A substrain of omicron is responsible for a growing number of infections.

BA.2.12.1, a subvariant of the COVID-19 Omicron strain, is now the cause of approximately 1 in 5 infections in the United States. Most U.S. infections are still linked to BA.2, the parent strain, which is responsible for 75% of domestic outbreaks.

“SARS-CoV-2, the virus that causes COVID-19, is constantly changing and accumulating mutations in its genetic code over time,” the CDC warned in a recent report. “New variants of SARS-CoV-2 are expected to continue to emerge. Some variants will emerge and disappear, while others will emerge and continue to spread and may replace previous variants.”