Nowadays, human oocytes/embryos are cryopreserved via slow freezing (SF) or vitrification (VIT). Due to the high survival rate guaranteed by VIT, this procedure is increasingly applied worldwide. Nevertheless, to date, perhaps millions of SF oocytes/embryos have already been stored in IVF cryobanks. The aim of this study was to evaluate a rapid warming protocol for SF human oocytes based on the standard warming procedure for VIT in order to optimize the SF survival rate and reduce costs by using the same solutions for both SF and VIT warming. Between December 2012 and January 2013, 216 SF oocytes donated for research were randomized for the rapid thawing (RT) conventionally used for SF oocytes, or rapid warming (RW) as for VIT protocols.

We observed that RW significantly increased the survival rate of SF oocytes (90.2% vs 74.6 % obtained with conventional thawing, P=0.005); furthermore, SF oocytes warmed via RW had better developmental competence than their sibling counterparts thawed with conventional rapid thawing. The application of this warming protocol allows us to increase the efficiency of oocyte SF procedure, enabling survival rates comparable with that reported in previous VIT studies. This finding re-defines the scenario of SF and VIT, confirming the pivotal role of warming and indicating that the results obtained until now with slow frozen oocytes/embryos can be improved; this could be the major breakthrough in human oocyte cryopreservation since the introduction of VIT. Furthermore, this finding allows IVF centres to reduce costs by using the same "universal warming protocol" for both SF and VIT reproductive cells.

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