Article

Attitudes of Medical Students toward COVID-19 Vaccination:
Who Is Willing to Receive a Third Dose of the Vaccine?
Norio Sugawara 1,2, * , Norio Yasui-Furukori 2 , Atsuhito Fukushima 3 and Kazutaka Shimoda 2
1

2

3

*

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Citation: Sugawara, N.;
Yasui-Furukori, N.; Fukushima, A.;
Shimoda, K. Attitudes of Medical
Students toward COVID-19
Vaccination: Who Is Willing toReceive a Third Dose of the
Vaccine? Vaccines 2021, 9, 1295.
https://doi.org/10.3390/vaccines
9111295
Academic Editor: Ralph
J. DiClemente
Received: 8 October 2021
Accepted: 5 November 2021

Health Services Center for Students and Staff, Dokkyo Medical University, 880 Kitakobayashi,
Mibu 321-0293, Tochigi, Japan
Department of Psychiatry, Dokkyo Medical University School of Medicine, 880 Kitakobayashi,
Mibu 321-0293, Tochigi, Japan; furukori@dokkyomed.ac.jp (N.Y.-F.); shimoda@dokkyomed.ac.jp (K.S.)
Department of Infection Control and Clinical Laboratory Medicine, Dokkyo Medical University School of
Medicine, 880 Kitakobayashi, Mibu 321-0293, Tochigi, Japan; atsufuku@dokkyomed.ac.jp
Correspondence: nsuga3@dokkyomed.ac.jp; Tel.: +81-28-287-2153; Fax: +81-28-286-5187

Abstract: Medical students may come in contact with individuals infected with COVID-19 in theirclinical rotations. A high level of acceptance of vaccination is needed for them to protect theirhealth and the health of patients from this disease. The objectives of this study were to (1) obtain
information on medical students’ attitudes toward COVID-19 vaccination, (2) assess factors associatedwith students’ attitudes, and (3) identify predictors of their willingness to receive a third dose of
the COVID-19 vaccine. Using a cross-sectional design, we conducted a questionnaire survey of
medical students in July 2021. For this survey, we employed a 15-item questionnaire specificallydeveloped to assess the students’ attitudes toward COVID-19 vaccination. Of the 742 distributed
questionnaires, 496 (294 males and 202 females) were completed. Among all the participants, 89.1%(442/496) received the second dose of the vaccine, and 90.7% (450/496) indicated that they would
hypothetically receive the COVID-19 vaccine in the future. Furthermore, 84.5% (419/496) of all theparticipants were willing to receive a third dose of the vaccine. Regarding willingness to receive athird dose of the COVID-19 vaccine, multiple logistic regression models showed that students’ grade
and their responses to Q1 (positive attitude toward vaccination), Q9 (belief in the protection offered
by COVID-19 vaccination), Q10 (concern about the excessively rapid development of COVID-19
vaccines), Q12 (need for aspects of pre-pandemic life), and Q14 (concern about the sustainabilityof immunity) had significant associations with this outcome. Confidence in vaccines, relaxationof mobility restrictions, and concern about the sustainability of immunity motivate willingness toreceive a third dose of the COVID-19 vaccine in medical students.
Keywords: COVID-19; SARS-CoV-2; vaccine; hesitancy; medical student; attitudes

Published: 8 November 2021

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1. Introduction
Since coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was first described based on a cluster of cases in China,
there have been more than 236 million confirmed cases of COVID-19, including 4.8 million
deaths, and the disease has caused an ongoing global pandemic [1]. Preventive measures
such as social distancing, quarantining, and wearing masks, have become an essential partof daily life, and this pandemic has affected a wide range of people’s lives, including mental, physical, and social aspects [2–5]. To eliminate this pandemic, widespread vaccination
against COVID-19 has been regarded as a promising measure.
In Japan, COVID-19 vaccination was launched on 17 February 2021 for healthcare
workers; the first vaccine approved in Japan, Comirnaty (produced by Pfizer/BioNTech),
was based on the use of nucleoside-modified RNA (modRNA), which encodes the spike
protein found on the surface of SARS-CoV-2 [6,7]. Medical students in Japan are not
involved in clinical practice against COVID-19. However, in clinical rotations, students

Vaccines 2021, 9, 1295. https://doi.org/10.3390/vaccines9111295

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may come in contact individuals infected with this disease, such as healthcare workers [8].
A high level of acceptance of vaccination is needed for medical students to protect theirhealth and the health of patients from COVID-19 [9,10]. Vaccination is successful only
when there are high rates of acceptance and coverage. Healthcare workers are considered
an important source of information on vaccination for the general population [11,12], andmedical students with a positive attitude toward vaccines might encourage widespread
vaccination [9].
Recently, the need for a third dose of the COVID-19 vaccine has been discussed due tothe threat of the B.1.617.2 (delta) variant and the need for the protection of immunocompromised individuals [13]. A three-dose regimen could reduce the risk of COVID-19 infection
and reverse the waning of neutralizing antibodies after two doses of the vaccine [14–16].
However, the best timing of a third dose or boosting with heterologous vaccines is controversial [13,17,18]. Furthermore, there are still many countries facing very large waves ofthe disease with large unvaccinated populations. In addition, rich countries’ three-dose
booster regimens could impede global vaccination [19]. At present, the administration of athird COVID-19 vaccine dose is still controversial.
The objectives of this investigation were to (1) obtain information on medical students’ attitudes toward COVID-19 vaccination, (2) assess factors associated with students’attitudes, and (3) identify predictors of their willingness to receive a third dose of the
COVID-19 vaccine. To the best of our knowledge, this article presents the first study on thewillingness to receive a third dose of the COVID-19 vaccine.
2. Methods
2.1. Participants and Measures
Based on the Health Services Center for Students and Staff (HSCSS) database, morethan 80% (648/742) of medical students at Dokkyo Medical University completed a twodose regimen of the Comirnaty vaccine between April and May in 2021. To assess theattitudes of medical students toward COVID-19 vaccination, the HSCSS conducted a questionnaire survey for medical students at the university in July 2021. For this survey, abrief 15-item questionnaire based on the relevant literature was developed. The surveys
were distributed to all medical students at Dokkyo Medical University School of Medicine.
Of the 742 distributed questionnaires, 496 (294 males and 202 females) were completed.
The response rates among students from Grade 1 to Grade 6, were 73.8% (93/126), 73.2%(90/123), 68.6% (83/121), 69.5% (89/128), 61.2% (63/103), and 55.3% (78/141), respectively. The detailed questions are summarized in Table 1. Students rated the item regarding their attitudes toward COVID-19 vaccination on a 4-point scale (1 = strongly agree,
2 = agree, 3 = disagree, 4 = strongly disagree). Those answering 1 or 2 were grouped
as answering “agree”, while those who answered 3 or 4 were considered as answering“disagree”. Sociodemographic data (age, grade, and sex) and history of allergic reaction,
anaphylaxis, asthma, atopic dermatitis, and COVID-19 vaccination were obtained from the
HSCSS database. Furthermore, we classified students by status of COVID-19 vaccination
and willingness to receive the vaccine (Q15) (Figure 1). Students who were unvaccinatedor dropped out after the first dose of the COVID-19 vaccine and were willing to receive thevaccine in the future were classified as Group A. On the other hand, those who completed
the second dose of the COVID-19 vaccine and were not willing to receive the vaccine in the
future were classified as Group B. In addition, those who completed the second dose of the
COVID-19 vaccine and were willing to receive the vaccine as the third dose in the futurewere classified as Group C.

Vaccines 2021, 9, 1295

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Table 1. Attitudes of medical students toward COVID-19 vaccination (n = 496).
Strongly

Q1
Q2
Q3
Q4

Q5

Q6

Q7

Q8

Q9
Q10
Q11

Q12

Q13
Q14
Q15

In principle, I approve of
vaccinations.
I have a fear of needles/shots.
If I get COVID-19, I will develop
severe symptoms.
Parents’ opinion affected mydecision of whether to receive
the COVID-19 vaccine.
Friends’ opinion affected mydecision of whether to receive
the COVID-19 vaccine.
News media affected mydecision of whether to receive
the COVID-19 vaccine.
Being free of charge affected mydecision of whether to receive
the COVID-19 vaccine.
In general, the COVID-19
vaccine has apparent side
effects.
The vaccines provide a highdegree of protection against
COVID-19.
The COVID-19 vaccine wasdeveloped too rapidly.
I have received sufficient
information about COVID-19
vaccination.
COVID-19 vaccination is needed
for me to travel or go out to eat
and drink as in pre-pandemiclife.
The COVID-19 pandemic willcontinue for a long time.
I have concerns about thesustainability of immunity by
the COVID-19 vaccine.
I am willing to get the
COVID-19 vaccine in the future.

Srtongly

Agree

(n)

Agree

(n)

Disagree (n)Disagree (n)

51.2%

(254)

47.2%

(234)

1.2%

(6)

0.4%

(2)

5.0%

(25)

24.8%

(123)

40.3%

(200)

29.8%

(148)

3.0%

(15)

27.6%

(137)

61.9%

(307)

7.5%

(37)

13.1%

(65)

32.7%

(162)

29.8%

(148)

24.4%

(121)

6.3%

(31)

33.3%

(165)

33.1%

(164)

27.4%

(136)

4.0%

(20)

33.1%

(164)

35.3%

(175)

27.6%

(137)

25.2%

(125)

34.1%

(169)

23.4%

(116)

17.3%

(86)

22.8%

(113)

51.4%

(255)

22.8%

(113)

3.0%

(15)

11.9%

(59)

63.7%

(316)

22.0%

(109)

2.4%

(12)

10.3%

(51)

35.7%

(177)

48.2%

(239)

5.8%

(29)

7.3%

(36)

49.6%

(246)

38.9%

(193)

4.2%

(21)

44.0%

(218)

48.8%

(242)

6.0%

(30)

1.2%

(6)

26.2%

(25)

62.7%

(25)

10.1%

(25)

1.0%

(25)

9.9%

(49)

57.5%

(285)

29.0%

(144)

3.6%

(18)

40.7%

(202)

50.0%

(248)

7.7%

(38)

1.6%

(8)

Abbreviations: COVID-19, Coronavirus disease 2019.

Vaccines 2021, 9, 1295

4 of 10Figure 1. Status of COVID-19 vaccination and willingness to receive the vaccine. Note. The box
shaded with vertical lines represents students (Group A) who were unvaccinated or dropped out after
first dose of the COVID-19 vaccine but who were willing to receive the vaccine in future. On the other
hand, the box shaded with horizontal lines represents those (Group B) who completed the second
dose of the COVID-19 vaccine but were not willing to receive the vaccine in future. Furthermore, thebox shaded with a gray background represents those (Group C) who were willing to receive a third
dose of the COVID-19 vaccine.

2.2. Ethics
This protocol received approval from the Ethics Committee of Dokkyo Medical University School of Medicine (Approval number: 2021–016), and it conformed to the provisions of
the Declaration of Helsinki. The requirement for written informed consent was waived by
the Ethics Committee since the study involved record review only. However, informationon the study was disclosed on our web page, and students were free to opt out.
2.3. Statistical Analysis
Descriptive analyses of the demographic and clinical characteristics were performed.
To compare the main variables between groups, unpaired Student’s t-tests were performed
to analyze the continuous variables, and chi-square tests were performed to analyze
the categorical variables. The data are presented as the means ± SDs or percentages.
Multivariate logistic regression analysis with a forward selection method was performed
to assess the effects of the students’ attitudes (Q1–Q14 for students) and covariates (age,grade, sex, and history of allergic reaction, anaphylaxis, asthma, and atopic dermatitis) on
the completion of COVID-19 vaccination with a two-dose regimen. In another analysis,
we employed multivariate logistic regression analysis with a forward selection method toassess the effects of the students’ attitudes and the same covariates on the willingness toreceive the COVID-19 vaccine in the future (Q15).
Furthermore, we also conducted multivariate logistic regression analysis with a forward selection method to assess the effects of students’ attitudes and the same covariates
on the changing willingness to receive the COVID-19 vaccine in the future (Groups A
and B). Regarding those who were willing to receive a third dose of the COVID-19 vac-

Vaccines 2021, 9, 1295

5 of 10cine (Group C), multivariate logistic regression analysis with a forward selection method
was employed as the dependent variable and students’ attitudes and the same covariates
were employed as independent variables. A value of p < 0.05 was considered significant.
The data were analyzed using SPSS software for Windows (Version 27).
3. Results
Among all the participants, 90.7% (450/496) indicated that they would hypotheticallyreceive the COVID-19 vaccine in the future (Q15) (Table 1). The mean age and grade of
all participants were 21.1 ± 2.5 years and 3.4 ± 1.7, respectively. In the analysis of theinfluence of students’ characteristics (Table 2) on their willingness to receive the COVID-19
vaccine in the future (Q15), no differences in any other characteristics were observed.
Table 2. Characteristics according to willingness to receive the COVID-19 vaccine in future.
Students’ Willingness to Receive
Yes
Age
Grade
Sex (male)
Past history of allergic reaction
due tofood
medication
animalpollen from plants and trees
house dust mites
unknown allergen
Past history of anaphylaxis
Past history of asthma
Past history of atopic dermatitis

No

58.9%

21.1 ± 2.4
3.4 ± 1.7
(264/450)65.2%

20.8 ± 2.6
3.0 ± 1.6
(30/46)

11.5%
5.1%
5.5%
25.1%
9.9%
3.9%
0.5%
3.4%
2.8%

(50/435)(22/435)(24/436)(109/437)(43/438)(17/435)(2/435)(15/435)(12/435)

6.7%
2.2%
4.4%
20.0%
6.7%
4.4%
0.0%
2.2%
0.0%

(3/45)(1/45)(2/46)(9/47)(3/45)(2/46)(0/435)(1/45)(0/435)To assess the influence of attitudes (Q1–Q14) toward COVID-19 vaccination as independent variables of students’ completion of COVID-19 vaccination with a two-dose
regimen, we performed a multivariate logistic regression analysis with a forward selection
method. Higher grade and students’ responses to Q1 (positive attitude toward vaccination),
Q4 (effect of parents’ opinions), Q10 (concern about the excessively rapid development of
COVID-19 vaccines), and Q12 (need for aspects of pre-pandemic life) showed significant
associations. In an analysis of students’ willingness to receive the COVID-19 vaccine in
the future as a dependent variable, patients’ responses to Q1, Q9 (belief in the protection
offered by COVID-19 vaccination), Q10, Q12, and Q14 (concern about the sustainability of
immunity) were significantly associated with willingness (Table 3).
Table 3. Factors associated with completion of COVID-19 vaccination with a two-dose regimen andwillingness to receive the COVID-19 vaccine in the future.
Odds
Ratio
Grade
Q1
Q4
Q10
Q12

95%

CI

Wald
Value

Completion of COVID-19 vaccination with a two-dose regimen
1.57
1.24
1.98
13.71
28.66
2.71
303.45
7.77
0.41
0.21
0.83
6.24
0.47
0.24
0.91
5.05
10.49
4.09
26.89
23.94

p Value<0.001
0.005
0.013
0.025
<0.001

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Table 3. Cont.
Odds
Ratio
Q1
Q9
Q10
Q12
Q14

95%

CI

Wald
Value

Willingness to receive the COVID-19 vaccine in future
8.61
1.44
51.55
5.56
2.52
1.23
5.18
6.32
0.26
0.12
0.55
12.25
5.14
2.18
12.09
14.02
2.53
1.22
5.28
6.15

p Value

0.018
0.012
<0.001<0.001
0.013

Multivariate logistic regression analysis using a forward selection method was carried out with completion of
COVID-19 vaccination with a two-dose regimen as the dependent variable and age, sex, grade, past historyof allergic reaction (due to food, medication, animal, pollen from plants and trees, house dust mite, unknown
allergen), anaphylaxis, asthma, atopic dermatitis, and attitudes toward COVID-19 vaccination (Q1–Q14) as
independent variables. Multivariate logistic regression analysis using a forward selection method was carried out
with willingness to receive COVID-19 vaccination in the future (Q15) as the dependent variable and the same
factors as independent variables. Abbreviations: COVID-19, Coronavirus disease 2019; CI, Confidence interval.

Table 4 shows the factors associated with the changed willingness to receive the
COVID-19 vaccine. Regarding Group A, responses to Q10 and Q14 showed a significant
association. In addition, students’ grade and responses to Q2 (fear of needles/shots), Q4,
and Q12 were significantly associated in Group B.
Table 4. Factors associated with changes in willingness to receive the COVID-19 vaccine in the future.
Odds
Ratio

95%

CI

Wald
Value

p ValueGroup A students who were unvaccinated or dropped out after the first dose of the COVID-19
vaccine but who are willing to receive the vaccine in the future.
Q10
4.50
1.69
11.96
9.11
0.003
Q14
0.31
0.13
0.74
6.95
0.008
Group B students who received a second dose of the COVID-19 vaccine but are not willing toreceive the vaccine in the future.
Grade
0.58
0.43
0.79
11.77
0.001
Q2
3.23
1.44
7.25
8.12
0.004
Q4
2.95
1.23
7.09
5.83
0.016
Q12
0.26
0.08
0.93
4.33
0.038
Multivariate logistic regression analysis using a forward selection method was carried out with changed willingness to receive COVID-19 vaccination in future (Groups A and B) as the dependent variable and age, sex, grade,
past history of allergic reaction (due to food, medication, animal, pollen from plants and trees, house dust mites,
unknown allergen), anaphylaxis, asthma, atopic dermatitis, and attitudes toward COVID-19 vaccination (Q1–Q14)
as independent variables. Abbreviations: COVID-19, Coronavirus disease 2019; CI, Confidence interval.

Regarding willingness to receive a third dose of the COVID-19 vaccine, multiple
logistic regression models showed that students’ grade and responses to Q1, Q9, Q10, Q12,
and Q14 had significant associations with this outcome (Table 5).
Table 5. Factors associated with willingness to receive a third dose of the COVID-19 vaccine among
medical students.

Grade
Q1
Q9
Q10
Q12
Q14

Odds Ratio

95%

1.41
9.85
2.18
0.32
5.83
2.20

1.16
1.08
1.19
0.17
2.53
1.20

-

CI

Wald Valuep Value

1.70
89.83
4.01
0.57
13.43
4.02

11.93
4.11
6.29
14.61
17.18
6.53

0.001
0.043
0.012
<0.001<0.001
0.011

Multivariate logistic regression analysis using a forward selection method was carried out with willingness toreceive a third dose of the COVID-19 vaccine in the future (Group C) as the dependent variable and age, sex, grade,
past history of allergic reaction (due to food, medication, animal, pollen from plants and trees, house dust mites,
unknown allergen), anaphylaxis, asthma, and atopic dermatitis, and attitudes toward COVID-19 vaccination(Q1-Q14) as independent variables. Abbreviations: COVID-19, Coronavirus disease 2019; CI, Confidence interval.

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4. Discussion
This study was conducted to investigate the attitudes of medical students toward
COVID-19 vaccination. In our survey, almost all the respondents (98.4%; 488/496) statedthat they approved of vaccinations in principle (Q1), and most of them (92.8%; 460/496)
considered COVID-19 vaccination to be necessary for them to travel or go out to eat
and drink as in pre-pandemic life (Q12). Among the respondents, 89.1% (442/496) hadreceived the second dose of the vaccine, and 90.7% (450/496) indicated that they would
hypothetically receive the COVID-19 vaccine in the future. Furthermore, 84.5% (419/496)
of all the participants were willing to receive a third dose of the vaccine. Although the
majority of the students (75.6%; 375/496) stated that the vaccines provide a high degree ofprotection against COVID-19 (Q9), 67.3% (334/496) were concerned about the sustainabilityof immunity by the vaccine (Q14). In addition, a nonnegligible number of the students(46.0%; 228/496) stated that the vaccine was developed too rapidly (Q10).
Vaccine hesitancy, which is defined as a delay in acceptance or refusal of vaccinations even though vaccination services are made available, poses a barrier to achieving
widespread vaccination against COVID-19. A previous international meta-analysis of the
proportion of vaccine hesitancy among the general population suggested that 20% of theparticipants intended to refuse COVID-19 vaccination [20]. Regarding medical students,
the rate of vaccine hesitancy reported in previous studies has varied considerably due tothe different definitions of the concept or timing of investigation in each study [9,10,21–23].
The availability of vaccination, or the percentage of vaccinated students might be associated with vaccination hesitancy. Several studies have shown that medical students
express higher intentions to be vaccinated than the general population [24], nonmedical
students [25,26], nursing students [27], and dental students [28]. A recent study conductedin Japan showed that 14 to 16% of the younger (15-39 years old) subgroup expressed
COVID-19 vaccine hesitancy in terms of their hypothetical intention to be vaccinated andthat vaccination intention was associated with several factors, including younger age,female sex, and living alone [29]. In our results, 10.9% (54/496) of the students did notcomplete the second dose of the vaccine. Although hypothetical intention is not a real-life
decision, the rate of vaccine hesitancy in the general population seems to be higher thanthat in medical students who were not vaccinated twice.
Fear of vaccination side effects has been reported to be associated with COVID-19
vaccine hesitancy in medical students as well as the general population [9,26,29,30]. In our
analysis, concern about the side effects of COVID-19 vaccination (Q8) was not associatedwith students’ attitudes toward or completion of vaccination. A possible explanation isthat the majority of our participants received the second dose of the vaccine, and their experience might have decreased their concern about the side effects of COVID-19 vaccination.
However, concern about the rapid development of COVID-19 vaccines (Q10) still had asignificant impact on students’ decision making regarding COVID-19 vaccinations.
Few studies have focused on the effect of parents’ opinions on medical students’decision making regarding COVID-19 vaccinations. We found significant associations
between the effect of parents’ opinion (Q4) and COVID-19 vaccine hesitancy in medical
students. Although recommendation from family or friends was indicated as a reason
for both receiving and not receiving vaccines [29], it is difficult to explain the association
between the effect of parents’ opinion and COVID-19 vaccine hesitancy in medical students.
The association found in Q4 might be related to a cultural issue.
Fear of needles has been associated with COVID-19 vaccine hesitancy [23,31] innonmedical student populations, and our results concerning Q2 might support previous
findings. However, our findings also indicated that even students with a fear of needles
received a second dose of the COVID-19 vaccine. In our results, students in higher grades
had lower vaccine hesitancy. The amount of medical education and need for protection
against COVID-19 in clinical rotations might motivate their willingness [32].
Most of our participants considered COVID-19 vaccination to be necessary to resume
aspects of pre-pandemic life (Q12). Mobility restrictions were enforced by the government

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to abate the spread of COVID-19, resulting in motivation to resume going-out activities after
the end of the pandemic [33]. However, even for vaccinated individuals, basic preventive
measures such as wearing masks are needed to prevent COVID-19 vaccine breakthroughinfections for a while [13].
Several limitations of this study should be acknowledged. First, our study is limitedby the fact that students were asked about behavioral intention to receive COVID-19 vaccination in the future. The responses of participants may not reflect implementation intention
when faced with the decision of actual vaccination. Second, the questionnaires were notstandardized, which could lead to problems in interpreting the results. Furthermore, we
employed a Likert scale, which leaves room for a subjective overevaluation of the selfperceived degree of comprehension and knowledge. Third, several potential confounding
factors, such as depressive symptoms and psychological distress, which may contribute towillingness to receive the COVID-19 vaccine, were not assessed in this study. Fourth, thestudy population consisted of students from one medical university; therefore, it may notreflect the characteristics of all Japanese medical students.
5. Conclusions
Most of the medical students completed the second dose of the COVID-19 vaccine
and were willing to receive a third dose. Confidence in vaccines, relaxation of mobility
restrictions, and concern about the sustainability of immunity motivated willingness toreceive the third dose of the COVID-19 vaccine in medical students. Our findings provide
key information related to medical student vaccination in Japan and education about
COVID-19 vaccination.
Author Contributions: Conceptualization, N.S.; methodology, N.S. and N.Y.-F.; validation, K.S.;
formal analysis, N.S.; writing—original draft preparation, N.S.; writing—review and editing, N.Y.-F.,
A.F. and K.S.; supervision, K.S.; project administration, N.S. All authors have read and agreed to the
published version of the manuscript.
Funding: The authors received no specific funding for this work.
Institutional Review Board Statement: This protocol received approval from the Ethics Committeeof Dokkyo Medical University School of Medicine (Approval number: 2021–016), and it conformed
to the provisions of the Declaration of Helsinki.
Informed Consent Statement: The requirement for written informed consent was waived by the
Ethics Committee since the study involved record review only. However, information on the study
was disclosed on our web page, and students were free to opt out.
Data Availability Statement: The Ethics Committee of Dokkyo Medical University School of
Medicine has set restrictions on data sharing because the data contain potentially identifying or sensitive student information. Please contact the institutional review board of the Ethics Committee ofDokkyo Medical University School of Medicine for data requests. Upon request, the ethics committee
will decide whether to share the data.
Acknowledgments: The authors would like to thank all of the coworkers (Kaoru Nakabayashi,
Yuriko Ami, Kyoko Nemoto, Yumiko Matsumoto, Sanae Abe, Yoko Kuroda, and Shiori Sakata) for
their skillful contributions to the data collection and management.
Conflicts of Interest: The authors declare no conflict of interest.

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