It has been almost a decade since the reactor antineutrino anomaly entered the stage, where the number of experimentally detected antineutrinos emerging from a nuclear power reactor interior was signi?cantly less than theoretically predicted from nuclear ? decay. This has, in turn, motivated the search for an eV-scale sterile neutrino in several very short baseline experiments, none of which have so far confi?rmed its existence. From the theory point of view, initial analyses introduced a signi?cant number of approximations, in particular for the treatment of so-called forbidden transitions. We report on the first large-scale calculation of the influence of ?first-forbidden ?transitions using state-of-the-art nuclear shell model calculations for a select number of highly-contributing ? branches. We use these results to propose a probability distribution for ?first-forbidden spectral shapes and employ Monte Carlo techniques to translate this into a detailed construction of theoretical uncertainties for the remaining forbidden transitions. We observed signi?cant changes in both the integrated
ux and spectral shape of the cumulative antineutrino spectra spectra for all ?ssion actinides [1, 2], and discuss both a mitigation of the so-called reactor shoulder and changes in the reactor antineutrino anomaly. Finally, we will comment how an improved treatment of allowed ? transitions [2, 3] can further signi?cantly change both ux and spectral shape.
 L. Hayen, J. Kostensalo, N. Severijns, and J. Suhonen, Physical Review C 99, 031301(R) (2019).
 L. Hayen, J. Kostensalo, N. Severijns, and J. Suhonen, Physical Review C 100, 054323 (2019), arXiv:1805.12259.
 L. Hayen, N. Severijns, K. Bodek, D. Rozpedzik, and X. Mougeot, Reviews of Modern Physics 90, 015008 (2018),