On Nov. 26, 2021, the World Health Organization (WHO) classified a new variant, B.1.1.529, as a variant of concern and named it Omicron. On Nov. 30, 2021, the United States also classified it as a variant of concern. Omicron was identified in Indiana on Dec. 19, 2021, and it now accounts for more than 99% of COVID-19 cases in the United States. Omicron has multiple sub lineages that are classified as variants of concern: B.1.1.529, BA.1, BA.1.1, BA.2, BA.3, BA.4 and BA.5. At the end of March, CDC estimated that the BA.2 Omicron variant was the dominant COVID-19 strain in the United States. BA.2 is a subvariant of Omicron and has some key genetic differences from the original Omicron variant, B.1.1.529, that may make it more infectious. CDC publishes information about the proportion of variants in the U.S. every week on its website.

The Omicron variant spreads more easily than the original virus that causes COVID-19 and the Delta variant. CDC expects that anyone with Omicron infection can spread the virus to others, even if they are vaccinated or don’t have symptoms. Persons infected with an Omicron variant can present with symptoms similar to previous variants.

Omicron infection generally causes less severe disease than infection with prior variants. Preliminary data suggest that Omicron may cause more mild disease, although some people may still have severe disease, need hospitalization, and could die from the infection with this variant. Even if only a small percentage of people with Omicron infection need hospitalization, the large volume of cases could overwhelm the healthcare system which is why it’s important to take steps to protect yourself.

COVID-19 vaccines remain the best public health measure to protect people from COVID-19 and reduce the likelihood of new variants emerging. This includes primary series, booster shots and additional doses for those who need them.

Scientists are still learning how effective COVID-19 vaccines are at preventing infection from Omicron. Current vaccines are expected to protect against severe illness, hospitalizations, and deaths due to infection with the Omicron variant. However, breakthrough infections in people who are vaccinated are likely to occur. People who are up to date with their COVID-19 vaccines and get COVID-19 are less likely to develop serious illness than those who are unvaccinated and get COVID-19.