Abstract
In this letter, we report a large-scale synchronized replacement pattern
of the Alpha strain by the Delta strain. We argue that this phenomenon
is associated with the invasion timing and transmissibility advantage of
the Delta strain. Alpha strain skipped some countries/regions, e.g.
India and neighboring countries/regions, which led to a mild first wave
before the invasion of the Delta strain, in term of reported
COVID-deaths per capita.
Keyword: variants of concern, Delta strain, Alpha variant,
synchronization
Dear Editor:
The coronavirus disease 2019 (COVID-19), caused by severe acute
respiratory syndrome coronavirus 2 (SARS-COV-2) has seriously affected
public health worldwide. As of February 2021, more than 100 million
people had been diagnosed with SARS-COV-2 and more than 2 million deaths
have been attributed to COVID-19 1. The virus evolved
rapidly and several variants have emerged. In March 2021, the Alpha
variant began to spread in Cambodia and Thailand, in some clusters in
Thailand, the positive rate of SARS-CoV-2 testing is as much as 60-90%.2 According to 3, besides a higher
transmissibility than the previous wild strain, patients infected with
the Alpha COVID-19 variant were at a higher risk of hospitalization than
those infected with the previous wild strain, reflecting the increased
virulence of the Alpha variant.
At the same time, Kappa (B.1.617.2) and Delta (B.1.617.2) variants
appeared in Maharashtra, India and resulting in a resurgence of cases in
the country. Different from Alpha variant, the Delta variant lineage is
defined by eight non-synonymous mutations in S protein. The Delta
variant which spread around 200 countries/regions has been classified as
a variant of concern by the CDC. 4 Besides a higher
transmissibility than Alpha strain, patients with the Delta variant were
more than twice as likely to be hospitalized as those with the Alpha
variant, according to5. The Delta variant is replacing
all the other SARS-COV-2 variants.
Starting from June 2020, India has implemented 11 stages of unlocking,
phase 11 of which was announced at the end of March, 202 and remains in
effect until April 30, 2020. But since mid-April, 2020, India has seen a
severe surge in the pandemic. As of May 10, 2020, more than 388,000
people had been affected.6 And according to another
report 7, characteristic mutations of the Delta
variant were observed in sequences obtained in India in April and May
2021, and the Delta variant became the dominant transmission variant in
May and June, 2021 in India.
In this letter, we visualize the replacement of previous strain with
Alpha strain and the replacement of Alpha strain with Delta strains
globally. We find that the Alpha strain only dominated for a short
period of 3-4 months and the replacement of Alpha with Delta show
surprisingly synchronous pattern in a large number of countries/regions.