Figure 1. Timing of the confirmations of Delta strain exceeding Alpha strain (the first time that the proportion of Delta > the proportion of Alpha), denoted as \(T_{\alpha\rightarrow\sigma}\)in 71 countries/regions, versus timing of the confirmations of Alpha strain exceeding the previous strain \(T_{w\rightarrow\alpha}\). (a) countries/regions are ordered from left to right according to\(T_{\alpha\rightarrow\sigma}\). (b) countries/regions are ordered from left to right according to \(T_{w\rightarrow\alpha}\).
From Figure 1, we can see that Delta strain was first found in India and spread first in neighboring countries/regions, gradually to other countries/regions. Alpha strain was first found in United Kingdom and spread first in neighboring countries/regions, gradually to the rest of the world. \(T_{\alpha\rightarrow\sigma}\) (Black circle) has a surprisingly synchrony patten across a large number of countries/regions (ie, a horizontal line across a large number of countries/regions in July 2021). This synchrony pattern is less evident in the\(T_{w\rightarrow\alpha}\) (red squares). In other words, the timing of Delta strain replacing Alpha strain occurred simultaneously in many countries/regions. In contrast, the timing of Alpha strain replacing the previous wild strain did not show a strong synchronous substitution trend. This could be related to a higher transmissibility of Delta strain compared to other previous strains. Delta strain also possesses shortened incubation period and increased viral load. In particular, the viral load is about 1000 times higher in patient who was infected with Delta strain than patient who was infected with the original strain. And the first detectable time for Delta strain is 4 days after infection which is longer than the average detectable time of original strain (6 days) 8
Also, the invasion time of Delta strain happened at the tail of the wave of Alpha strain and coincided with a relax of social distancing in many countries (see Figure S1 in supplementary).
The dominance time of the Alpha strain is between\(T_{w\rightarrow\alpha}\) and \(T_{\alpha\rightarrow\sigma}\). In South and Southeast Asia, e.g. India, the Alpha strain failed to dominate, which could be associated to a mild first wave (with low deaths per capita in India in 2020).
In Scotland, BNT162b2 and Astrazeneca vaccines were 79% and 60% effective at preventing SARS-COV-2 Delta infection after two doses, respectively. However, the Pfizer-Biontech and Astrazeneca vaccines maintain a high degree of protection against any infection similar to Alpha strain 9.
In summary, we reported a large-scale synchronized replacement of Alpha strain by the Delta strain which could be due to the invasion timing of Delta strain and its relatively high transmissibility. Also, we note that these countries/regions in South and Southeast Asia experienced a mild 2020 year was largely skipped by the Alpha strain.