Introduction

In the last decade, publication performances in tropical countries has received much attention. A series studies were presented for Latin America including Costa Rica (^{Monge-Nájera &Ho, 2012}), Panama (^{Monge-Nájera & Ho, 2015}), Nicaragua (^{Monge-Nájera & Ho, 2017}a), Honduras (^{Monge-Nájera & Ho, 2017}b), El Salvador (^{Monge-Nájera & Ho, 2017}c), Guatemala (^{Monge-Nájera & Ho, 2018}), and Ecuador (^{Calahorrano, Monge-Nájera, Wang, & Ho, 2020}). Similar investigations were also done in Africa, including Gambia (^{Bah, Fu, & Ho, 2019}), Cameroon (^{Tchuifon Tchuifon, Fu, & Ho, 2017}) and Ghana (^{Boamah & Ho, 2018}). A compared bibliometric study of Brunei (an Asian country) and Neotropical countries was also recently published (^{Ho et al., 2018}). Other approaches have been used to evaluate country publication performance, for example, six publication indicators such as total, independent, collaboration, first-author, corresponding-author, and single-author publications (^{Ho & Kahn, 2014}); and citation indicators, for instance, total citations from Web of Science Core Collection since publication to the end of the most recent year (^{Wang, Fu, & Ho, 2011}; ^{Chuang, Wang, & Ho, 2011}), citations in the most recent year only (^{Ho, 2012}), and citations per publication (^{Fu, Wang, & Ho, 2012}; ^{Ho, 2018}).

Located in Southeast Asia, Vietnam is the 15th most populous country in the world, with a mean yearly per capita gross domestic product of $ 8 000 and a medium human development index of 0.69; despite widespread poverty, the country’s economy has grown significantly in recent years (^{Fforde, 2019}). The Vietnam State Science Committee was established in 1959 with social, applied and basic science branches. In the early 1970s, scientific research facilities were also established in Nghia Do to cover Mathematics, Physics, and Marine Studies (vast.ac.vn). The 1980s were marked by close work with the Soviet Union, followed in 1993 by an emphasis on using high technological research for socio-economic development and more recently by the formal establishment of the Vietnam Academy of Science and Technology (vast.ac.vn, chinhphu.vn).

A study done a decade ago found that Vietnamese researchers were active in fields of pure and applied mathematics, theoretical and applied physics, public health and infectious diseases (^{Nguyen & Pham, 2011}). A recent study reported that most Vietnam papers in the SCI are attributable to international collaboration, and that Vietnam is building up research capacity (^{Nguyen, Ho-Le, & Le, 2017}). The country also has a growing academic interest in eco-compensation (^{Yu et al., 2020}) and has good citation rates for Mekong River research (^{Sui, Chen, Lu, & Chen, 2015}) and dengue research (^{Dwivedi, 2017}), as well as patents for the printing industry, which show capacity for innovation (^{Kwon, Li, & Sohn, 2019}).

The aim of this study was to analyze publications by Vietnamese researchers, in all fields of science covered by the Science Citation Index Expanded (SCI-EXPANDED) from 1991 to 2018, and to compare Vietnam with other tropical countries.

Materials and methods

The data refer not only to tropical biology, but to all areas of science covered by the Science Citation Index Expanded (last updated October 03, 2019). We searched for “Vietnam” in the country field, 1991 to 2018. The impact factors (IF ₂₀₁₈), used to compare the impact of different document types, and of locally-lead versus foreign-lead projects, were extracted from the 2018 Journal Citation Reports (JCR).

The database uses the term “reprint author”, but here we use “corresponding-author”. Single authors are considered both first- and corresponding- authors in this study. In documents with several “corresponding-authors” we used the last “corresponding-author”. We used three citation indicators. The first was the number of citations from the Web of Science Core Collection in a particular year, for instance, C ₂₀₁₈ means the number of citations in 2018 (for a justification, see ^{Ho, 2012}). The second was total citations from the Web of Science Core Collection since publication to the end of 2018 was recorded as TC ₂₀₁₈. The third was mean citations per publication (CPP ₂₀₁₈ = TC ₂₀₁₈/TP).

Results

Results are presented below according to type of publication, language, impact, subject and collaboration. Note: Figures 4-7, and Tables 3-9, appear as supplementary files in Digital Appendix 1 (figures) and Digital Appendix 2 (tables).

Document type and language of publication: The index records 17 document types and 34 790 publications from Vietnam for the period 1991-2018. The majority were articles (90 % of 34 790 publications), followed distantly by meeting abstracts (5.4 %), proceedings papers (3.8 %), and reviews (2.5 %) (Table 1).

The maximal citations per publication value (CPP ₂₀₁₈) was in reviews with 34, which was 2.6 times the citation rate of common articles (Table 1). The mean number of authors per publication (APP) was 93 in data papers, 16 in articles and 9.2 in reviews (Table 1). Web of Science can classify a document in more than one type, for example, 1 332 proceedings papers were also classified as articles, and thus the sum of percentages can be higher than 100 %. We chose only journal articles (31 216 articles) for further analysis because they represented the majority of document types, as well as whole research ideas and results (^{Ho, Satoh, & Lin, 2010}).

Language of publication is one of the basic concerns in bibliometric studies of big data analysis (^{Wang & Ho, 2011}). Vietnam research was published in a total of 12 languages (Table 8 in Appendix 1), but 99 % were in English (31 017 articles), followed distantly by French (94; 0.30 %), Russian (64; 0.21 %), and German (19; 0.061 %). Other languages were Chinese (8; 0.026 %), Japanese (5; 0.016 %), Czech (2; 0.0064 %), and Dutch (2; 0.0064 %). One article was bilingual (English and French). Articles not in English had fewer citations, with CPP ₂₀₁₈ of 2.7, versus a CPP ₂₀₁₈ of 13 for English.

Publication output and citation impact: The CPP ₂₀₁₈ increased more rapidly in the first yearafter publication. The initial value for citations per publication was 0.63 and reached a peak with CPP ₂₀₁₈ of 3.0 in the 2nd year (Fig. 1, Table 9 in Appendix 1).

Fig. 1 Citation life span for Vietnam articles. 

The annual publication output and mean CPP ₂₀₁₈ for the years 1991-2018 appear in Fig. 2; they have increased steadily from 119 articles in 1991 to 4 992 articles in 2018; while the highest CPP ₂₀₁₈ was found in 2001 and 2004 with CPP ₂₀₁₈ of 31 for isolated articles produced by large international research projects.

Fig. 2 Number of articles and citations per publication by year. 

Web of Science categories and journals: To assess development among research fields andtheir interactions, we used a relationship between the number of articles in each category and publication year (^{Ho, Satoh, & Lin, 2010}) (Fig. 4 in Appendix 2). The Vietnamese articles were published in 4 783 journals among all 178 SCI-EXPANDED categories. The top ten productive Web of Science categories are listed in Table 2, and Fig. 5 in Appendix 2.

A total of 3 129 articles (10 % of articles) were published in materials science-related categories including multidisciplinary materials science (8.2 %), composites materials science (0.75 %), coatings and films materials science (0.46 %), ceramics materials science (0.31 %), biomaterials materials science (0.29 %), characterization and testing materials science (0.18 %), paper and wood materials science (0.10 %), and textiles materials science (0.048 %). Similarly, a total of 2 625 articles (8.4 % of articles) were published in mathematics-related categories including mathematics (7.5 %), applied mathematics (7.5 %), and interdisciplinary applications mathematics (10 %).

The top 10 most productive journals are listed in Table 3 in Appendix 1 (Fig. 6 in Appendix 2). PLoS One (IF ₂₀₁₈= 2.776) in the category of multidisciplinary sciences, published the mostarticles with 393 (1.3 % of articles). Articles published in Physical Review B and Journal ofHigh Energy Physics had higher CPP ₂₀₁₈of 18 and 17 respectively. In addition, articlespublished in Nature (IF ₂₀₁₈ = 43.070) had a CPP ₂₀₁₈ of 1 112. These tend to be journals that publish well financed research from large international projects, often in the fields of physics and medicine articles (Long, Huang, &^{Ho, 2014}) with TC ₂₀₁₈ of 1 000 or more ^{Koboldt et al. (2012}) with TC ₂₀₁₈ of 4 577; ^{Bhatt et al. (2013}) with TC ₂₀₁₈ of 2 742; ^{Altshuler et al. (2015}) with TC ₂₀₁₈ of 2 385; ^{Hammerman et al. (2012}) with TC ₂₀₁₈ of 1 725; and ^{Weinstein et al. (2014}) with TC ₂₀₁₈ of 1 121. The large size of these projects is reflected in the number of authors, which range from 18 to 774. The journal with the highest IF ₂₀₁₈ (5.833) was the Journal of High Energy Physics with 155 articles, followed by Physical Review B (IF ₂₀₁₈ = 3.736) with 148 articles, and PLoS One (IF ₂₀₁₈ = 2.776) with 393 articles. Journals with the lower impact factors but still high in the rank included Zootaxa with an IF ₂₀₁₈ of 0.990, and the Journal ofthe Korean Physical Society with an IF ₂₀₁₈of 0.63.

Collaborative countries and institutes: There were 24 214 (78 % of 31 216) internationallycollaborative articles and 7 002 (22 % of 31 216) Vietnam independent articles; of these 3 104 (9.9 % of 31 216 articles) nationally collaborative articles and 3 898 (12 %) institutional independent articles. Overall, the internationally collaborative articles had a higher CPP ₂₀₁₈ of 15, while Vietnam independent articles had a CPP ₂₀₁₈ of 5.6 (Fig. 3). Articles published with Vietnamese researchers as first- or corresponding-authors had lower CPP ₂₀₁₈ of 7.5 and 7.4, respectively (Fig. 3). Neither the first nor the corresponding-authors of the most cited articles are Vietnamese, with a mean of 18 citations per publication (Fig. 3).

Fig. 3 Characteristics of publication type and their citations per publication. TP: total articles, NFR: both first and corresponding-authors are not from Vietnam, NR: corresponding-author is not from Vietnam, NF: first-author is not from Vietnam, IC: internationally collaborative articles, NC: nationally collaborative articles, II: institutional independent articles, CI: Vietnam independent articles, FP: first-author is from Vietnam, RP: corresponding-author is from Vietnam, FR: both first and corresponding-authors are from Vietnam. 

The top ten most collaborative countries are listed in Table 4 in Appendix 1 (Fig. 7 in Appendix 2). Vietnamese researchers collaborated the most with the USA including 3 949 internationally collaborative articles (13 % of articles). Nevertheless, Japan had the most first- and corresponding-author articles (7.1 and 7.6 %, respectively). Articles with Italy, the UK, and the Netherlands had higher citations per publication values (lower with South Korea and Japan).

A total of 3 898 (12 %) articles were single institution articles (IP) and 88 % were inter-institutionally collaborative articles (CP). Table 5 in Appendix 1 presents the 10 most productive institutions in Vietnam. The Vietnam Academy of Science and Technology ranked top in the all six publication indicators with TP of 5 673 articles (18 % of articles), IP of 835 articles (21 % of 3 898 institute independent articles), a CP of 4 838 articles (18 % of 27 318 inter-institutionally collaborative articles), a FP of 2 496 articles (8.0 % of 31 216 first-author articles), a RP of 2 343 articles (7.6 % of 30 911 corresponding-author articles), and an SP of 395 articles (20 % of 1 951 single-author articles). The Oxford University Clinical Research Unit published 782 articles, therein three single-institute articles and two single-author articles had a CPP ₂₀₁₈ of 58, the highest value.

Table 6 in Appendix 1 presents the top 10 oversea institutions that collaborated with Vietnam in research publications with three publication indicators TP, FP, and RP, and citation indicator CPP ₂₀₁₈.Vietnam collaborated the most articles with University of Oxford in the UK, at 1 151 articles. The Russian Academy of Sciences had 692 collaborative articles with Vietnam including 280 first-authors articles and 286 corresponding-author articles. Collaborative articles with Heidelberg University in Germany had the highest CPP ₂₀₁₈ (63), followed by University of Oxford in the UK with a CPP ₂₀₁₈ of 52 and the University of Edinburgh, also in the UK, with a CPP ₂₀₁₈ of 50.

Citation life cycles of the most frequently cited articles: A total of 446 articles had a TC ₂₀₁₈ of 100, 52 % with a first-author from the USA (17 %), the UK (12 %), France (6.3 %), Japan (6.1 %), Switzerland (5.8 %), and Netherlands (4.5 %), while 50 % were published by a corresponding-author from the USA (19 %), the UK (13 %), France (7.2 %), Switzerland (5.8 %), and Japan (5.6 %). Table 7 in Appendix 1 shows the ten classic articles with a TC ₂₀₁₈ of 1 000 or more (^{Long et al., 2014}). Nine had first- and corresponding-authors from the USA, in the tenth, they were from the UK. Eight articles were published by a big group with 357 to 774 authors. Furthermore, the Vietnam independent article with the highest TC ₂₀₁₈ (238 total citations) was “Expanding applications of metal-organic frameworks: Zeolite imidazolate framework ZIF-8 as an efficient heterogeneous catalyst for the Knoevenagel reaction” (^{Tran, Le & Phan, 2011}) by U.P.N. Tran, K.K.A. Le, and N.T.S. Phan from the Department of Chemical Engineering at the HCMC University of Technology.

Discussion

Vietnam published 17 document types, above other tropical countries, for example, 11 in Brunei (^{Ho et al., 2018}), 12 in Gambia (^{Bah et al., 2019}), 13 in Honduras (^{Monge-Nájera & Ho, 2017}b), and 15 in Panama (^{Monge-Nájera & Ho, 2015}).

Vietnam was colonized by France and for many years French was a second language in the country, but its publications in this database are 99 % in English, closer to countries like Gambia (^{Bah et al., 2019}), Ghana (^{Boamah & Ho, 2018}), and Brunei (^{Ho et al., 2018}) which were colonized by the British. A probable reason is that journals covered by this particular index publish mostly in English. Other trends may occur in journals that publish in French or Vietnamese, but these are not covered by the index, even if they are important for the local development of research in Vietnam. In fact, the Science Citation Index Expanded is far from giving a representative view of science in Vietnam, because 95 % of all articles are published in Vietnamese and not covered by the index (^{Hien, 2010}).

The increase in the number of publications, in all fields of science, is a general trend of tropical countries in the last decade, the same trend has been found in Asia (^{Ho et al., 2018}), Africa (^{Boamah & Ho, 2018}; ^{Bah et al., 2019}), and Latin America (^{Monge-Nájera & Ho, 2018}; ^{Calahorrano et al., 2020}).

Vietnamese research published in collaboration with other countries was more cited in this database (CPP ₂₀₁₈) than local articles. Similarly, articles published by first-authors or corresponding-authors from Vietnam also had lower CPP ₂₀₁₈ values. For the 446 highly cited articles (TC ₂₀₁₈ 100), 53 % had a first-author from the USA, the UK, France, Japan, Switzerland, or the Netherlands, while 50% were published by a corresponding-author from the USA, the UK, France, Switzerland, or Japan. International collaboration with developed countries produced higher citation rates. The USA contributed the most citations to Vietnam publications. The USA, Japan, South Korea, and France were the most frequent research partners. Collaborative articles with Italy, the UK, and Netherlands had higher citations. These patterns can be also found in other tropical countries (^{Monge-Nájera & Ho, 2017}a; 2017b; 2017c; ^{Tchuifon Tchuifon et al., 2017}; Ho et al., 2018; ^{Boamah & Ho, 2018}; ^{Bah et al., 2019}; ^{Calahorrano et al., 2020}).

Vietnam published more articles than other tropical countries. ^{However, its independent articles had lower citation rates (CPP ₂₀₁₈} 5.6) than tropical countries in Latin America such as Guatemala with a CPP ₂₀₁₈ of 13 (^{Monge-Nájera & Ho, 2018}), Ecuador (8.5) (^{Calahorrano et al., 2020}), Honduras (7.5) (^{Monge-Nájera & Ho, 2017}b), Nicaragua (6.4) (^{Monge-Nájera & Ho, 2017}a), El Salvador (5.9) (^{Monge-Nájera & Ho, 2017}c), and Ghana (6.4) in Africa (^{Boamah & Ho, 2018}) and Brunei (7.9) in Asia (^{Ho et al., 2018}).

Vietnam publications were dominated by materials science, which is different from other tropical countries, for example, ecology in Brunei (^{Ho et al., 2018}), Ecuador (^{Calahorrano et al., 2020}), and Panama (^{Monge-Nájera & Ho, 2015}); public, environmental and occupational health in Cameroon (^{Tchuifon et al., 2017}), El Salvador, Gambia (^{Bah, Fu, & Ho, 2019}), Ghana (^{Boamah & Ho, 2018}), Guatemala (^{Monge-Nájera & Ho, 2018}), Honduras (^{Monge-Nájera & Ho, 2017}b), and Nicaragua (^{Monge-Nájera & Ho, 2017}a).

Tropical medicine, plant sciences, immunology, infectious diseases, and nutrition and dietetics also dominated research in tropical countries (^{Tchuifon et al., 2017}; ^{Bah et al., 2019}; ^{Boamah & Ho, 2018}; ^{Monge-Nájera & Ho, 2018}; ^{Monge-Nájera & Ho, 2017}b). However, Costa Rica published the most articles in biology (^{Monge-Nájera & Ho, 2012}).

The reason for the importance of materials science research in Vietnam is that the subject was included as a priority by the government (^{Zink, 2009}).

In conclusion, the tropical countries of Latin America, Africa, and Asia studied have some similarities in the importance of applied research subjects, and their dependence on foreign initiatives, but there are also crucial differences. Although science in Vietnam is developing and shows a positive trend in number of papers and citations, most of the impact found in this study results from minor participation of Vietnamese authors in large international projects. This important conclusion that we obtain from our data sadly matches the findings and call made over a decade ago by Hien, who stated that Vietnam depended too much on foreign authors, and called for more multidisciplinary work and internationally-recognized standards (^{Hien, 2010}). For a more robust development of local science, the country should significantly enhance collaboration among Vietnamese institutes in order to lessen its dependence on overseas projects, even if at the beginning this causes a fall in the impact of their work as measured by this particular database.

Ethical statement: authors declare that they all agree with this publication and made significant contributions; that there is no conflict of interest of any kind; and that we followed all pertinent ethical and legal procedures and requirements. All financial sources are fully and clearly stated in the acknowledgements section. A signed document has been filed in the journal archives.