In order to produce proteins, our cells need to make an RNA copy of certain pieces of DNA, a process called transcription. Reverse-transcription is the opposite activity, by which RNA is used as a template to generate its DNA equivalent [5]. Reverse transcription happens in some viruses whose genome is made by RNA, instead of DNA like that of humans. Evidence of reverse-transcription happening in human cells has been reported as well [6].

The mentioned study by Aldén and colleagues is entitled “Intracellular Reverse Transcription of Pfizer BioNTech COVID-19 mRNA Vaccine BNT162b2 In Vitro in Human Liver Cell Line”. When reading a (scientific) publication, it is important to define its context and especially the model system that has been used for the study. Here, liver cells derived from a cancerous cell line have been exposed to different doses of mRNA contained in the Pfizer-BioNTech COVID-19 vaccine, showing that they are able to reverse-transcribe it into its equivalent DNA sequence. Cancerous cell lines are often used to perform in vitro studies, as their alterations confer favourable characteristics for research purposes: they grow easily, are very resistant and therefore easy to maintain. However, such alterations do not allow a direct comparison with healthy systems. Therefore, translating the findings of this study into the general notion that mRNA from COVID-19 vaccines is converted into DNA in human liver cells – as stated in the title of the publication – is inaccurate and falls into the false equivalence fallacy. Furthermore, as discussed in our previous article about the scientific method, scientific observations need to be extensively tested by different research groups and using different settings. Only if the same outcome is achieved can those findings be translated into a solid theory or conclusion. 

The mentioned publication [1] appeared in a scientific journal belonging to a controversial publisher named MDPI, criticised in the past for different reasons, including the quality of its peer-reviewing process [7]. Other experts in the field reported that the study cannot properly support some of its own claims [8], with one important connection completely missing: the study reports no evidence of the fact that the reverse-transcribed DNA can be transported from the cytoplasm – where the mRNA is firstly delivered and the reverse-transcription happens – to the nucleus of the cell – where instead the genetic material is stored. Without this step, there is no chance that any newly formed DNA can be incorporated into the host cell genome, being the two inside different compartments of the cell. A comment clarifying the significance of the mentioned study even appeared in the same journal less than two months after its publication [9]. This shows that the topic is still debated within the scientific community itself and cannot yet be translated into solid conclusions. Moreover, results from a previously published article (claiming that reverse-transcribed SARS-CoV-2 RNA can be incorporated into the genome of a cell infected by the virus [10]) which was used as a reference for the currently analysed one, were widely debated by the scientific community [11]. Furthermore, those findings could not be reproduced by other studies published by a different group, in a different journal [12].

Finally, before drawing strong conclusions, the results presented by this study will need to be confirmed with experiments in vivo, as the authors themselves state. Living systems are in fact composed of different organs, tissues and cell types communicating with each other and obviously display much more complex behaviours than a cell line cultured in a laboratory (i.e. studied in vitro).