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Table of Contents: 2020 NOVEMBER–DECEMBER No. 437

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MEDLINE Data Changes—2021

Tybaert S. MEDLINE Data Changes—2021. NLM Tech Bull. 2020 Nov-Dec;(437):e3a.

2020 December 04 [posted]

This article collects the notable data changes made to MEDLINE during the National Library of Medicine (NLM) annual maintenance known as Year-End Processing (YEP) for 2021:

MeSH Vocabulary Updated for 2021

The MeSH Browser currently points to the 2020 MeSH vocabulary with a link to the 2021 MeSH vocabulary. On December 3rd, the default will switch to 2021 and the alternate link will be routed to 2020 MeSH. Searchers should consult the Browser to find MeSH headings of interest and their relationships to other headings. The Browser contains MeSH heading records that may include Scope Notes, Annotations, Entry Terms, History Notes, Allowable Qualifiers (Subheadings), Previous Indexing, and other information. It also includes Subheading records and Supplementary Concept Records (SCRs) for substances, diseases, and organisms that are not MeSH headings.

You can download 2021 MeSH from links on the NLM Data Distribution page for MeSH Data.

For highlights about 2021 MeSH, see What's New for 2021 MeSH (forthcoming).

The PubMed MeSH database and translation tables will be updated to reflect 2021 MeSH by the middle of December when YEP activities are complete and the newly maintained MEDLINE data are available in PubMed.

Updated MeSH in MEDLINE Citations

MEDLINE citations with updated MeSH are anticipated to be in PubMed by mid-December 2020. See "Modifying a Saved Search Strategy" for details on revising My NCBI saved searches.

Changes to MeSH Headings

For 2021, fourteen MeSH headings were either changed or deleted and replaced with more up-to-date terminology. During YEP, NLM updates these MeSH headings on MEDLINE citations.

Replaced Term Replaced-By Term
Alaska Natives Alaskan Natives
Aldehyde Dehydrogenase 1 Aldehyde Dehydrogenase 1 Family
Bonding, Human-Pet Human-Animal Bond
Clostridium difficile Clostridioides difficile
Debaromyces Debaryomyces
Infantile Apparent Life-Threatening Event Brief, Resolved, Unexplained Event
Mentally Disabled Persons Persons with Mental Disabilities
Pectobacterium chrysanthemi Dickeya chrysanthemi
Pimenta Genus Pimenta
Pyruvate Dehydrogenase (Acetyl-Transferring) Kinase Pyruvate Dehydrogenase Acetyl-Transferring Kinase
Respiratory Distress Syndrome, Adult Respiratory Distress Syndrome
RNA Replicase RNA-Dependent RNA Polymerase
Site-Specific DNA-Methyltransferase (Cytosine-N(4)-Specific) Site-Specific DNA-Methyltransferase (Cytosine-N4-Specific)
Social Distance Psychological Distance

New MeSH Headings and Concepts

277 new MeSH headings were added to MeSH in 2021.

A complete list of the new 2021 MeSH headings is available in PDF format, see New Headings with Scope Notes, Annotations and Tree Locations.

Typically, NLM does not retrospectively re-index MEDLINE citations with new MeSH heading concepts. Therefore, searching PubMed for a new MeSH term tagged with [mh] or [majr] effectively limits retrieval to citations indexed after the term was introduced. PubMed Automatic Term Mapping (ATM) expands an untagged subject search to include both MeSH Terms and All Fields index terms and may retrieve relevant citations indexed before the introduction of a new MeSH term. Searchers may consult the MeSH Browser or the MeSH database to see the previous indexing terms most likely used for a particular concept before the new MeSH heading was introduced.

MeSH Tree Changes

For a list of new MeSH headings by tree hierarchy, see 2021 New MeSH Headings by Subcategory available in PDF format.

For a complete list of all MeSH headings by tree hierarchy, see 2021 MeSH Tree Locations available in PDF format.

Changes of particular interest to searchers:

The new MeSH heading Working Dogs will also be indexed with the check tag Dog.

Three of the entry terms for the qualifier (subheading) “metabolism” were deleted because they were considered multi-meaning: incorporation, mobilization, and turnover.

COVID-19 Terminology:

Starting in January 2020, new terms related to the 2019 novel coronavirus (SARS-CoV-2) and the resulting disease (COVID-19) were added to the MeSH vocabulary as SCRs.

Many of the SCRs related to SARS-Cov-2, COVID-19, and other older relevant SCRs were promoted to MeSH headings for 2021.

New MeSH headings and SCRs were created for 2021 as well. For a list of terms that might be of interest to searchers, see COVID-19 and SARS-CoV-2 MeSH Terms—2021.

Additional SCRs will be created as needed throughout 2021. Please continue to check the MeSH Browser for new SCRs.

Special MeSH Projects

Several special projects resulted in the following new 2021 MeSH headings:

New and Updated Behavioral and Social Science Headings:

As part of an ongoing project to incorporate terminology from the NIH Office of Behavioral and Social Sciences Research into the MeSH vocabulary, new entry terms and MeSH headings were created.

For example, new entry terms:

For example, new Headings:

New Cardiovascular Headings:

New Food and Beverage Headings:

New Infection Headings:

New and Updated Organism Headings:


  • As a part of an ongoing project that began in 2018, bacteria terms were updated. New genera of bacteria were added and some bacteria were reclassified.


  • New fungus terms were updated as part of an ongoing project to update this vocabulary.
  • New entry terms were added to existing fungal infection headings and SCRs. For example,
  • Note that MeSH follows the NCBI Taxonomy convention of grouping various nomenclatural and taxonomic synonyms together in one MeSH record. This includes grouping together homotypic and heterotypic synonyms and anamorphs and teleomorphs.


New Persons Headings:

Do Not Confuse:

Pets, Service Animals, Therapy Animals, and Working Dogs with one another.

  • Pets are kept by humans for companionship and enjoyment.
  • Service Animals are trained to do work or perform tasks for an individual with a disability.
  • Therapy Animals are generally registered, trained, and screened, but are not covered by laws as are service animals.
  • Working Dogs are trained to perform practical tasks, as opposed pets or companion dogs.

Physical Distancing (has the entry term Social Distancing) with Psychological Distance, which replaced the MeSH heading Social Distance in 2021.

Rappaport, which is a genus of gram-negative bacteria, with the author last name Rappaport.

Recurrence with Reinfection, which is infection by the same infectious agent following a recovery. Reinfection may be by a different strain or reactivation of the same strain.

Other Changes

In addition to changes and deletions of MeSH terms on MEDLINE citations, YEP includes other adjustments to reflect 2021 MeSH vocabulary and enhance search retrieval. These follow-on adjustments are largely meant to add additional MeSH headings or Supplementary Concept Record Names to citations in order to help searchers refine retrieval. In some cases, the changes clarify areas where a single concept existed before but is now represented by two or more specific concepts.

Entry Combination Revisions

This year during YEP, NLM will again retrospectively replace certain MeSH heading/subheading combinations, known as Entry Combinations, with the new pre-coordinated MeSH heading. If you get no retrieval for a MeSH heading/subheading combination, check the heading in the 2020 MeSH Browser to see if the Entry Combination information indicates a different term.

There were two new Entry Combinations for 2021.

Previous MeSH Heading/Subheading
(Entry Combination)
Replaced-by Heading for 2021
Immune Checkpoint Proteins/antagonists & inhibitors Immune Checkpoint Inhibitors
Viral Proteases/antagonists & inhibitors Viral Protease Inhibitors

Other News:

Data Distribution Notes

There is no change to the PubMed DTD for 2021. The DTD dated 010119 is still valid.

Because there are no changes to the PubMed DTD, NLM will not create new sample data for 2020.

More information and announcements can be found at Download MEDLINE/PubMed Data.

NCBI Taxonomy Identifiers

Beginning in 2020, NCBI Taxonomy Identifiers (txid) have been added to MeSH records as Registry Numbers (RNs) for SCR-Class 4 Organisms. This project has been expanded to add these identifiers to MeSH headings in Category B1-B4 (Organisms) for the 2021 MeSH vocabulary.

At this time, the NCBI Taxonomy Identifiers will not be carried as metadata on MEDLINE citations.

The NCBI Taxonomy Identifiers are available to view in the MeSH Browser where they are displayed in the Registry Number field and denoted by the prefix of txid. NCBI Taxonomy Identifiers are unique numerical identifiers between one and seven digits that are assigned by NCBI to organisms. For example, the NCBI Taxonomy Identifier for SARS-Cov-2 is displayed as Registry Number txid2697049.

For more information on the NCBI Taxonomy Identifiers, see NCBI Taxonomy Identifiers Added to MeSH.

By Sara Tybaert
MEDLARS Management Section

NLM Technical Bulletin National Library of Medicine National Institutes of Health