Potential SARS-CoV-2 infectiousness among asymptomatic healthcare workers.

A majority of SARS-CoV-2 infections are transmitted from a minority of infected subjects, some of which may be symptomatic or pre-symptomatic. We aimed to quantify potential infectiousness among asymptomatic healthcare workers (HCWs) in relation to prior or later symptomatic disease. We previously (at the onset of the SARS-CoV-2 epidemic) performed a cohort study of SARS-CoV-2 infections among 27,000 healthcare workers (HCWs) at work in the capital region of Sweden. We performed both SARS-CoV-2 RT-PCR and serology. Furthermore, the cohort was comprehensively followed for sick leave, both before and after sampling. In the present report, we used the cohort database to quantify potential infectiousness among HCWs at work. Those who had sick leave either before or after sampling were classified as post-symptomatic or pre-symptomatic, whereas the virus-positive subjects with no sick leave were considered asymptomatic. About 0.2% (19/9449) of HCW at work were potentially infectious and pre-symptomatic (later had disease) and 0.17% (16/9449) were potentially infectious and asymptomatic (never had sick leave either before nor after sampling). Thus, 33% and 28% of all the 57 potentially infectious subjects were pre-symptomatic or asymptomatic, respectively. When a questionnaire was administered to HCWs with past infection, only 10,5% of HCWs had had no indication at all of having had SARS-CoV-2 infection ("truly asymptomatic"). Our findings provide a unique quantification of the different groups of asymptomatic, potentially infectious HCWs.

Introduction

Transmission modelling has indicated that most (about 59%) of SARS-CoV-2 transmission occurs from non-symptomatic individuals with pre-symptomatic spread being more important (about 35% of transmissions) than spread from individuals who never develop any symptoms (i.e. asymptomatic) (about 24% of transmissions) [1-3]. The individual contribution to the SARS-CoV-2 transmission is variable between infectious subjects, with about 20% of infected subjects responsible for about 80% of viral transmissions [4, 5]. When screening non-symptomatic individuals, the proportion of positives that are pre-symptomatic or will remain asymptomatic is not known.

The first wave of the SARS-CoV-2 epidemic affected Sweden in March-June 2020, particularly in the Stockholm region [6]. The Stockholm HCW study of past or present SARS-CoV-2 invited all healthcare workers (HCWs) in the region for PCR and antibody testing in April-June 2020, with about 27,000 HCWs enrolled [7-9]. Whereas antibodies and low amounts of virus in PCR (high Ct values) were associated with past disease (post-symptomatic subjects) [7, 8], high amounts of virus in PCR predicted future disease in the next 1–2 weeks [7]. The pattern with no symptoms, high amounts of virus and no antibodies was unfortunately particularly common among HCWs working with home care for older persons [10].

The high-level independent expert investigation has emphasized that a failure to recognize the importance of asymptomatic transmissions was a decisive moment that furthered the spread of the pandemic [11]. The few studies that have quantified the importance of asymptomatic spread have been systematically reviewed [12], but it was clear that more data on this point was needed. We realized that the database of our previously performed Stockholm HCW cohort study provided a unique possibility for quantitative estimation of the proportion of pre-symptomatic and asymptomatic subjects among potentially infectious non-symptomatic HCWs. For this ad hoc study, we analyzed the 9449 HCWs who had complete testing and sick leave data, and in addition, compared with a group of 3981 HCWs who completed a questionnaire on symptoms.

Material and methods

In the previously described Stockholm HCW cohort, all healthcare providers in the capital region of Sweden were asked about participation in a study of past or present SARS-CoV-2 infections [8, 9]. Among the major healthcare providers who agreed to participate, a very high proportion (>90%) of the HCWs at work were enrolled, following written informed consent [8, 9]. This report focuses on the two largest healthcare providers, Karolinska University Hospital and Stockholm South General Hospital. At Karolinska (Fig 1), there were 9449 employees with complete testing data who were also followed for past or future sick leave [7]. At South Hospital the participants instead completed a questionnaire about symptoms (3981 HCWs) [9]. All HCWs at work were eligible for inclusion (physicians, nurses, assistant nurses, psychologists, social workers, physiotherapists, care administrators, medical secretaries, occupational therapists, speech therapists and nutritionists etc). Rules were strict that HCWs were not allowed to work in case of possible symptoms. We have previously reported that antibodies are strongly associated with past disease but not with future disease [8]. Similarly, we found that presence of low amounts of virus in PCR was associated with past disease, but not with future disease [7]. Conversely, presence of high amounts of virus associated with future disease, but not past disease [7]. For detailed patient flow charts of the cohort study, please see references [7, 9]. In the present report, we considered HCWs who tested positive but did not have the post-symptomatic testing pattern (presence of antibodies or only low amounts of virus) as potentially infectious, as it is well established that subjects are not infectious when returning to work after the stipulated number of days have passed after symptoms have resolved. The study was approved by the Swedish Ethical Review Agency and registered at clinicaltrials.gov (NCT04411576). Informed consents were collected from each participant prior start of the sampling (S1 File).

Fig 1

CONSORT flowchart.

Study flow chart according to the Standard Reporting of Observational Studies (STROBE) guidelines. Abbreviation: SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Statistical analysis

Proportion and 95% bootstrapped-confidence interval of potentially infectious non-symptomatic HCWs with different clinical characteristics was calculated. The association between age and status as a potentially infectious non-symptomatic HCW was examined using a logistic regression model adjusting for sex and patient contact. Moreover, the association between status as a potentially infectious non-symptomatic HCW and sex, patient contact and sick leave information was independently estimated. All statistical analysis and figure plotting were conducted using either SPSS or R [13].

Results

The potentially infectious HCWs tended to be more common among younger (<30 years of age) HCWs (OR 2.9 in logistic regression; Table 1). There was a 2.8-fold and 13.5-fold relative risk, respectively, for the potentially infectious subjects of being sick 1–3 weeks before testing (post-symtoomatic) or to become sick in the next two weeks (pre-symptomatic) (Table 2). There were 28.1% (16/57) potentially infectious HCWs who had no sick leave at all (asymptomatic) either before nor after testing (Table 2).

Table 1

Proportion of HCWs who are potentially infectious when screening of non-symptomatic HCWs (A) or who have had SARS-CoV-2 infection without experiencing any symptoms (B).

A.	Potentially infectious non-symptomatic HCWs					B.	Seropositives		Asymptomatic		Total
									among seropositives
Age categories	n	N	%	95% CI	Adjusted OR (95% CI)	Age categories in years	n	%	n	%	N
< 30 years	12	1143	1.05	0.46–1.64	2.9 (1.0–8.4)	<40	266	20.2	32	12.0	1314
≥30 to <40 years	15	2347	0.64	0.32–0.96	1.8 (0.6–4.8)
≥40 to <50 years	17	2334	0.73	0.38–1.07	2.0 (0.7–5.5)	≥40 to <50	178	18.0	17	9.6	989
≥50 to <60 years	8	2215	0.36	0.11–0.61	1.0 (0.3–3.1)	≥50 to <60	156	16.0	12	7.7	975
≥60 years	5	1410	0.36	0.04–0.66	Ref	≥60	104	14.8	13	12.5	703
Total	57	9449	0.60	0.45–0.76		Total	704	17.7	74	10.5	3981

n = potentially infectious healthcare workers; N = total of participants.

Table 2

Clinical characteristics associated to the potentially infectious healthcare workers (n = 57) in a cohort of 9,449 study participants.

	Potentially infectious non-symptomatic
	n (%)	Total %	95% CI	N	OR (95% CI)
Sex
Female	44 (77.2%)	0.59	0.41–0.76	7488	Ref
Male	13 (22.8%)	0.66	0.3–1.02	1961	1.1 (0.6–2.1)
Patient contact
No	15 (26.3%)	0.49	0.24–0.73	3073	Ref
Yes	42 (73.7%)	0.66	0.46–0.86	6376	1.3 (0.7–2.4)
Sick leave
No sick leave	16 (28.1%)	0.29	0.15–0.42	5614	0.5 (0.2–1.1)
1–2 weeks after testing	19 (33.3%)	3.8	2.12–5.48	500	6.9 (3.3–14.6)
1–3 weeks before testing	11 (19.3%)	0.81	0.33–1.29	1356	1.2 (0.7–2.2)
4–6 weeks before testing	11 (19.3%)	0.56	0.23–0.88	1979	Ref

n = potentially infectious healthcare workers; Total% = proportion from total, 95% CI = 95% Confidence Interval, N = total of participants; Ref = reference group.

We found no noteworthy differences between men and women or between HCWs with or without patient contact (Table 2). Analysis of the amounts of virus detected in PCR (CT values) found a significantly higher amount of virus among the HCWs who were younger than 40-years of age (Fig 2).

Fig 2

SARS-CoV-2 RT-PCR CT score distribution among the potential infectious non-symptomatic healthcare workers (HCWs).

(A) Distributions of CT score values among the 57 potentially infectious HCWs. (B) Significant difference of the CT score mean between below 40-year-old and 40 or older age potentially infectious non-symptomatic HCWs.

Among 3981 HCWs who completed serology testing and a questionnaire about symptoms, 704 subjects tested positive for SARS-CoV-2 antibodies. Among those there were 74 seropositive HCWs (10.5% of seropositives) who responded “No” to the question if they had had any symptoms suggesting that they might have had SARS-CoV-2 infection (“truly asymptomatic”). There was a moderate variability between age groups (ranging from 8–12%), which was not statistically significant (Table 1). Among all HCWs under the age of 40 years, 2.4% were seropositive and asymptomatic.

Discussion

Our study significantly adds up to the topic of what proportion of potentially infectious, non-symptomatic HCWs are either pre-symptomatic, asymptomatic or post-symptomatic. The importance of this topic has been clearly highlighted by the WHO-commissioned independent expert investigation, which highlighted this topic as a decisive factor on the spread of the pandemic [11].

The phenomenon of being non-symptomatic and potentially infectious is more common among younger HCWs, who also are positive for larger amounts of virus, than older HCWs. Although it is significantly more common for these potentially infectious HCWs to be pre-symptomatic (becoming sick in the next few weeks), a meaningful fraction (0.17% of HCWs) had no sick leave at all. That is, when screening non-symptomatic HCWs, about 0.2% (19/9449) were potentially infectious and pre-symptomatic (later had disease) and 0.17% (16/9449) were potentially infectious but never had sick leave neither before nor after sampling corresponding. When a questionnaire was administered to HCWs with past infection, only 10,5% of HCWs had had no indication at all of having had SARS-CoV-2 infection (“truly asymptomatic”). It was strictly forbidden to work in case of symptoms and rules were clear that it was not allowed to just work from home without reporting sick leave in case of symptoms. The lower proportion of subjects being entirely unaware of having had the infection might be due to very minimal symptoms that the subjects did not suspect to be infection at the time they had it.

Modelling studies have indicated that non-symptomatic subjects are responsible for a majority of the transmissions, with transmissions from pre-symptomatic subjects being more important than transmission from subjects who never develop symptoms. However, there is uncertainty in our basic knowledge about how common pre-symptomatic and asymptomatic infection is and our study provides a real-life estimation of this fundamental feature of the infection.

A major strength is that the entire cohort was comprehensively followed with sick leave data, both before and after sampling, a strategy that (as far as we have been able to determine) is internationally unique to our cohort. Other strengths include the fact that we performed both serology and antibody testing and administered questionnaires. By assessing both past and future sick leave we were able to classify the infected HCWs as either pre-symptomatic (coming down with disease in the near future), a-symptomatic (never developing disease) and post-symptomatic (had returned to work after resolution of symptoms). Our study has also some limitations. Firstly, not all HCWs were queried for past symptoms. Secondly, there was no PCR verification of whether sickness was due to SARS-CoV-2 or not, because PCR testing was not generally available at the time.

In this study, we estimate that at a given timepoint during the first wave of COVID-19 outbreak only 57 out of 9449 (0.6%, 95% CI 0.45–0.76) HCWs in Stockholm region were potentially infectious and that only 16/9449 were potentially infectious and remaining healthy (asymptomatic). In addition, our other estimate suggests that only few subjects (10,5%) with past SARS-CoV-2 had never experienced any symptoms9. Although the proportion of asymptomatic potential spreaders is low, the fact that they are still at work could result in a relatively large importance for the spread of the epidemic. Compared to the input values assumed in the transmission dynamic modelling, our real-life estimates of how common asymptomatic potentially infectious subjects may be are in good agreement with the input values used in the modelling [1].

Importantly, we find that potentially infectious non-symptomatic HCWs are particularly common among younger HCWs and that the amount of virus also tends to be higher among younger HCWs. A study serially following young (18–20 years of age) men found a minority of non-symptomatic men who repeatedly tested positive over time [14]. Such long duration and/or serial re-infection without symptoms could be important in the spread of the epidemic.

Taken together, significant SARS-CoV-2 infectivity prior to the onset of symptoms (i.e. pre-symptomatic), and a significant fraction of infections that are also asymptomatic [3] is in agreement with clinical evidence showing that HCWs are often exposed to SARS-CoV-2 also outside COVID-19 wards and may become infectious without symptoms [15-17]. Our finding that potentially infectious HCWs might be more common among younger HCWs suggests that it is important to vaccinate this young target population, if not yet vaccinated [18].

Study protocol in original language.

(PDF)

Click here for additional data file.

Transfer Alert

This paper was transferred from another journal. As a result, its full editorial history (including decision letters, peer reviews and author responses) may not be present.

29 Oct 2021

PONE-D-21-25144Potential SARS-CoV-2 infectiousness among asymptomatic healthcare workersPLOS ONE

Dear Dr. Pimenoff,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please make sure that the revised version will be prepared considering all the suggestions from reviewer 1, with a very careful consideration of the statistical aspects, as recommended.

Please submit your revised manuscript by Dec 13 2021 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: https://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Cristian Apetrei, MD, PhD

Academic Editor

PLOS ONE

Journal Requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and

https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. Thank you for stating the following in the Acknowledgments Section of your manuscript:

"This work was supported by the Karolinska University Hospital (KUH); the County Council of Stockholm; Erling-Persson family foundation; KTH Royal Institute of Technology; Creades and SciLifeLab. There were no roles for the funders in design or execution of the study, in analysis and interpretation of data or in the decision to submit for publication"

We note that you have provided funding information that is not currently declared in your Funding Statement. However, funding information should not appear in the Acknowledgments section or other areas of your manuscript. We will only publish funding information present in the Funding Statement section of the online submission form.

Please remove any funding-related text from the manuscript and let us know how you would like to update your Funding Statement. Currently, your Funding Statement reads as follows:

"This work was supported by the Karolinska University Hospital (KUH); the County Council of Stockholm; Erling-Persson family foundation; KTH Royal Institute of Technology; Creades and SciLifeLab. There were no roles for the funders in design or execution of the study, in analysis and interpretation of data or in the decision to submit for publication."

Please include your amended statements within your cover letter; we will change the online submission form on your behalf

3. We note that you have stated that you will provide repository information for your data at acceptance. Should your manuscript be accepted for publication, we will hold it until you provide the relevant accession numbers or DOIs necessary to access your data. If you wish to make changes to your Data Availability statement, please describe these changes in your cover letter and we will update your Data Availability statement to reflect the information you provide

4. Your ethics statement should only appear in the Methods section of your manuscript. If your ethics statement is written in any section besides the Methods, please move it to the Methods section and delete it from any other section. Please ensure that your ethics statement is included in your manuscript, as the ethics statement entered into the online submission form will not be published alongside your manuscript

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

[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: No

Reviewer #2: Yes

**********

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

Reviewer #1: No

Reviewer #2: I Don't Know

**********

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: No

Reviewer #2: Yes

**********

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

**********

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 primary objective of a secondary study was to quantify the proportion of healthcare workers who were potentially infectious. Their sick leave was classified as post-symptomatic or pre-symptomatic, and virus-positive subjects with no sick leave were considered asymptomatic. About 0.2% of he HCW at work were potentially infectious and pre-symptomatic (later had disease) and 0.17% were potentially infectious and asymptomatic (never took sick leave). In summary, 33% and 28% of all the 57 potentially infectious subjects were pre-symptomatic or asymptomatic, respectively.

Major revisions:

Include a statistical analysis section which lists and describes all the statistical methods used to analysis the data; cite the statistical software used.

Minor revisions:

1- Abstract: Replace “neither” with “either.”

2- Provide the percentage that corresponds to 16/57.

3- Table 2: Provide corresponding percentages for categorical factors. For instance, for sex provide the percentage corresponding to 44/(44+13) Female & 13/(44+13) Male, and 7488/(7488+1961) Female & 1961/(7488+1961) Male.

4- To assist in the review process, please add page and line numbers to the document.

Reviewer #2: This study identified the proportion of potentially infectious, nonsymptomatic HCWs which are either pre-symptomatic, asymptomatic or post-symptomatic. This topic is highlighted by WHO which mention the importance of this decisive factor on the spread of the pandemic.

The authors tested an impressive number of HCWs from Karolinska University hospital, using methods described in detail in references 7 and 8: serology and RT PCR. Previously published studies (references 6, 9 and 10) by the same team of researchers, support the authors' interest in the real-time pandemic situation in Stockholm.

The results of this study that the younger HCWs are potentially more infectious, based on real laboratory data, and the conclusion that they should get vaccinated, could be used like a model for other EU countries, to motivate their HCWs and general population to vaccinate themselves.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

2 Nov 2021

Following the Editors request and guidelines we have addressed the four points below, followed by our response to the reviewers comments.

1. We note that you have stated that you will provide repository information for your data at acceptance. Should your manuscript be accepted for publication, we will hold it until you provide the relevant accession numbers or DOIs necessary to access your data. If you wish to make changes to your Data Availability statement, please describe these changes in your cover letter and we will update your Data Availability statement to reflect the information you provide.

Response: The anonymized individual-level HCWs COVID-19 results (RT-PCR and serology) data (N=9449) with clinical features used in this study are deposited into b2share (https://b2share.eudat.eu/). The separate clinical dataset of 3981 HCWs who completed serology testing and a questionnaire about symptoms is only available in aggregated age-group format but made also available in the b2share portal. We will make the data files publicly available as soon as the paper is accepted for publication.

2. Your ethics statement should only appear in the Methods section of your manuscript. If your ethics statement is written in any section besides the Methods, please move it to the Methods section and delete it from any other section. Please ensure that your ethics statement is included in your manuscript, as the ethics statement entered into the online submission form will not be published alongside your manuscript.

Response: Modified accordingly

3. Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

Response: Revised and corrected one in press – article with the up-to-date citation information.

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

Reviewer #1: No

Reviewer #2: Yes

Response: Please, see our response to the Q1 above.

Please, see below our on-by-one responses to the reviewers comments:

Reviewer #1:

The primary objective of a secondary study was to quantify the proportion of healthcare workers who were potentially infectious. Their sick leave was classified as post-symptomatic or pre-symptomatic, and virus-positive subjects with no sick leave were considered asymptomatic. About 0.2% of he HCW at work were potentially infectious and pre-symptomatic (later had disease) and 0.17% were potentially infectious and asymptomatic (never took sick leave). In summary, 33% and 28% of all the 57 potentially infectious subjects were pre-symptomatic or asymptomatic, respectively.

Major revisions:

1. Include a statistical analysis section which lists and describes all the statistical methods used to analysis the data; cite the statistical software used.

Response: Statistical analysis paragraph was included into the methods section. Line 100.

Minor revisions:

2. Abstract: Replace “neither” with “either.”

Response: Modified accordingly. Line 31.

3. Provide the percentage that corresponds to 16/57.

Response: Modified accordingly. Line 109.

4. Table 2: Provide corresponding percentages for categorical factors. For instance, for sex provide the percentage corresponding to 44/(44+13) Female & 13/(44+13) Male, and 7488/(7488+1961) Female & 1961/(7488+1961) Male.

Response: Modified accordingly. Table 2.

5. To assist in the review process, please add page and line numbers to the document.

Response: Added accordingly.

Reviewer #2:

This study identified the proportion of potentially infectious, nonsymptomatic HCWs which are either pre-symptomatic, asymptomatic or post-symptomatic. This topic is highlighted by WHO which mention the importance of this decisive factor on the spread of the pandemic. The authors tested an impressive number of HCWs from Karolinska University hospital, using methods described in detail in references 7 and 8: serology and RT PCR. Previously published studies (references 6, 9 and 10) by the same team of researchers, support the authors' interest in the real-time pandemic situation in Stockholm. The results of this study that the younger HCWs are potentially more infectious, based on real laboratory data, and the conclusion that they should get vaccinated, could be used like a model for other EU countries, to motivate their HCWs and general population to vaccinate themselves.

Response: We thank the reviewer for this important comment. We also believe that these results should motivate intervention policies for vaccinating the healthcare personnel at all ages in a systematic fashion.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

10 Nov 2021

Potential SARS-CoV-2 infectiousness among asymptomatic healthcare workers

PONE-D-21-25144R1

Dear Dr. Pimenoff,

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.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Cristian Apetrei, MD, PhD

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

9 Dec 2021

PONE-D-21-25144R1

Potential SARS-CoV-2 infectiousness among asymptomatic healthcare workers

Dear Dr. Pimenoff:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Cristian Apetrei

Academic Editor

PLOS ONE