authors’ result rather shows that the WHO definition of high-risk contact is inadequate: long distance (greater than one meter) and short time (less than 15 minutes) can still constitute high risk of infection. This is because, the SARS-CoV-2 virus can be transmitted in airborne way [1-3].

The highly contagious nature of the SARS-CoV-2 viral infection shows that although facial mask may slow down the transmission of the virus, in long run, infection is inevitable, and we have to live with this SARS-CoV-2 virus. Epidemiologic data show that most of the COVID-19 cases are asymptomatic and mild [4] and the SARS-CoV-2 virus is self-limiting [5]. The severe cases of COVID-19 is dominantly caused by the immunopathology, i.e., the overreaction of the host immune system and the cytokine storm [6,7] instead of viral infection. So the public health authorities may focus more on the curing of the noncommunicable diseases like morbid obesity, type 2 diabetes mellitus and other metabolic syndromes than the concern regarding the coronavirus infection and transmission.

Reference

1. Huang J, Jones P, Zhang A, Hou SS, Hang J and Spengler JD (2021) Outdoor Airborne Transmission of Coronavirus Among Apartments in High-Density Cities. Front. Built Environ. 7:666923. doi: 10.3389/fbuil.2021.666923

2. Yu ITS, Li Y, Wong TW (2004) Evidence of airborne transmission of the severe acute respiratory syndrome virus. N. Engl. J. Med. 350(17):1731–1739. DOI: 10.1056/NEJMoa032867.

3. Zhang R, Li Y, Zhang AL, Wang Y, Molina MJ (2020) Identifying airborne transmission as the dominant route for the spread of COVID-19. Proc Natl Acad Sci U S A. 117(26):14857-14863. DOI: 10.1073/pnas.2009637117.

4. Wu Z, McGoogan JM (2020) Characteristics of and Important Lessons From the Coronavirus Disease 2019 (COVID-19) Outbreak in China: Summary of a Report of 72 314 Cases From the Chinese Center for Disease Control and Prevention. JAMA. 323(13):1239-1242. DOI: 10.1001/jama.2020.2648

5. Zhu CC, Zhu J (2021) The effect of self-limiting on the prevention and control of the diffuse COVID-19 epidemic with delayed and temporal-spatial heterogeneous. BMC Infect Dis 21, 1145. DOI: 10.1186/s12879-021-06670-y

6. Cao, X (2020) COVID-19: immunopathology and its implications for therapy. Nat Rev Immunol 20, 269–270. DOI: 10.1038/s41577-020-0308-3

7. van Eijk LE, Binkhorst M, Bourgonje AR, Offringa AK, Mulder DJ, Bos EM, Kolundzic N, Abdulle AE, van der Voort PH, Olde Rikkert MG, van der Hoeven JG, den Dunnen WF, Hillebrands JL, van Goor H (2021) COVID-19: immunopathology, pathophysiological mechanisms, and treatment options. J Pathol. 254(4):307-331. DOI: 10.1002/path.5642.

Reviewer #3: In this paper entitled " How common are high-risk coronavirus contacts? A video-observational analysis of outdoor public place behavior during the COVID-19 pandemic", the authors used video-observed high-risk contacts in outdoor public places. The study has 7814 individuals and only 0.26 % of individuals display high-risk contact. The manuscript alleviates public health concerns regarding coronavirus transmission in outdoor places based on a low number of high-risk individuals. In addition, the manuscript is fascinating and easy to understand. However, there are a few concerns in the manuscript.

Minor Comments:

1) The English may be polished. There are grammatical errors and spelling mistakes in the manuscript.

2) The link provided in the manuscript in line 100 is not working. Find a way to make the data available to readers.

3) The cameras were located in busy settings such as shopping streets, public transportation. Will footage from pubs, grocery shops, coffee shops change the results?. Why did authors not consider these settings for their study as these sites are closer and human interaction is more?.

4) The footage is from 1 p.m and 1.30 p.m; for half an hour in the afternoon. During this time, the rush in public transport is minor. Why did the authors not consider another time for their study?. Will footage time like office hours and school hours affect the results?

5) Please mention the limitation of the study?

6) The authors may additionally provide one Figure as a summary in results section.

7) The author may elaborate on the results of observational video analysis in the discussion.

9) Discuss similar studies in the introduction and few information on social distancing, nature biomolecules for their treatment, and importance of health (few references i.e. doi: 10.1007/s12088-020-00908-0; doi: 10.1007/s12088-020-00893-4).

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28 Jan 2022

*** Response to reviews of ‘How common are high-risk COVID-19 contacts?’****

***Editors feedback***

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***REPLY: Validated.

2. In your Data Availability statement, you have not specified where the minimal data set underlying the results described in your manuscript can be found. PLOS defines a study's minimal data set as the underlying data used to reach the conclusions drawn in the manuscript and any additional data required to replicate the reported study findings in their entirety. All PLOS journals require that the minimal data set be made fully available. For more information about our data policy, please see http://journals.plos.org/plosone/s/data-availability.

Upon re-submitting your revised manuscript, please upload your study’s minimal underlying data set as either Supporting Information files or to a stable, public repository and include the relevant URLs, DOIs, or accession numbers within your revised cover letter. For a list of acceptable repositories, please see http://journals.plos.org/plosone/s/data-availability#loc-recommended-repositories. Any potentially identifying patient information must be fully anonymized.

Important: If there are ethical or legal restrictions to sharing your data publicly, please explain these restrictions in detail. Please see our guidelines for more information on what we consider unacceptable restrictions to publicly sharing data: http://journals.plos.org/plosone/s/data-availability#loc-unacceptable-data-access-restrictions. Note that it is not acceptable for the authors to be the sole named individuals responsible for ensuring data access.

***REPLY: Replication data and materials are available at https://osf.io/7ek9d/.

3. We note that you have indicated that data from this study are available upon request. PLOS only allows data to be available upon request if there are legal or ethical restrictions on sharing data publicly. For more information on unacceptable data access restrictions, please see http://journals.plos.org/plosone/s/data-availability#loc-unacceptable-data-access-restrictions.

In your revised cover letter, please address the following prompts:

a) If there are ethical or legal restrictions on sharing a de-identified data set, please explain them in detail (e.g., data contain potentially sensitive information, data are owned by a third-party organization, etc.) and who has imposed them (e.g., an ethics committee). Please also provide contact information for a data access committee, ethics committee, or other institutional body to which data requests may be sent.

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***REPLY: We would like to stress that all data scripts, transcripts, and materials are publicly available at https://osf.io/7ek9d/. Only the raw data, consisting of video recordings of individuals moving through public space, is available upon request. Ethical and legal restrictions for sharing this data are mentioned in the revised Data Availability statement.

***Reviewer #1***

4. Authors have performed this study at outdoor public places in Amsterdam, it would be interesting to analyze other available data from other geographical location in the Netherlands.

***REPLY: We agree that this would be interesting and relevant. However, in the current study, we only have access to CCTV camera footage from Amsterdam. In fact, we were only able to collect this data because of our long-term collaboration with the Amsterdam Police Department—there exists no centralized, nationwide archive of CCTV data. In the revised paper (p. 9), we have further flagged this geographical issue as a study limitation.

5. Authors should provide their data graphical representable format.

***REPLY: We agree that this would add clarity to the argument and have thus summarized the data patterns in Figure 1 (p. 5) and Figure 2 (p. 6).

6. Authors should discuss their study by putting/citing some current research.

***REPLY: We now cite and discuss other recent studies relating to the topic at hand, both in the introduction (p. 3) and the discussion section (p. 8-9).

7. At least one supplementary Figure as illustration may be afforded as to highlight the summary or prospect of this study.

***REPLY: In the exercise of trying to visualize our main results, we decided to create two figures. Figure 1 (p. 5) is a descriptive figure which summarizes the main findings with regard to the frequency of high-risk contacts, and Figure 2 (p. 6) is modelling based and details the difference between low and high risk distancing contacts. We have revised the Results section as part of the inclusion of these figures.

8. What was the infection rate in Netherlands when this study was conducted and what might be the proposed reason for that?

***REPLY: In the revised paper (p. 4), we now specify that the Netherlands case largely followed the European pattern, with fluctuating infection rates across the data collections period lasting one full year.

***Reviewer #2:***

9. The authors thus conclude that their results alleviate the public health concern regarding coronavirus transmission in outdoor public places.

This conclusion may mislead. From the several waves of SARS-CoV-2 virus infection outbreak in the past two years in almost all major countries in the world, we can see that although the rate of outdoor person-to-person viral transmission is much lower than that of the indoor transmission, outdoor transmission is still a considerable form of transmission. The authors’ result rather shows that the WHO definition of high-risk contact is inadequate: long distance (greater than one meter) and short time (less than 15 minutes) can still constitute high risk of infection. This is because, the SARS-CoV-2 virus can be transmitted in airborne way [1-3].

The highly contagious nature of the SARS-CoV-2 viral infection shows that although facial mask may slow down the transmission of the virus, in long run, infection is inevitable, and we have to live with this SARS-CoV-2 virus. Epidemiologic data show that most of the COVID-19 cases are asymptomatic and mild [4] and the SARS-CoV-2 virus is self-limiting [5]. The severe cases of COVID-19 is dominantly caused by the immunopathology, i.e., the overreaction of the host immune system and the cytokine storm [6,7] instead of viral infection. So the public health authorities may focus more on the curing of the noncommunicable diseases like morbid obesity, type 2 diabetes mellitus and other metabolic syndromes than the concern regarding the coronavirus infection and transmission.

***REPLY: We agree that the implications of our results could be nuanced along the suggested lines. The revised paper (p. 8-9) now unpacks these potential implications and considerations in a paragraph.

***Reviewer #3:***

10. The English may be polished. There are grammatical errors and spelling mistakes in the manuscript.

***REPLY: We have copy-edited the manuscript.

11. The link provided in the manuscript in line 100 is not working. Find a way to make the data available to readers.

***REPLY: We thank reviewer #3 for flagging this; the link is now updated.

12. The cameras were located in busy settings such as shopping streets, public transportation. Will footage from pubs, grocery shops, coffee shops change the results?. Why did authors not consider these settings for their study as these sites are closer and human interaction is more?.

***REPLY. We agree that this would be interesting, but the CCTV data systems we have obtained access to are only installed in outdoor public places. However, in the revised paper (p. 9), we now flag this as a study limitation. Further, in the discussion (p. 9), we now also consider the current result with respect to interaction in indoor vis-à-vis outdoor settings.

13. The footage is from 1 p.m and 1.30 p.m; for half an hour in the afternoon. During this time, the rush in public transport is minor. Why did the authors not consider another time for their study?. Will footage time like office hours and school hours affect the results?

***REPLY: We thank reviewer #3 for this comment, we now add a comment on why this period was selected, and furthermore, we acknowledge this as a study limitation that may have influenced the current results (p. 9).

14. Please mention the limitation of the study?

***REPLY: We have further detailed the study limitations (presented in the last paragraphs of the paper (p. 9).

15. The authors may additionally provide one Figure as a summary in results section.

***REPLY: In the exercise of trying to visualize our main results, we decided to create 2 figures. Figure 1 (p. 5) is a descriptive figure which summarizes the main findings with regard to the frequency of high-risk contacts, and Figure 2 (p. 6) is modelling based and details the difference between low and high risk distancing contacts. We have revised the Results section as part of the inclusion of these figures.

16. The author may elaborate on the results of observational video analysis in the discussion.

***REPLY: Thanks to this comment, the revised paper now elaborates/discusses the results of the analysis in more detail (p. 8). Relatedly, we have also expanded the introduction with citations and considerations of other research so as to clarify the contribution of the paper (p. 2-3).

17. The authors discuss similar studies in the introduction and few information on social distancing, nature biomolecules for their treatment, and importance of health (few references i.e. doi: 10.1007/s12088-020-00908-0; doi: 10.1007/s12088-020-00893-4).

***REPLY: Throughout the paper revision, we have considered additional relevant citations and have included several. We hope that this helps position our contribution more clearly within the emerging social distancing and related Covid-19 literature.

Submitted filename: Response to Reviewers (2).docx

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7 Mar 2022

How common are high-risk coronavirus contacts? A video-observational analysis of outdoor public place behavior during the COVID-19 pandemic

PONE-D-21-34926R1

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Reviewer #1: The manuscript entitled "How common are high-risk coronavirus contacts? A video-observational analysis of outdoor public place behavior during the COVID-19 pandemic " has been improved form its previous draft.

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9 Mar 2022

PONE-D-21-34926R1

How common are high-risk coronavirus contacts? A video-observational analysis of outdoor public place behavior during the COVID-19 pandemic

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