Omicron subvariants BA.4 and BA.5 are on the rise in the European Union, spurring officials there to warn that a surge of COVID-19 cases will likely follow in the coming weeks.

In an alert Monday, the European Center for Disease Prevention and Control cautioned that various factors would influence how bad the expected BA.4/BA.5 wave will be. Those factors include the extent of vaccination and past infection in the population, as well as timing since those events because protection from both wanes over time.

BA.4 and BA.5 are clumped together because they share the same mutations in the genetic coding for their spike proteins, though they have differing mutations elsewhere in their genome. Both have a transmission advantage over the initial omicron subvariant, BA.1, as well as subvariants BA.2 and BA.2.12.1.

So far, there's no indication that BA.4 or BA.5 cause more severe infections than the currently circulating omicron subvariants—specifically BA.2 and BA.2.12.1. But, the pair appear better able to evade immune protection from vaccines and prior omicron infections, possibly leading to more breakthrough infections. "As in previous waves," the ECDC writes, "an increase in COVID-19 cases can result in a rise in hospitalizations, ICU admissions, and deaths."

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Rising subvariants

BA.4 and BA.5 were first seen in South Africa in January and February and arrived in the EU in March. Recently, its spread has picked up speed. Portugal is the first EU country to see a wave, with BA.5 accounting for 87 percent of cases as of May 30. Now, BA.4 and BA.5 are increasing in Austria, Belgium, Denmark, France, Germany, Ireland, Italy, Netherlands, Spain, and Sweden.

In Belgium, BA.5 accounted for 19 percent of samples recently, and BA.4 accounted for 7.5 percent. In Spain, BA.4 and BA.5 accounted for more than 10 percent. In the Netherlands, BA.5 reached 8 percent recently, while BA.4 was close to about 5 percent.

The US is facing a similar outlook: BA.4 and BA.5 are gaining ground close on the heels of BA.2.12.1, which achieved dominance in the US just at the end of May. Currently, BA.2.12.1 accounts for an estimated 62.2 percent of US cases, while BA.4 accounts for 5.4 percent, and BA.5 is at 7.6 percent, according to the latest data from the US Centers for Disease Control and Prevention. Less than a month ago, the pair were accounting for about 2 percent of cases collectively.

The pair have significant potential to spur a new wave of infection in the US. Though more than 28 million Americans were infected amid the BA.1 wave that peaked in January, BA.4 and BA.5 can evade BA.1-derived neutralizing antibodies. And while the Food and Drug Administration in March authorized a second COVID-19 booster dose for those ages 50 and above, only 15 million people in that age group got a second booster so far. That's about 25 percent of people who received the first booster. Only 47 percent of fully vaccinated people in all age groups—about 104 million—received a first booster since last fall.