Why the coronavirus’s delta variant dominated 2021

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2021 was a year of coronavirus variants.

Alpha and beta kicked off the year, and several worrisome variants later, omicron is closing it out. How omicron may come to define the pandemic’s future remains uncertain. But even as omicron comes on strong, one variant, which rose to global dominance midyear in a way variants like alpha and beta never did, continues to largely define the pandemic right now: delta.

Things had actually seemed to be looking up in some parts of the world in the late spring and early summer of 2021, a year and a half into the COVID-19 pandemic. In the United States, for instance, millions of people were vaccinated, cases of the disease were falling, and people were beginning to socialize and resume normal activities.

But then delta hit hard. First spotted in India in October 2020, this variant of SARS-CoV-2, the coronavirus that causes COVID-19, quickly swept around the world, supplanting other versions of the virus in 2021 (SN: 7/2/21). Delta overwhelmed health care systems, tore through unvaccinated populations and showed that even the vaccinated were vulnerable, causing some breakthrough cases.

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