The COVID-19 virus is constantly changing. Sometimes these changes, called variants, make the virus more or less contagious or cause people to get sicker or have different symptoms. CDC tracks these variations and works to slow the spread of the viruses by educating people, offering tests and vaccinations, and monitoring the community for more outbreaks.

When the coronavirus that causes COVID-19 infects a person it copies itself. During this process, some of these copies may have genetic changes, or mutations. These changes can affect how the virus spreads, how sick a person gets or how well vaccines work against it.

COVID-19 can spread through respiratory droplets from coughing, sneezing or talking, and through contact with an infected person. The virus can also spread when a person touches a surface that has viral particles on it, like a door handle or keyboard.

The most recent change in the circulating virus was the emergence of the Delta variant, which has more potential to spread and may cause severe illness in more people. The Delta variant is also more difficult to detect by lab tests. People who are infected with the Delta variant do not show the same symptoms as those who are infected with Alpha, Gamma or Omicron.

Viruses evolve all the time, but some of these changes are more significant than others. Scientists carefully study virus lineages in the laboratory and in real life, and classify them based on their characteristics. For example, some mutations improve a virus’s ability to infect cells or to travel from one cell to another (transmission rates) while other changes might affect how sick a person will be or how well a vaccine or treatment works against it.