COVID cases likely rising in half of states, CDC estimates

Cases of COVID-19 are now likely growing in 25 states, according to estimates published by the Centers for Disease Control and Prevention on Friday, as this year's summer wave of the virus appears to be getting underway.

The agency's modeling suggests that the uptick is in "many" Southeast, Southern and West Coast states, the CDC said in its weekly update.

COVID-19 activity nationwide, however, is still considered "low," based on data from wastewater samples, although that is up from "very low" the week before.

Some of the highest rates of COVID-19 in emergency rooms are in the Pacific Northwest, which saw a steep rise in ER visits during June, and in the Southeast, which has seen rates steadily inch up for weeks. Rates are now the highest they've been since February and March in those regions.

Health officials have been watching for an increase in COVID-19 this summer, as new variants have spread to the U.S. after fueling surges overseas.

A CDC analysis published Thursday concluded that the virus has settled into twice-a-year surges nationwide, typically peaking first in July through September and then a second time later in December through February.

Those surges look to be linked to cycles of genetic changes to a part of the virus that scientists call the S1 region, a key part of the spike protein of the virus that binds to human cells.

"Our analysis revealed biannual COVID-19 peaks in late summer and winter, a pattern that is expected to persist as long as the rapid evolution of SARS-CoV-2 and cyclical S1 diversity continues," the scientists wrote in the CDC analysis.

COVID-19's rise comes as health officials have been probing reports of an unusual uptick in another respiratory virus called parvovirus B19. While many people develop no symptoms from parvovirus B19 infections, some people get sick with signs like fever and muscle pains, followed by a rash.

The CDC warned late last month of a "sustained increase" in transmission of parvovirus B19, and suggested that pregnant women consider masking to curb the risk of severe complications. Some local health departments have also tracked a rise in emergency rooms treating cases of this virus.

"Several of the most recent weeks saw the highest percentage of B19-associated ED visits compared to the same week in all years since 2015," health officials in Chicago warned on Thursday.