The spread of the Delta variant and the risk of wave IV.

We present epidemic spreading scenarios for different variants of the population resistance level in the case of the delta variant.

Scenario assumptions:

• Population immunity level – the percentage of recoveries in the population as of June 1 is around 45% or around 57%.
• All scenarios take into account the opening of schools and the almost complete removing of restrictions on September 1. We understand this as a practical lack of administrative restrictions, apart from e.g. wearing face masks in closed rooms and possible hygiene habits acquired by the society during the epidemic.
• We are considering scenarios assuming a different share of the Delta variant in new infections on June 1 (1%, 3%, 6% respectively).
• The relative infectivity of the Delta variant with respect to the variant Alpha is taken as the relative infectivity of the variant Alpha with the wild-type variant.

The virus variants in the model are characterized by relative infectivity. Relative infectivity is a value that describes the biological characteristics of a virus that affects the rate of host cell colonization.

We also assume that each age group approved for vaccination will no longer get vaccinated once the target vaccination coverage is achieved. This assumption results from the fact that not every person in the age group declares the willingness/possibility of being vaccinated. There is an upper limit of 70% for this scenario.
In the model, this limit will be reached in the second half of July 2021.

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As can be seen in the presented graphs, at a lower level of immunization of the population, the appearance of the Delta variant causes the fourth wave. The assumed share of the variant affects only the time of wave occurrence, not it’s height.

It should be noted that the occurrence of a higher wave in this scenario is associated with several “pessimistic” assumptions, such as the lack of restrictions, very high relative infectivity of the variant and the vaccination limit in age groups.