The ‘Deltacron’ sequences were generated from virus samples obtained by Kostrikis and his team in December as part of an effort to track the spread of SARS-CoV-2 variants in Cyprus. While examining some of their sequences, the researchers noticed an Omicron-like genetic signature in the gene for the spike protein, which helps the virus to enter cells.

In an e-mail to Nature, Kostrikis explains that his initial hypothesis was that some Delta virus particles had independently evolved mutations in the spike gene similar to those common in Omicron. But after the wide news coverage, other scientists working on genetic sequencing and COVID-19 pointed out another possibility: a lab error.

Sequencing any genome depends on primers — short bits of manufactured DNA that serve as the starting point for sequencing by binding to the target sequence.

Delta, however, has a mutation in the spike gene that reduces some primers’ ability to bind to it, making it harder to sequence this region of the genome. Omicron doesn’t share this mutation, so if any Omicron particles were mixed into the sample owing to contamination, it might make the sequenced spike gene seem to be similar to that in Omicron, says Jeremy Kamil, a virologist at Louisiana State University Health Shreveport.