The National Library of Medicine is currently involved in MEDLINE year-end processing (YEP) activities. These include changing the Medical Subject Headings (MeSH) main headings and subheadings as well as Supplementary Concept Records that standardize names and associated numbers for chemicals, protocols, diseases and organisms that are not main headings. The MeSH edits include maintaining existing MEDLINE citations to conform with the 2020 version of MeSH, and other global changes.
For details about the impact on searching from December 4 to mid-December, visit: Annual MEDLINE/PubMed Year-End Processing (YEP): Impact on Searching During Fall 2019.
For background information on the general kinds of changes made annually, visit: Annual MEDLINE/PubMed Year-End Processing (YEP): Background Information.