The New England Journal of Medicine seems to publish two kinds of material under the general submission rubric of “Letter” or “Correspondence”—critical comments about articles published previously in the New England Journal of Medicine (including original research, perspectives, and commentaries) and “research letters,” which report original data about topics that are “timely.” An example of the first type of “Correspondence” (critical comment) is here.
Comment about a publication in the NEJM.
Following are selected examples of data published in the “Correspondence” section of the New England Journal of Medicine about COVID-19 over the last 18 months and the number of citations to the Correspondence (AKA Research Letter) through July 29, 2021. All three examples are about COVID-19/SARS-CoV-2 because only this correspondence is reliably available online in full-text form for free.
Lu X, Zhang L, Du H, et al. SARS-CoV-2 Infection in Children. N Engl J Med. 2020;382(17):1663-1665. doi:10.1056/NEJMc2005073
This March 18, 2020 research letter had 804 PMC citations on July 29, 2021.
Solomon IH, Normandin E, Bhattacharyya S, et al. Neuropathological Features of Covid-19. N Engl J Med. 2020;383(10):989-992. doi:10.1056/NEJMc2019373
This June 12, 2020 research letter had 222 PMC citations on July 29, 2021.
Widge AT, Rouphael NG, Jackson LA, et al. Durability of Responses after SARS-CoV-2 mRNA-1273 Vaccination. N Engl J Med. 2021;384(1):80-82. doi:10.1056/NEJMc2032195
This December 3, 2020 research letter had 134 PMC citations on July 29, 2021.
Information about the citation history of these three research letters shows the potential impact of even a short publication in the New England Journal of Medicine.
OTHER PUBLICATION OF SAME MATERIAL FOR THREE RESEARCH LETTERS
This is clearly a small (very small) sample. But, for the first two research letter publications (Lu, Zhang, Du, et al. and Solomon), no subsequent publication reporting the same data could be identified (as of July 29, 2021). For the third research letter (Widge et al.), a second research letter with different co-authors but based on the same 34 (or 33) subjects in an on-going follow-up of antibody responses in people vaccinated with the mRNA-1273 vaccine (Moderna) was published in the New England Journal of Medicine June, 2021.
Doria-Rose N, Suthar MS, Makowski M, et al. Antibody Persistence through 6 Months after the Second Dose of mRNA-1273 Vaccine for Covid-19. N Engl J Med. 2021;384:2259-2261. doi: 10.1056/NEJMc2103916. Epub 2021 Apr 6. PMID: 33822494.
Using Doria-Rose et al. as an example of publishing longer term follow-up of subjects in an existing study in the same journal (here, the New England Journal of Medicine), the practice if "publishing more about the same people" is clearly acceptable to the editors of this journal. Whether publishing the longer-term follow-up in another journal would be considered a breach of publication ethics is unclear.
Also, considering these examples, publishing a follow-up “full” paper for a Research Letter is not (small sample) universal.
INTERNATIONAL COMMITTEE OF MEDICAL JOURNAL EDITORS (ICMJE)
Historically, in medical journals, publication of exactly the same data in another publication would be considered to be duplicate publication and has been generally discouraged/frowned upon. Simultaneous submission of exactly the same manuscript to two different publications without disclosure (see below) is prohibited.
The International Committee of Medical Journal Editors (ICMJE) provides detailed guidance on the topic of “overlapping publication” for variety of situations.
Pertinent to your question regarding submission of a research letter to the New England Journal of Medicine with a plan to submit a “full manuscript” to another journal, the ICMJE states:
“When authors submit a manuscript reporting work that has already been
reported in large part in a published article or is contained in or
closely related to another paper that has been submitted or accepted
for publication elsewhere, the letter of submission should clearly say
so and the authors should provide copies of the related material to
help the editor decide how to handle the submission.”
A sequential publication might be justified best in cases where the “full manuscript” based on the same subjects with the same general findings includes more (and important) methodologic detail, more detailed results, a more comprehensive review of the pertinent literature (5 references is not a lot), and an expanded discussion of the implications of the findings and limitations of the data. If the methodologic detail and the literature synthesis will support future work, the plan for an expanded manuscript and a "new" publication find further justification (to the science community and to journal editors).
Questions of copyright may come in to play, especially in publications that involve Figures and photographs. If a subsequent publication is submitted, pay attention to copyright issues.
BOTTOM LINE (OPINION)
Bottom line (this is an opinion). A nice result that can be conveyed clearly in the Research Letter format gets a lot of mileage when published in the New England Journal of Medicine.
Consider carefully what you (and the world) gain (or lose) by publishing "the same thing" someplace else. Will readers be "confused" by having two publications about "the same thing?" Will readers think the publications are (prohibited) duplicate publications? How much time will the second publication (submission) take from other work?