The data of the number of publication and citations presented in Table 1 show the impact of the explosion of research on Covid-19. The highest number of citations for the 2020 articles reached 8488 on Covid-19, compared to 1446 for cancer. That places that most cited cancer article around position 1250 in the all-time ranking of articles on cancer, as the only 2020 articles that enters the h-core of the cancer articles. With 8488 citations, an article would be ranked at place 30 in the overall cancer ranking. In order to attain the top 1% in the citation distribution for the year 2020, an article on Covid-19 needs 116 citations, while only 11 for cancer. 10% of the articles on Covid-19 have at minimum 11 citations, compared to 3 for cancer.
The h-index for the 2020 sample climbs to 259 for Covid-19 and to 74 for cancer. 35 papers on Covid-19 gathered more than 1000 citations while only 2 articles on cancer realized this achievement. 15 articles on coronavirus before 2020 attained the 1000 citations. In fact, the figures of Table 1 effectively illustrate how fast the citation performance of the 2020 Covid-19 articles has overpassed the older coronavirus articles, and largely overpassed the 2020 articles on cancer.
Journals on Covid-19
The lower part of Table 1 provides the data of the 2020 citation distribution for the most important journals that have published on Covid-19. While these data give the counts for all their articles, nearly all articles with more than 500 citations are on Covid-19; multidisciplinary journals as Nature and Science have other highly-cited articles in 2020 on other topics or fields than on Covid-19.
The evolution of the total citations received each year shows a smooth, gradual growth at an average rate around 20–25% for Science and NEJM, from 300 to 400.000 citations per year for NEJM, and from 600.000 to 700.000 for Science, but more than a doubling for JMV from 9000 to 21.000 citations.
h-indexes and h-core
The next Table 2 presents the h-index for the selected journals and the h-index for the 2020 articles. Then follow, for each journal, the number of publications on Covid-19, complemented with the number of articles within the 10% (requiring for this sample at minimum 11 citations), and the number of articles within the h-core of Covid-19 (259).
The following column shows the number of Covid-19 articles that reach the journal’s h-core, and the place that the most cited Covid-19 article takes in the ranking of that journal obtained already within its incomplete first year of publication. Finally, the Journal Impact Factor is provided with the position of the medical journal in the Journal Citation ReportFootnote 1 (JCR) classification of the Web of Science. Three top medical journals are in the top 11 of the JCR, the multidisciplinary journals on places 14 and 15; two journals are situated around the 50th position, the two specialized journals in virology are placed in the range around the 1000th and the 5000th position.
Evolution of citations in the last 5 years
The data for the last 5 years from 2015 to 2020 (and 2010) are compared in a detailed analysis at the level of the individual journal. Table 3 shows the number of NEJM articles (of all type, including letters) per year, the number of citations for the highest cited article, the number of the maximum citations in the first 2 years. Then follow the number of NEJM articles, the number of NEJM articles in classes of more than respectively 1000 citations, 500, 250, 50, and 10 for the year of its publication, and for the year immediately after publication. It also shows the average number of citations per year for its first 2 years (for the first year, the average number of citations equals the total citations of the first year).
For the period from 2015 to 2019, the maximum citation for the first year reaches 300 for the first year, and 1100 for the first 2 years; only 8 articles reach more than 250 citations, while 27 articles already overpass that limit in 2020, all articles on Covid-19. In the second year, only one article has overpassed 1000 citations and 40 articles got more than 500 citations; 8 and respectively 14 articles have already reached those thresholds during their first year, and with the prudent hypotheses of doubling the number of citations during the second year, one can estimate that the first 2 years distribution may largely overpass all previous records.
A similar analysis can be executed for Science, where only two articles (of 2018) got more than 200 citations in their first year of publication, while already 8 articles have reached 250 citations in 2020, all Covid-19 articles. One article ends higher than 1000 in its first year (published on 13th March, so only 9 months) and 3 higher than 500 (published in March and in May). In previous years only 2 articles achieved more than 1000 citations by their second year and thus more than an average of 500 citations per year (the one 1107 in June 2015 and the other 1426 in June 2017 both on photovoltaic technology). Over the same period of 5 years, only 12 articles obtained more than 500 citations during their second year.
The comparative figures are even more impressive for the more specialized Journal of Virology. Until 2019, no JMV articles got more than 26 citations in their first year of publication, and only one JMV article obtained more than 50 citations after the second year, 59 exactly. For 2020, the most cited article reached 551 citations; 5 articles reached more than 250 citations and 40 papers more than 50 citations.
Evolution of h-indexes over the years
Table 4 exhibits the evolution of the h-index over the years from 2015 to 2020, recalculated from the WoS data for the selected journals, complemented with an older reference for the h-index in 2010.
In the period 2015–2019, the h-index of those journals raised by approximately 3% a year, and with a similar range in 2020, except for JMV where the rise of the h-index is around 6%. The steady growth is due to the global increase of the publications in the Web of Science (Hu, Leydesdorff & Rousseau, 2020) and to the intensified usage of larger lists of references (Varga, 2019), a practice encouraged by the easiness of export tools from databases.
NEJM, Lancet and JAMA, have high h-indexes, over 675; it is therefore extremely difficult to enter the h-core of the journal’s dataset before a few years, and none succeeded this performance during the last 5 years. The exceptional character of Covid-19 research with fast citation reactions has waived this limit: 7 Covid-19 articles entered the NEJM h-core (1076), 10 the Lancet h-core (804) and 7 the JAMA h-core (675). With its much lower h-index of 122, JMV had 12 articles that reached the journal’s h-core in their first year. With many more articles in the range just under the h-core, a higher growth of the h-index might be expected for 2021 and 2022, especially for the specialized journals.
Evolution ha-index over the years
The evolution of the ha-indexes of the selected journals is presented in Table 5. The ha-index has the advantage to be more stable. While the h-index of journals seems to rise linearly, the ha-index ‘s yearly increase is somewhat lower and it gradually and asymptotically approaches to a limit. The ha-index acknowledges an exemplary contribution at an earlier stage; highly-cited papers can join the ha-core much sooner than the h-core, in 2 or 3 years time.
35 NEJM articles join the journal’s ha-core (214) from their first year, 13 Lancet articles (163) and 14 in JAMA (116). 39 JMV articles joined and increased the ha-core from 14 to 56, while 76 other Covid-19 articles have the potential to join this ha-core for the next year if they receive double citations of 2020.
General rules of thumbs can change under exceptional circumstances. With huge increases of citations for the Covid-19 research, the higher stability of the ha-average comes under question, especially for specialized journals with a lower ha-index. The average ha-index can raise substantially, to set a new limit at a higher plateau. For the medical journals, the ha-average increased from approximately 3% to 5.5% for JAMA, to 7% for NEJM, to 12% for The Lancet. The increase is phenomenal for the Journal of Medical Virology where the ha-average had reached a plateau of 14–15 over the last 10 years, it suddenly quadrupled to 56.
As general journals, with multiple highly-cited articles from various fields, Nature and Science have not experienced the same impact from the Covid-19 publication on their h-indexes.
The h- and ha-indexes in cohorts per year
Table 6 gives, for each year cohort, the evolution of the h-index in the year of publication, for NEJM, Science and JMV.
The NEJM immediacy h-indexFootnote 2 (using the publication year and the same year as citation period) lies between 34 and 44 and climbs up to 74 for 2020, approaching the range the h-index normally attains by the second year. The impact on Science results in an increase by 50% to 46. Interesting is the calculation of the h-index for the 2020 Covid-19 articles in Science that reaches 41 compared to 31 for the non-Covid-19 articles in Science in 2020. But as already predicted by the quantum leap of JMV’s h-index in 2020, the immediacy h-index of JMV explodes from around 5 to 55. Remarkable is the fact that medical journals as NEJM largely overpass the immediacy h-index of Science that can benefit from all top articles in all sciences. The phenomenon reveals a tsunami for specialized journals, that suddenly experience much more attention and interest to their themes of research.
The impact on the journal impact factor JIF
It is interesting to further analyze the impact of the phenomenon on the JIF of a few medical journals and to examine its evolution and forecast.
According to the Journal Citation Report, the NEJM has the second highest JIF of all scientific articles, in recent years around 75. The impact of the highly-cited articles will be reflected in the 2021 JIF-factors, to be released early 2022. By that time, the high citations of 2020 will be integrated in the JIF calculation for the first time. In our simulation for NEJM, a raise is expected from 75 to 100 to 150 in 2021 and to 115 to 160 in 2022. Lancet with a JIF of around 60 can expect to increase its JIF to the range of 90 to 130 and to stabilize around 100–120 in 2022. The lower ranked JMV that has grown most with the Covid-19 publications will see its JIF sextupled from 2 to the range 11–16 by 2021 and even more in 2022. Depending from a stabilizing or declining interest in the theme, and possible lower amounts of publications once the vaccines have been implemented and the pandemic under control, one could see the journals ulteriorly return to a lower range of JIF.
In the following Table 7 the evolution is shown of the JIF of the three journals over the last 3 years (as retrieved in the JCR WoS) and a simulation of the future JIF-factors for 2021 and 2022 (to be released by mid 2022 and by mid 2023). The lower 2023 estimate is based on the hypotheses of a decline in new citations after the hype, but can occur later.
The simulation shows how NEJM and Lancet, two of the already best ranked journals in function of their JIF, on the 2nd and 5th place, may possibly double their current JIF which is an exceptional phenomenon in bibliometrics. With its explosion of JIF, the lower ranked JMV will raise in the JCR ranking from place 1353 to the range between 300 and 100. This simulation elucidates the disruptive character of the Covid-19 research for bibliometrics.
Science and Nature that have a larger and wide spread of articles in various research areas with a smoother distribution curve will not be affected that much by a few highly-cited papers on Covid-19. It is possible that a few highly-cited papers on Covid-19 will have a raise on the total number of citations in 2022 or 2023 but they fall within what can be expected by the law of large numbers. With 700–1000 articles per year, Science and Nature’s JIF factors will remain more stable.
The impact on the journal immediacy index
The Immediacy Indexes of the three journals explode thanks to the tsunami of citations within the year 2020. NEJM’s immediacy index jumps from around 20 to 127, Lancet from the range of 15 to 155 and JMV from 0.8 to 15.