Omicron and its subvariants have ranked as the predominant SARS-CoV-2 strains in the U.S. for almost two years now. While the original Omicron strain (BA.1) is no longer circulating, Omicron subvariants are now driving most of the country’s SARS-CoV-2 infections. Omicron was first identified in Botswana and South Africa in late November 2021, and cases quickly began to surface and multiply in other countries. By December of that year, Omicron was causing daily case numbers in the U.S. to skyrocket to over a million. In 2022, it had spawned a number of subvariants. In 2023, a new Omicron strain called EG.5 (nicknamed “Eris”) is the dominant strain in the U.S., and experts are monitoring another new strain called BA.2.86 (nicknamed “Pirola”).

How contagious is it? Omicron’s subvariants are considered to be especially efficient spreaders of the disease. The original strain of Omicron was more transmissible than Delta was. One explanation was that more than 30 of Omicron’s mutations are on the virus’s spike protein, the part that attaches to human cells, and several of those are believed to increase the probability of infection.

Severity: Scientists are still working to learn more about whether the current Omicron strains cause more severe disease than their predecessors. Data has suggested that the original Omicron strain was less severe, in general, than previous variants, according to the CDC. But it has also been noted that surges in cases may lead to significant increases in hospitalizations and deaths, as they did during the variant’s spread at the beginning of 2022, when the estimated death rates went as high or higher than they were at the time of the Delta variant surge in the previous autumn.

Can vaccination prevent it? The CDC says that while breakthrough infections in vaccinated people are expected, staying up to date with vaccinations is the best protection against Omicron. Scientists are evaluating the effectiveness of a new fall 2023 updated COVID-19 booster against EG.5 and BA.2.86, according to the CDC. Currently, the CDC says the updated vaccine is expected to be effective at reducing severe disease and hospitalization from the two recent subvariants.