CDC says 'There's lots of flu in lots of places.' And it's not going away anytime soon.

A nasty flu season is in full swing across the United States, with a sharp increase in the number of older people and young children getting hospitalized, federal health officials said Friday.

The latest weekly data from the Centers for Disease Control and Prevention show influenza has spread across the country.

"Flu is everywhere in the United States right now. There's lots of flu in lots of places," said Daniel Jernigan, director of CDC's influenza division.

Of particular concern, he said, is the "very rapid increase" in the number of people hospitalized with laboratory-confirmed cases. The overall hospitalization rate for the week ending Jan. 6 - 22.7 per 100,000 - is almost double that of the previous week. Seven children died in the first week of January, bringing the total number of pediatric deaths to 20.

This increase comes at a time when hospital workers are scrambling to deal with an ongoing shortage of intravenous fluids used to deliver medicine and treat dehydrated patients. Supplies from factories in hurricane-ravaged Puerto Rico have been slow to rebound, the Associated Press reported.

CDC officials said there are also spot shortages of antiviral medicines in parts of the country with lots of flu. The agency has been in regular contact with manufacturers, and while the national supply should be sufficient to meet demand, some manufacturers are reporting delays in filling orders, Jernigan said.

He urged pharmacies to call more than one distributor and said patients filling prescriptions for antivirals may want to check ahead with their pharmacies to make sure the medicine is available.

Although he noted the anecdotal reports of healthy young people who have died from the flu, the individuals most at risk remain the elderly and young children. People who are very sick or at high risk of complications should be treated as soon as possible with antiviral medicines, without clinicians waiting for confirmatory testing, he said.

The main culprit for this harsh flu season is the predominant strain, H3N2, which causes the worst outbreaks of the two influenza A viruses and two types of influenza B viruses circulating. Seasons where H3N2 dominates typically result in the most complications, especially for the very young, the elderly and people with certain chronic health conditions, experts say.

Even though flu activity has probably peaked, the forecast for next three months is grim.

"Even if we have hit the top of the curve, it still means there's lots more flu to go," he said in a briefing for reporters Friday. "If we look at similar seasons, there's at least 11 to 13 more weeks of influenza to go."

The data suggests this season is closely tracking the 2014-2015 flu season, when H3N2 was also dominant, Jernigan said. But the overall hospitalization rate then was even higher, almost 30 per 100,000 people for the same time frame. "At this point, [the current season] does not appear to be as severe," he said.

There are indications that the H1N1 virus, which caused the 2009-2010 swine flu pandemic but is now a regular human flu virus, is also starting to show up, Jernigan said. In addition, influenza B viruses tend to become more active later in a flu season. "We may have a pretty robust season with these others as well," he said.

For those reasons, vaccination is still the best way to protect against the flu. The CDC recommends an injectable flu vaccine for everyone 6 months or older as soon as possible because the body takes about two weeks to produce a full immune response.

In seasons when the predominant strain is H3N2, vaccine effectiveness tends to be lower than that against H1N1 and influenza B viruses, Jernigan said. So far, the data suggest that this season's vaccine will be about 32 percent effective against circulating H3N2 viruses.

"We have a lot of flu season to get through still," Jernigan said.