(CNN) Sixteen flu-related deaths were reported Friday by the US Centers for Disease Control and Prevention in its weekly flu report . This brings the total number of pediatric flu-related deaths to 53 for the season, which began in October.

According to the report, influenza activity is now widespread in 48 states and Puerto Rico, down from 49 states in the previous two weeks. Oregon joined Hawaii in lower activity levels for the week ending January 27.

"Our latest tracking data indicate that flu activity is still high and widespread across most of the nation and increasing overall," said Dr. Anne Schuchat, the acting CDC director. "So far this year, the cumulative rate of hospitalizations is the highest since we've been tracking in this way, which goes back to 2010.

"This is a very difficult season," she said.

Caused by viruses, flu is a contagious respiratory illness with mild to severe symptoms that can sometimes lead to death.

The CDC also recorded an uptick in the percentage of patients who visited health care providers complaining of influenza-like illness across the nation: 7.1% of patients for the week ending January 27, up from 6.5%, the newly revised estimated from the previous week.

"We have not hit our peak yet, unfortunately," CDC spokeswoman Kristen Nordlund said. "Really, the bottom line is, there is still likely many more weeks to go."

Schuchat said there are hopeful signals within the CDC's latest report.

"For the second week in a row, there are signs that activity in the West may be easing up," she said. "However, we are by no means out of the woods."

One of the worst in years

This year's flu season is rivaling the worst in recent years, according to Dr. Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases.

Overall, the data showed 17,024 new laboratory-confirmed cases of illness during the week ending January 27, bringing the season total to 126,117. These numbers do not include all the people who have had the flu, as many do not see a doctor when sick.

The CDC has received reports of antiviral drug shortages in some places, Schuchat said. "However, the manufacturers say that there's product available. Pharmacists may want to increase supply on their shelves, and patients may have to call more than one pharmacy to fill their prescription."

US Food and Drug Administration Commissioner Dr. Scott Gottlieb said in statement Thursday that some antivirals as well as flu tests are in short supply. "However, at this time, there is no nationwide shortage of these products," Gottlieb said.

Flu vaccines also have been reported in short supply, but they are still available, Gottlieb said. "I strongly encourage anyone who hasn't had a flu shot to get one and anyone experiencing flu-like symptoms to promptly consult with their health care provider about appropriate treatment options."

Circulating virus strains

Circulating virus strains included both B strains (Yamagata and Victoria), H1N1 and H3N2, according to the CDC. "It's an H3N2 year," Fauci said.

Historically, H3N2 strains have been "bad actors," he said, known to cause more complications, hospitalizations and deaths.

This virus strain is also a challenge to this year's flu vaccine. Researchers in Canada reported in a recent study that the vaccine proved only 10% effective against the H3N2 virus, though the flu shot provided more protection against other currently circulating strains.

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"News from Canada is similar to what Australia reported," Schuchat said, adding that the CDC's interim report on preliminary vaccine effectiveness will be published in a couple of weeks. "This is consistent with our concerns."

The hospitalization rate for the fourth week of 2018 is about 51 people per 100,000, higher than in the fourth week of the 2014-15 season, which recorded about 43 people hospitalized per 100,000. The 2014-15 flu season was considered "moderately severe" by the CDC and has been used as a comparison to the current season.

Dr. Dan Jernigan, director of the influenza division at the National Center for Immunization and Respiratory Diseases, said "lots of flu occurring nationwide simultaneously over several weeks -- this is an unusual pattern for flu in the US. This season is turning out to be a particularly challenging one."

The 2014-15 season had an estimated 710,000 hospitalizations by its end, he said. This season, cumulative hospitalizations rates in California are four times higher than at this time in 2014-15, while rates in Oregon and Minnesota are double those seen in 2014-15.

If this trend continues, the 2017-18 season "may well exceed" total hospitalizations during the 2014-15 season, Jernigan said.

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About half of the children who have been hospitalized this season for flu-related causes have no underlying conditions, and Jernigan said about half of the 53 children who died were "otherwise healthy."

The CDC has tracked pediatric deaths since 2004, he said, and "since that time, they've ranged from 37 to 171 during regular seasons; the highest was during the 2009 pandemic, when 358 pediatric deaths were reported."

The 2017-18 flu season "unquestionably falls into the bucket of a severe year," Fauci said.

"This year, 2017-18, for a strikingly long part of the season, completely parallels the 2014-15 year," he said. "Except that last week or the week before, 2014-15 started to plateau and turn around -- but 2017-18 continued to go up.

"We very well may start to see it peak and turn around," Fauci said of the current flu season. "I hope it does, because if it doesn't, it will be an even worse year than we're thinking."