You've probably heard about the Delta variant, but for the sake of getting us all on the same page, a little recap: The COVID pandemic is caused by a virus called SARS-CoV-2, which is constantly changing as it replicates and spreads.
These changes, or mutations, in the virus's genetic code result in new variants of the virus.
Most of these have been found to either be weaker or benign versions of the original strain, meaning that the mutations didn't cause the virus to be more harmful in any way, but a handful of variants carry mutations that are associated with increased transmissibility and more severe cases.
And those are the variants that experts are keeping close tabs on.
So far, four of these have been identified as "variants of concern" by the World Health Organization: Alpha, Beta, Gamma, and Delta, which is the most recent addition to the list.
Together, these variants are shown to be either more contagious, more deadly, or more resistant to current treatments.
But of the four, Delta is especially concerning.
According to the WHO, it's the "fastest and fittest" variant yet.
Recent work out of Imperial College London suggests that it's actually up to 60% more transmissible than the Alpha variant — which was already 50% more transmissible than the original strain— and many scientists expect Delta will soon replace Alpha as the dominant variant worldwide.
We're already feeling the effects, particularly with rates of infection steadily rising amongst unvaccinated populations.