Objective Living systematic reviews (LSRs) are systematic reviews that are constantly updated to incorporate new evidence once it becomes available. 1 The aim of this study was to assess the frequency of updates to LSRs and the influence of COVID-19 on updates.

Design A cohort study of all LSRs indexed in PubMed, from inception to 2020, was performed, which were followed up until 2021. A PubMed search was conducted on September 2021 using the term living systematic review to retrieve articles published until December 31, 2020. Only the first version of the LSR was considered; protocols, conceptual papers, and other types of research were excluded. In January 2022, updates of included LSRs until December 31, 2021, were searched; the 2020 Journal Impact Factor (JIF) was collected from Journal Citation Reports; the open-access status of the articles (if publicly available in full text or not) was checked; and the LSR subject (COVID-19 or not) was assessed. The number of months from publication of the original LSR to its first update stratified by COVID-19 subject was calculated and a Kaplan-Meier survival analysis was performed. Regression analysis using Cox proportional hazards models was conducted to calculate the hazard ratios (HRs) with 95% CIs adjusted by JIF and open-access status to minimize potential confounding.

Results Of 104 reports retrieved, 52 were original LSRs and were included in the study. Most LSRs were published in 2020 (33 of 52), and 19 were published between 2004 and 2019. Most articles were open access (50 of 52) and half had COVID-19 as the subject (28 of 52). As of December 31, 2021, a total of 60% (31 of 52) of the LSRs had been updated, taking a mean (SD) of 14 (3) months for the first update; 21 LSRs were not updated after a mean (SD) of 25 (4) months of publication. LSRs about COVID-19 were updated faster (n = 28; mean SD, 9 1 months) than LSRs about other subjects (n = 24; mean SD, 30 4 months; Figure 7). COVID-19 subject (HR, 3.64 95% CI, 1.38-9.63; P = .009) and higher JIF (HR, 1.10 95% CI, 1.05-1.17; P < .001) were associated with a higher probability of updating LSRs over time, while open-access status had no association with LSR updates (HR, 1.27 95% CI, 0.16-10.25; P = .82).

Conclusions This study identified a limited number of LSRs (52), and 60% of them were updated in about 1 year. Nearly 2 dozen were never updated despite having been published for 2 years, on average; future research may assess the reasons involved. The rapidly evolving COVID-19 pandemic and available research potentially favored more updated LSRs. LSRs published in high-impact journals were also more likely to be updated.

Reference 1. Elliott JH, Synnot A, Turner T, et al. Living systematic review: 1. introduction: the why, what, when, and how. J Clin Epidemiol. 2017;91:23-30. doi:10.1016/j.jclinepi.2017.08.010

Conflict of Interest Disclosures None reported.