Introduction
There are seven known coronaviruses that cause diseases in humans. HCoV-229E, HCoV-NL63, HCoV-OC43, and HCoV-HKU1 lead to mild upper respiratory infections. Severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are three highly pathogenic coronaviruses that can infect humans and are responsible for the pandemic diseases SARS, MERS and COVID-19 [1].
SARS circulated in 32 countries and territories from November 2002 to August 2003; the cumulative number of infections was 8,422, and the mortality rate reached 10.87%. MERS spread to 27 countries from April 2012 to December 2019; there were 2,496 cases of infections, and the mortality rate was as high as 34.77% [2]. COVID-19 has caused a worldwide pandemic that started in December 2019 and spread with surprising speed. The pattern of the COVID-19 pandemic has changed from the first stage in January and February 2020, in which there was a single epidemic centre (China), to the second stage in March 2020, in which there were multiple epidemic centres (Italy, Iran, and South Korea). By the end of March, the global number of cases was rapidly increasing, with an estimated 50,000 cases confirmed per day [3]. By 26 July 2020, COVID-19 had been reported in 215 countries and territories, and there were 15,785,641 reported cases, with a mortality rate of 4.05% [4].
SARS-CoV, MERS-CoV and SARS-CoV-2 share certain biological, clinical and epidemiological features. However, according to gene sequencing, the main differences among them are in the open reading frame 1a (ORF1a) gene and the spike coding protein gene [5]. The spike protein of SARS-CoV-2 has the largest sequence divergence, with 380 amino acid sequence substitutions [6], contributing to its affinity. With regard to the clinical characteristics, COVID-19 varied from mild cases to severe cases, and most cases are mild. In contrast, SARS and MERS tend to have an urgent onset and rapidly progress to severe illness [7]. COVID-19 seemingly has a more insidious onset. Due to the abovementioned characteristics, COVID-19, SARS and MERS have different morbidity and mortality rates. COVID-19 has the highest prevalence but lowest fatality rate, while MERS had the highest fatality rate. SARS was characterized by superspreading events, while MERS seemed to be less transmissible, and COVID-19 is unique for its indiscriminate transmission among the general public [5]. Owing to its unique biological, clinical and epidemiological characteristics, SARS-CoV-2 has led to a more prolonged epidemic.
Bibliometrics is an effective tool for evaluating research trends in different scientific fields. Coronaviruses did not attract worldwide attention until the 2003 SARS epidemic, followed by the 2012 MERS outbreak and the recent COVID-19 pandemic. To investigate in more detail the rapid expansion of research stimulated by coronavirus epidemics, we evaluated 25,835 publications to identify the research trends and core contents in the field of coronavirus research between 1991 and 2020. Our article focused on analysing the trends in research on the characteristics of coronavirus infections, which may provide further guidance for present and future studies.