Also known as B.1.617.2, the variant was first identified in India in late 2020. It has spread to at least 85 countries, according to the World Health Organization, and has itself mutated to produce a further version that is causing concern in India.
Vaccines from Moderna Inc., Johnson & Johnson and Pfizer Inc. and partner BioNTech SE—the three shots authorized in the U.S.—were primarily tested in large clinical trials for their efficacy at preventing symptomatic disease, rather than their ability to prevent infection completely. While the vaccines seem to be slightly less effective at preventing symptomatic Covid-19 from the Delta variant, research has shown that they remain highly effective at preventing serious illness and hospitalization.
Overall, the portion of eligible Americans fully vaccinated against Covid-19 has been stalled for weeks at around 57%.
Here is what we know so far about the Delta variant and vaccines.