Last updated: January 12, 2022

SARS-CoV-2 is the name of the virus that causes coronavirus disease 2019 (COVID-19). The disease is believed to have originally occurred from animal-to-person contact, and it spreads person-to-person.

COVID-19 can be mild or severe, and it can affect anyone. However, older people with other health conditions seem to be at higher risk for severe illness and death. Our understanding of COVID-19 continues to evolve.


Every virus changes over time. These “changed” viruses are called variants. Several common SARS-CoV-2 variants have emerged. Scientists study these variants to see if they make people sicker and if they spread more easily. They also look at data to confirm if COVID-19 vaccines still work to protect people who have a particular variant.

At least five variants of concern are known to be circulating in the United States:

  • B.1.1.7 (Alpha): Found in the United States in December 2020.
  • B.1.351 (Beta): Found in the United States at the end of January 2021.
  • P.1 (Gamma): Found in the United States in January 2021.
  • B.1.617.2 (Delta): Found in the United States in March 2021.
  • B.1.1.529 (Omicron): Found in the United States in December 2021.

These variants seem to spread more easily, particularly among those who are not vaccinated. Among those who are vaccinated, immunity may decrease over time and due to changes in variants. Studies suggest that boosters increase immunity to COVID-19, which can:

  • Help you avoid getting a COVID-19 infection
  • Help prevent serious illness and hospitalization if you do get infected

Learn more about COVID-19 vaccines and boosters.