The question of which SARS-CoV-2 strain was the “strongest” is complex because viral strength is not a single, easily defined metric. Scientists assess a variant’s threat based on a combination of factors, not just the ability to cause severe illness. The Delta variant became a major concern because it effectively combined several dangerous attributes, dominating the pandemic landscape for a significant period. Understanding a virus’s true power requires analyzing its biological and epidemiological performance against its predecessors and successors.
Defining the Delta Variant (B.1.617.2)
The Delta variant, officially designated as B.1.617.2, was first detected in India in late 2020. The World Health Organization (WHO) classified it as a Variant of Concern in May 2021, marking its potential threat to global health. It rapidly spread across the world, becoming the globally dominant strain by mid-2021.
The variant’s success was linked to key genetic changes, particularly several mutations in the spike protein gene. Noteworthy mutations included L452R, T478K, and P681R. The P681R mutation was thought to enhance the virus’s ability to enter human cells. These alterations suggested increased transmissibility and a potential for immune evasion.
How Viral Strength is Measured
Scientists assess a variant’s “strength” using three primary metrics that reflect its public health impact.
Transmissibility
This measures how easily and quickly the virus spreads from person to person. It is often quantified using the basic reproduction number (\(R_0\)), which represents the average number of new infections generated by one infected person in a completely susceptible population.
Pathogenicity (Severity)
This is gauged by the risk of severe outcomes, such as hospitalization, intensive care unit (ICU) admission, or death. Severity is often expressed through the case fatality rate or the hospitalization rate relative to previous strains.
Immune Evasion
This is the variant’s ability to bypass existing immunity from prior infection or vaccination. This is determined by testing how effectively antibodies neutralize the new variant compared to older strains.
Why Delta Overtook Earlier Variants
Delta’s dominance was rooted in a substantial biological advantage over earlier strains, including the original Wuhan strain and the Alpha variant (B.1.1.7). The Delta variant was estimated to be significantly more transmissible than its predecessors, with some models suggesting it was about 40% to 60% more transmissible than the already highly contagious Alpha variant.
This increase in transmissibility was partly due to an increased viral load in infected individuals. Studies found that patients infected with the Delta variant had viral loads that were up to 1,260 times greater than those seen with the ancestral strain. This higher concentration of virus meant that infected individuals shed more virus, making transmission more likely. The incubation period also appeared to shorten, which further accelerated its spread.
Beyond its high infectivity, Delta also demonstrated increased pathogenicity compared to earlier strains, particularly in unvaccinated individuals. Infection with the Delta variant was associated with a higher risk of severe pneumonia and a greater chance of hospitalization. Some studies linked Delta to a significantly increased risk of hospitalization and ICU admission compared to non-Delta strains.
Delta’s Comparison to Dominant Successor Variants
Delta’s reign was eventually challenged by the emergence of the Omicron variant and its subsequent sub-lineages in late 2021. The definition of viral strength shifted, moving away from high severity to focus on extreme transmissibility and immune evasion. Delta retained high pathogenicity, meaning it was more likely to cause severe illness, hospitalization, and death compared to Omicron.
Omicron proved superior at spreading and bypassing existing immunity. It replicated much faster in the upper respiratory tract and possessed a significantly higher number of spike protein mutations than Delta. This allowed Omicron to spread more easily, often causing breakthrough infections in vaccinated or previously infected individuals.
Delta was surpassed because it was outcompeted in the race for transmissibility and immune escape, despite its inherent severity. Delta was the strongest in terms of severe disease potential, but not in its long-term ability to evade population immunity and spread.