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Additional Editor Comments:

This manuscript examines the association between government financial support and influenza vaccination coverage using nationally representative KNHANES 2020 data. The authors conduct multivariable logistic regression and subgroup analyses to assess socioeconomic determinants of vaccination among financially supported and non-supported populations. The topic is relevant to public health policy; however, several important methodological and interpretational issues require substantial revision

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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?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Partly

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

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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

The PLOS Data policy

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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Reviewer #1: To improve the manuscript, I recommend that you explicitly address the "blind spots" identified in your analysis within the Discussion and Policy Recommendations sections. Specifically, while the National Immunization Program (NIP) effectively bridges the gap for the elderly and children, your data shows that low-income adults who fall outside these eligibility criteria remain highly vulnerable.

I suggest proposing a "tiered support model" or "income-based vouchers" for those in the lowest income quartiles who are currently in the non-supported group. Furthermore, since your study uses 2020 data, a brief discussion on how the COVID-19 pandemic may have uniquely influenced the vaccination behavior of these "self-paid" individuals would provide essential context. Finally, confirming whether KNHANES sampling weights were applied will strengthen the statistical validity of your conclusions for a national audience. These additions will transform your findings into a more actionable framework for achieving true health equity.

Reviewer #2: The topic of submitted paper, "Socioeconomic differences in influenza vaccination coverage by government financial support status: A population-based study in Korea," effectively explains Korea's national policy, and the section describing the differences in influenza vaccination coverage based on socioeconomic status is a crucial element for policy development. I believe that providing financial support to high-risk and socioeconomically disadvantaged groups is effective for influenza vaccination coverage. Therefore, I consider the results of this paper to be meaningful and should be published, as they can serve as a good case study for other countries.

Reviewer #3: The definition of the financial support group requires clarification. The manuscript appears to classify individuals based on eligibility criteria such as age, pregnancy, and chronic disease status, rather than confirmed receipt of free government-sponsored vaccination. This distinction should be clearly stated. In addition, it is unclear whether all individuals with chronic diseases were eligible for free vaccination under the 2020–2021 national program, as some groups may be recommended for vaccination but not necessarily covered by financial support. Furthermore, depending on the type of vaccine selected, individuals may have received vaccination through paid options even if they were eligible for free vaccination. These issues raise concerns about potential misclassification of the exposure.

The Abstract describes the adjusted odds ratio as a difference in vaccination coverage, which may be misleading and should instead be expressed in terms of odds.

The Introduction is somewhat lengthy and would benefit from being streamlined to focus more clearly on the rationale and study objective.

In the Methods section, descriptions of statistical analyses are included within the Study design subsection. For clarity, it would be preferable to separate the conceptual study design from the statistical analysis and present the analytical methods in a dedicated Statistical analysis section.

The interpretation of the findings in the Discussion appears stronger than can be supported by the study design and exposure definition. The financial support group is defined based on eligibility criteria for the national immunization program, which largely includes specific high-risk populations such as children and older adults, whereas the non-support group represents a substantially different population with distinct baseline characteristics. Therefore, the comparison between these groups may reflect differences in underlying population structure rather than the effect of financial support itself.

As the exposure is based on eligibility rather than actual receipt of free vaccination, the observed differences in vaccination coverage may be driven by these underlying characteristics. Accordingly, interpreting the findings as evidence of the effectiveness of government financial support, as well as the policy implications presented, may go beyond what can be supported by the data. Given the cross-sectional design and the potential for residual confounding and exposure misclassification, a more cautious interpretation is warranted.

These methodological considerations should be more explicitly addressed in the limitations section, particularly with respect to the distinction between eligibility and actual uptake, differences in population structure between comparison groups, and the constraints in drawing policy-relevant conclusions.

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Reviewer #1: Yes: Yahya Abodea

Reviewer #2: No

Reviewer #3: No

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Attachments

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Submitted filename: Major revision_001.docx

Socioeconomic differences in influenza vaccination coverage by government financial support status: A population-based study in Korea

PONE-D-26-08153R1

Dear Dr. Kim,

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.

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Kind regards,

Vasuki Rajaguru, PhD

Academic Editor

PLOS One