On November 18, 2009, the National Library of Medicine (NLM) began its annual year-end-processing activities that are projected to end mid-December. During this time, NLM temporarily halts the addition of fully-indexed MEDLINE citations to PubMed and the delivery of the fully indexed MEDLINE files to Ovid and other third parties. Annual year-end activities include changing the Medical Subject Headings (MeSH) and Supplementary Concept Substance Names on existing MEDLINE citations to conform with the 2010 version of MeSH, and other global changes. The release of author name and other corrections into PubMed and Ovid MEDLINE are also halted during year-end-processing.
What does this mean for searchers?
PubMed: During end-of-year activities, "in process" citations will continue to be added to PubMed. As a result, the number of PubMed records labeled [PubMed - in process] will temporarily increase. These citations do not have MeSH terms, but they have undergone issue level and subsequent citation level quality control reviews.
Searches that use MeSH terms only (e.g. MeSH Database-initiated searches) rather than a combination of MESH and All Fields searches (which would be the norm if you search from the PubMed home page) do not retrieve in process or publisher-supplied records. Those MeSH-only searches may not retrieve the latest results during the update. This will also affect users who have saved searches in PubMed and set up email alerts using MeSH terms only.
Ovid MEDLINE: As the NLM updates its MEDLINE files to accommodate subject heading and other changes, Ovid and other third party MEDLINE providers must also perform global updates on their sites. Auto-alerts and currency of the Ovid MEDLINE files (those with date ranges) may be affected, but the Ovid MEDLINE In-Process file segment will continue to be updated. (The Ovid MEDLINE In-Process file segment may be accessed from the list of databases retrieved from the "Change Databases" tab on the main Ovid search screen.)
The points outlined here and more information can be found in the NLM fact sheet: Annual MEDLINE / PubMed Year-End Processing (YEP): Impact on Searching During Fall 2009.