A pair of highly transmissible omicron subvariants are deepening their hold on the U.S.

According to updated data from the Centers for Disease Control and Prevention, BA.4 and BA.5 were responsible for 70% of new coronavirus cases last week. That’s up from 55% of cases the week before, when the pair became the dominant strains circulating in the U.S.

The pair made up just 1% of cases at the start of May. The rapid rise of the subvariants could spur a slight increase in coronavirus cases or, at the very least, a longer plateau for the latest coronavirus wave, according to experts.

While coronavirus infections have hovered around 100,000 on average each day for the past month and a half, many believe that number to be a significant undercount of the true spread.

BA.4 and BA.5 are also spreading across the world. The World Health Organization last week reported an 18% increase in new, weekly coronavirus infections worldwide with BA.5 driving the most cases of any omicron subvariant.

In the U.S., the two previous omicron subvariants to become dominant – BA.2.12.1 and BA.2 – have declined dramatically. BA.2, or “stealth omicron,” made up 3% of cases last week while BA.2.12.1 was responsible for 27% of infections.

The trend raises a question of whether BA.4 and BA.5 will take over the variant scene completely in the coming weeks.

Moderna and Pfizer have said their omicron-specific booster shots appear to work on BA.4 and BA.5, but those shots aren’t expected to be available until the fall. The subvariants are likely to fuel hundreds of thousands of coronavirus infections before then.

Coronavirus hospitalizations in the U.S. are on the rise while deaths have plateaued at around 300 each day. Meanwhile, the CDC recommends nearly 70% of Americans should either be wearing masks indoors in public spaces or considering the measure based on their risk for severe COVID-19.