A flu epidemic in the United States is more severe than usual, and 29 children have died of complications, said health authorities.
With the nation only about halfway through the season, complications are likely to worsen for those who caught the flu, said Tom Frieden, chief of the Centers for Disease Control and Prevention.
"We expect to see both the number and rates of hospitalization and deaths to rise further," Frieden told reporters.
The CDC said 30 states and New York City are reporting high influenza rates, up from 24 states last week. And more than 5,000 people have required hospitalization to treat their flu symptoms. New York has declared a state of emergency over the crisis.
Widespread geographic flu activity was also reported in 48 states for the week ending January 12, up from 47 states the previous week.
Nationwide, influenza rates dropped slightly to 4.6 percent, down from 4.8 percent the previous week.
There is no national reporting system for flu-related deaths among adults, but the CDC said that 8.3 percent of deaths reported through the 122 Cities Mortality Reporting System were due to pneumonia and influenza.
That exceeds the epidemic threshold of 7.2 percent. The rate of flu- and pneumonia-linked deaths the week before was 7.3 percent.
Nine children died last week alone, bringing the total to 29 since the season began in early December. The flu kills an average of about a hundred children in the United States each year. The toll was 34 in 2011-2012.
"It's shaping up to be a worse than average season and a bad season particularly for the elderly," Frieden said, stressing that there was still time to get vaccinated and that early treatment was "critically important."
He predicted that hospitalizations and deaths would continue to rise as the flu epidemic spreads further.
The severity of the symptoms this year may be explained by the season experiencing a dominant strain of influenza A(H3N2), historically blamed for more serious cases of the virus.
So far, about half of confirmed flu cases concern people aged 65 and older, with a high hospitalization rate of 82 per 100,000.
People older than 65 usually account for about 90 percent of the 36,000 annual flu deaths around the country.
Flu strikes every year across the United States, bringing chills, fever, coughing and achy misery to millions.
Health officials said the flu vaccine is a good match for the strain of influenza circulating around the nation, and confers about 62 percent protection against the illness this season.
The CDC recommends that everyone older than six months get vaccinated, particularly those who are at risk for serious complications, such as babies, senior citizens, pregnant women and those with chronic health issues including asthma, diabetes or lung disease.
In addition to being inoculated against the flu, health officials recommend such basic prophylactic measures as frequent hand washing, and coughing or sneezing into one's sleeve to keep the illness at bay.