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Two new omicron subvariants are spreading in the U.S., making up 13% of all new COVID-19 cases, according to estimates from the Centers for Disease Control and Prevention.

The cases involving the subvariants, omicron BA.4 and BA.5, have nearly doubled from last week, and have grown from being responsible for only 1% or U.S. cases in early May, The New York Times reported.

The CDC released the new figures this week and forecast that BA.4 and BA.5 could push out the two other subvariants, BA.2 and BA.2.12.1, that moved swiftly across the country in the past few months.

According to some researchers, the new subvariants are more contagious than the original omicron virus and could escape immunity from past infections and vaccinations. However, scientists say there is no current evidence suggesting that the subvariants would cause more severe disease.

Preliminary laboratory research suggests that unvaccinated people who were infected with the original version of omicron could be reinfected with BA.4 or BA.5, the Times reported. Those who have been vaccinated could also be infected with the subvariant but are less likely than the unvaccinated to either get the virus or get a severe version of it, the study suggested.

BA.4 and BA.5 were first detected in South Africa in January and February of 2022, respectively.