Keyword frequency analysis 

Bibliographic Items Co-occurrence Matrix Builder (BICOMB) software (version 2.0, School of Medical Information, China Medical University, Shenyang, China) was used to extract and analyse publication years, journal distributions, and keyword frequencies [15]. This software was developed by the Medical Information Department of China Medical University. It has good compatibility with Chinese journals and the ability to replace keywords with synonyms. For example, ‘imported case’ and ‘imported patient’ are unified as ‘imported case’.

This study defined the ‘K/A ratio’ as the frequency of a keyword among all the articles within a certain period. The absolute frequencies of keywords in different periods could not be directly compared. The K/A ratio eliminated the impact of the difference in the total number of journal articles in different periods. It was used as an indicator to compare the popularity of the same keyword in different periods. Word clouds were constructed to show the top 100 keywords in each period [18]. Microsoft Excel was used to calculate and display the change trend of the K/A ratio. A heat map was produced based on the rank of K/A ratio.