ABSTRACT
The first SARS-CoV-2 variant of concern (VOC) to be designated was lineage B.1.1.7, later labelled by the World Health Organisation (WHO) as Alpha. Originating in early Autumn but discovered in December 2020, it spread rapidly and caused large waves of infections worldwide. The Alpha variant is notable for being defined by a long ancestral phylogenetic branch with an increased evolutionary rate, along which only two sequences have been sampled. Alpha genomes comprise a well-supported monophyletic clade within which the evolutionary rate is more typical of SARS-CoV-2. The Alpha epidemic continued to grow despite the continued restrictions on social mixing across the UK, and the imposition of new restrictions, in particular the English national lockdown in November 2020. While from a case-number perspective these interventions succeeded in reducing the absolute number of cases of SARS-CoV-2 in the UK, the impact of these non-pharmaceutical interventions was predominantly to drive the decline of those SARS-CoV-2 lineages that preceded Alpha. We investigate the only two sampled sequences that fall on the branch ancestral to Alpha. We find that one is likely to be a true intermediate sequence, providing information about the order of mutational events that led to Alpha. We explore alternate hypotheses that can explain how Alpha acquired a large number of mutations yet remained largely unobserved in a region of high genomic surveillance: an under-sampled geographical location, a non-human animal population, or a chronically-infected individual. We conclude that the last hypothesis provides the best explanation of the observed behaviour and dynamics of the variant, although we find that the individual need not be immunocompromised, as persistently-infected immunocompetent hosts also display a higher within-host rate of evolution. Finally, we compare the ancestral branches and mutation profiles of other VOCs to each other, and identify that Delta appears to be an outlier both in terms of the genomic locations of its defining mutations, and its lack of rapid evolutionary rate on the ancestral branch. As new variants, such as Omicron, continue to evolve (potentially through similar mechanisms) it remains important to investigate the origins of other variants to identify ways to potentially disrupt their evolution and emergence.
ABSTRACT
SARS-CoV-2 Spike amino acid replacements in the receptor binding domain (RBD) occur relatively frequently and some have a consequence for immune recognition. Here we report recurrent emergence and significant onward transmission of a six-nucleotide deletion in the S gene, which results in loss of two amino acids: H69 and V70. Of particular note this deletion, 𝚫H69/V70, often co-occurs with the receptor binding motif amino acid replacements N501Y, N439K and Y453F. One of the 𝚫H69/V70+ N501Y lineages, B.1.1.7, is comprised of over 4000 SARS-CoV-2 genome sequences from the UK and includes eight other S gene mutations: RBD (N501Y and A570D), S1 (𝚫H69/V70 and 𝚫144/145) and S2 (P681H, T716I, S982A and D1118H). Some of these mutations have presumably arisen as a result of the virus evolving from immune selection pressure in infected individuals and at least one, lineage B.1.1.7, potentially from a chronic infection. Given our recent evidence that 𝚫H69/V70 enhances viral infectivity (Kemp et al. 2020), its effect on virus fitness appears to be independent to the RBD changes. Enhanced surveillance for the 𝚫H69/V70 deletion with and without RBD mutations should be considered as a priority. Permissive mutations such as 𝚫H69/V70 have the potential to enhance the ability of SARS-CoV-2 to generate new variants, including vaccine escape variants, that would have otherwise significantly reduced viral fitness.