What's the concern about COVID-19 variants? Are they more contagious?
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.
Getting a COVID-19 vaccine protects against serious illness, the need for hospital care due to COVID-19 and death from COVID-19. Staying up to date with the latest vaccine is most important for people at higher risk. That includes adults over age 65, those with weakened immune systems, people who are pregnant, and people with chronic conditions, such as heart disease, lung disease or obesity.
Serious side effects from the COVID-19 vaccine are extremely rare. Because of that, even healthy adults benefit from the COVID-19 vaccine, which lowers the risk of serious illness at a rate similar to that of the flu vaccine.
History of infectious disease outbreaks and vaccines timeline.
Learn about the history of major disease outbreaks, epidemics and pandemics, as well as the impact vaccines and research had on many infectious diseases.
Find out more at History of infectious disease outbreaks and vaccines timeline.
Oct. 24, 2025
- COVID data tracker: Variant proportions. Centers for Disease Control and Prevention. https://covid.cdc.gov/covid-data-tracker/#variant-proportions. Accessed Nov. 2, 2023.
- SARS-CoV-2 variant classifications and definitions. Centers for Disease Control and Prevention. https://www.cdc.gov/coronavirus/2019-ncov/variants/variant-classifications.html. Accessed April 14, 2023.
- Statement on Omicron sublineage BA.2. World Health Organization. https://www.who.int/news/item/22-02-2022-statement-on-omicron-sublineage-ba.2. Accessed April 17, 2023.
- Stay up to date with your vaccines. Centers for Disease Control and Prevention. https://www.cdc.gov/coronavirus/2019-ncov/vaccines/stay-up-to-date.html. Accessed April 17, 2023.
- What is genomic surveillance? Centers for Disease Control and Prevention. https://www.cdc.gov/coronavirus/2019-ncov/variants/genomic-surveillance.html. Accessed April 14, 2023.
- Summary of variant surveillance. Centers for Disease Control and Prevention. https://covid.cdc.gov/covid-data-tracker/#variant-summary. Accessed Nov. 2, 2023.
- Medical review (expert opinion). Mayo Clinic. Sept. 25, 2025.