Several US states are fighting record cases and hospitalizations of this season's new 'super flu,' which experts warn will continue to intensify.
During the week of December 27, 831 Colorado residents were hospitalized with the flu, the highest number since the state began tracking two decades ago.
While the rate declined 12 percent during the week of January 3, figures are still higher than this time last year, and doctors practicing in Colorado have called this year's flu season the 'worst' they have ever seen.
About one in four influenza tests in The Centennial State came back positive during the week of January 3.
According to the latest CDC data, Colorado is one of 14 states with the agency's highest level of flu activity, level 13, which is considered 'very high.'
California is facing a similar situation. Health officials Tuesday announced hospitalizations are increasing statewide as the 'super flu,' or H3N2 subclade K, has been detected.
The California Department of Public Health reported the current hospitalization rate is four per 100,000 people, up from three per 100,000 the week prior, and 15 percent of influenza tests in the state came back positive the week of January 3, a slight dip from 17 percent the week before.
This is a significant drop from this time last year, when 28 percent of tests came back positive and the hospitalization rate was 10 in 100,000, but the CDC still lists viral flu activity in California as 'high.'
California also reported its second pediatric flu-related death last week for the current season, in an unvaccinated adolescent from San Mateo county.
Dr Monica Gandhi, an infectious disease expert at the University of California San Francisco, told SFGATE that super flu 'may start rising in California now, because people were sick when they went places... and there needs to be an incubation period.'
Dr Jean Hoffman, an emergency medicine and critical care physician at the University of Colorado, wrote for The Conversation: 'In my 18 years of practicing clinical medicine, this year is one of the worst I have seen.
'Our emergency department hit a record number of single-day total visits over the holidays, and visit volumes have stayed high. Flu is likely contributing to this trend.'
She added: While there is always a season where respiratory viruses hit hard, this year influenza is making patients miserable and wreaking havoc on both the state and national health care system.'
Nationwide, the latest CDC data shows weekly hospitalizations have increased from 31.3 per 100,000 people to 54.1 per 100,000, a 53 percent jump compared to the week before.
Cumulative hospitalizations, which measures the rate for the season, are also up from 28.1 to 40.6 per 100,000, a 37 percent increase from the week before.
However, test positivity is starting to decline after several weeks of surges. During the week of January 3, one in four tests came back positive compared to one in three the week before.
The number of states reporting 'very high' influenza activity has also decreased from 34 the week of December 27 to the week of January 3.
Along with Colorado, areas with the CDC's highest activity level, level 13, include: New York state, New Hampshire, Massachusetts, New Jersey, Ohio, Michigan, Missouri, Tennessee, North Carolina, South Carolina, Georgia, Louisiana and New Mexico.
This year's dominant flu strain is brand new to people's immune systems, making them highly vulnerable and at an increased risk of severe, hospitalizing illness.
While symptoms are the same as other flu strains, experts warn the 'super flu' variant is leading to more severe illness, especially in children, with signs including week-long fevers and a cough that leads to trouble breathing.
In one case, two-year-old Sarah Lopez from north Georgia has been hospitalized for weeks after she was infected with the flu.
Her mother Kenia told WSB-TV 2: 'When I would say come here, she wouldn't do it either. She would just throw her upper torso.
'Overnight, everything got worse. She couldn’t sit up on her own; she couldn’t talk; she couldn’t move pretty much anything; just her head a little bit.'
Lopez was hospitalized with the flu but then developed transverse myelitis, a rare condition where a patient suffers from inflammation in part of the spinal cord. Symptoms include back pain, difficulty moving and temporary paralysis.
At the hospital, doctors put her on a ventilator to help her breathe and a feeding tube to give her nutrients.
Speaking last week, her mother said that Lopez has now recuperated some strength and movement in her legs but still struggles to stand up. She has been taken off the ventilator and the hospital is now considering transferring her to a rehabilitation unit.
Dr Hoffman wrote: 'Flu tends to cause fever, body aches and maybe a cough. But this so-called super flu has also caused vomiting and diarrhea, which has made people feel much worse than isolated respiratory symptoms alone.
'When people are feeling worse, they seek emergency care, which is part of why our emergency department is seeing so many people.'
The annual flu vaccine is the best way to prevent the disease and is typically between 30 to 75 percent effective.
As of mid December, 42 percent of American adults have received this season's flu shot, which is in line with last year's figures.