Colorado “stuck on this plateau” as rise of BA.5 cancels out other variants’ decline

Colorado’s COVID-19 trajectory is still on a plateau, and there aren’t many clues to what may happen next.

The Colorado Department of Public Health and Environment reported 324 people were hospitalized with COVID-19 across the state as of Tuesday afternoon, which wasn’t meaningfully different from 320 at the same time the previous week.

In June, the state’s modeling team had projected a decline in cases and hospitalizations as the BA.2 and BA.2.12.1 variants of the virus ran out of people to infect. It appears that’s happened to some degree, but it’s being canceled out by growth driven by the BA.5 variant, said Beth Carlton, an associate professor of environmental and occupational health at the Colorado School of Public Health.

“I would guess we’re stuck on this plateau because of the arrival of BA.5,” she said.

All of the BA variants are relatives of the original omicron, which caused massive disruption over the winter. There’s no evidence that any of the newer versions are more likely to cause severe illness, but they have evolved to be more contagious, better at evading the immune system, or both.

BA.5 has a distinct advantage over previous variants, but it’s not totally clear how much is due to immune evasion, versus contagiousness, Carlton said.

That matters for projecting the future because there are very few people who have no immunity from vaccination or prior infection — a virus that’s just more contagious would rapidly run out of hosts. How good this variant is at reinfecting survivors, and how quickly immunity against it wanes, will determine how many people could get sick again in the coming weeks, she said.

As of June 19, the most recent data available, the state reported about 41% of sequenced samples contained BA.5, making it the most common variant in the state. It likely accounts for a majority of cases at this point, Carlton said.

There’s relatively little information about what might happen next, because other states are dealing with the arrival of BA.5 at about the same time Colorado is, and fewer people are researching variants at this point in the pandemic, Carlton said. Other countries saw some increase in hospitalizations as BA.5 took over, but they have different vaccination rates and histories with other variants.

“We don’t have this leading indicator state out ahead of us” to watch, she said.

The number of newly reported cases dropped by about 12%, to 12,798, in the week ending Sunday. That’s likely an undercount, however, because the rate of tests coming back positive is about twice the 5% level where public health watchers start worrying about missed cases.

In some parts of the state, it’s considerably higher. San Miguel County reported more than 35% of tests were coming back positive in the last week. Only 26 of the state’s 64 counties had rates below 10%.

As of Wednesday, the Centers for Disease Control and Prevention listed 16 counties in Colorado at the high risk level: Adams, Arapahoe, Bent, Boulder, Broomfield, Denver, Douglas, Jefferson, Kiowa, La Plata, Larimer, Montezuma, Pitkin, Rio Blanco, San Juan and Summit. Counties are considered high risk if they have at least 200 new cases per 100,000 people and some signs of rising hospitalizations.

The state reported 378 active outbreaks as of Wednesday. That’s not comparable to previous weeks, because the state is now only reporting outbreaks in correctional settings; nursing homes; assisted living facilities; child care centers; some facilities serving people with disabilities; homeless shelters; and overnight camps.

Deaths hit a peak of 55 in the first week of June, dropped in the second week, and then started rising again through the end of the month. The data for more recent weeks is still incomplete.