Question: An article just mentioned the existence of a hybrid delta plus variant. What is it? And is there specific relevance in calling it a hybrid instead of just giving it the next letter of the Greek alphabet?

Answer: As a reminder, SARS-CoV-2 is an RNA virus (only one strand of genetic instructions, Figure) and every time the virus enters a cell, it replicates.  Because it is an RNA virus, it doesn’t have a back-up copy of genetic instructions.  SARS-CoV-2 is therefore more prone to flaws in the replication, which cause mutations.  A group of coronaviruses that share the same set of mutations is called a variant.  You can visualize each variant like a branch on a family tree.  If accumulated mutations are such that they change how the SARS-CoV-2 virus functions (like making it more transmissible), WHO gives the new variant a name after letters in the Greek alphabet. The Delta variant (B.1.617.2) was first detected in September 2020 in India.  It has more than a dozen mutations, but two mutations in particular make the Delta variant more transmissible (E484Q and L452R). Given its increased transmissibility, the Delta variant is quickly spreading around the world and will soon become the dominant variant in the US.  Meanwhile the recently detected “Delta Plus” variant (B.1.617.2.1 or AY.1) is a further mutated version of the Delta variant that likely began circulating in mid-March and was discovered through analysis of the GISAID database.  The “Delta Plus” name is currently a nickname; it’s not official WHO nomenclature.  And “hybrid” is an unfortunate, inaccurate descriptor.  Basically, the “Delta Plus” variant includes a third potentially important mutation (K417N) that is also found in the Beta variant originally detected in South Africa and the Gamma variant originally detected in Brazil.  This mutation helps the virus more tightly bind to cells.  It’s unclear whether the additional mutation gives the virus new/changed functions, so WHO has not given “Delta Plus” an official new name (yet).  “Delta Plus” is hybrid in that it includes mutations found in other variants of concern, but these mutations are naturally occuring and are not the result of two variants merging or being melded, which is what I envision when I hear “hybrid.”  If you’d like to read more, National Geographic published an informative article a few days ago, How dangerous is the new Delta Plus variant? Here’s what we know.

Figure. Diagram of the Coronavirus Genome (from NY Times)