Concern over variants, sometimes called strains, of the virus that causes COVID-19 is based on how the virus might change. A virus could get better at infecting people, spread faster or cause people to get sicker.

As a virus infects a group of people, the virus copies itself. During this process the genetic code can randomly change in each copy. These changes are called mutations.

Some mutations don't have any effect on the virus.

But other mutations can:

• Make the virus better at infecting a person's cells, causing serious illness.
• Make the virus better at avoiding the immune system.
• Cause tests for the virus to be less accurate.
• Cause vaccines to not work as well.
• Make medicine used to prevent or treat COVID-19 stop working or not work as well.

If a mutation changes how a virus acts in a group of people, it's called a variant. Scientists across the world track the changes in the virus variants that cause COVID-19.

Omicron

The main variant in the United States is omicron. This variant spreads more easily than the original virus that causes COVID-19 and the delta variant. But omicron seems to cause less severe disease.

Omicron has a few major offshoots, also called sublineages. Together the omicron variants make up nearly all COVID-19 infections in the United States.

People who are up to date on their vaccines can get breakthrough infections. They can then spread the virus to others. But the COVID-19 vaccines can work to prevent severe illness. To strengthen your protection against the virus that causes COVID-19, the CDC recommends staying up to date with COVID-19 vaccines for which you are eligible.