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(NEXSTAR) – If you’re stuck at home with COVID-19 right now, you’re far from alone. The U.S. is undergoing another major surge in coronavirus cases, driven by two new types of the omicron variant.

BA.4 and BA.5, both subvariants of the highly contagious omicron, make up more than 90% of new cases in the country, according to tracking done by the Centers for Disease Control and Prevention.

There are a few reasons these two subvariants – especially the dominant BA.5 – are spreading like wildfire and making such a big impact in the U.S. right now.

BA.4 and BA.5 are sneakier

Early evidence suggests BA.4 and BA.5 aren’t more contagious than the original strain of omicron. However, it appears they are better at evading prior immunity.

That means people with some immunity – either from vaccines or past COVID infection – are still susceptible to infection from BA.5.

Plus, it’s been more than six months since our last major national surge of the first omicron variant. People infected in that wave may likely have gotten some boosted immunity, but that window has passed for most of us.

“The good news is that the vast majority of breakthrough infections now are outpatient illnesses,” said Dean Blumberg, UC Davis Children’s Hospital’s chief of pediatric infectious diseases. “They are not resulting in the kind of severe illness that we saw earlier in the pandemic when no one had immunity, which led to increased hospitalizations and deaths.”

People are testing at home, if at all

While you may feel like you know more people catching COVID than ever before, case rates tracked by public health departments don’t seem to reflect a massive surge. The New York Times’ thorough COVID tracker shows a rise in new cases, but the latest curve is dwarfed by last winter’s surge.

Epidemiologists agree that COVID-19 cases are severely undercounted right now.

“With the home testing, we’ve lost our ability to track based on reported cases,” said Dr. George Rutherford, an epidemiologist at the University of California, San Francisco.

When someone tests positive for the virus at home, that case often goes unreported to public health officials, leaving us with a murkier idea of how widely BA.5 is spreading.

On top of that, milder symptoms mean fewer people may be testing themselves. If they never test for the virus, they may continue to spread it unknowingly in the community.

COVID restrictions are gone

The original wave of coronavirus cases – before we had vaccines – triggered widespread restrictions, closures and masking requirements. The delta surge of summer 2021 and the original omicron wave of last winter had more cautious localities and states reinstate mask mandates. Now, there are nearly no COVID-19 restrictions left nationwide.

With less masking and more intermixing, the virus can spread more quickly through a community.

The CDC recommends wearing a mask indoors if your county is experiencing a “high” level of COVID transmission. As of Tuesday, more than a third of U.S. counties were in that category. You can check your county’s status on the CDC’s map.