What Makes the Delta Variant Different Than Previous Strains? The biggest difference between the delta variant and other strains is that it is significantly more contagious. According to scientists at Yale Medicine, the delta variant is 50% more contagious than the alpha variant, which originated in the United Kingdom and is the most common strain of the virus seen in Illinois. The alpha variant was 50% more contagious than the original strain of COVID-19, meaning that the number of people that could potentially be infected by each COVID-positive individual is significantly higher. According to experts, the average COVID-positive individual, in a completely unmitigated environment without masks or other barriers, would infect an average of 2.5 people.