Flu is widespread in 43 US states, up from 36 states in the prior week, the US Centers for Disease Control and Prevention reported on Monday.
Six children died from the flu during the last full week in December, bringing the total flu deaths to 21 this season, the report showed.
Last week the CDC reported for the first time that deaths from flu and pneumonia reached an epidemic level, comprising 6.8% of all deaths. That figure slipped slightly below the epidemic level in this week's report, the CDC said.
"Last week was the first week that this particular number exceeded the epidemic threshold, but we've been in a flu epidemic for weeks now," said Dr. Michael Jhung, medical officer in the Centers for Disease Control and Prevention's Influenza Division.
The statistic is just one of many clues the CDC uses to gauge the severity of flu in the United States, which has a widespread outbreak - or an epidemic - every year.
Jhung reviews several factors when determining whether the flu season has started, suggesting the nation is in a period of epidemic. Initially, he considers the percentage of positive flu tests. When that exceeds 10% for 2 weeks, the season is starting. That happened around mid-November this season.
He also looks at the proportion of patients seeking care for influenza-related illnesses. When that exceeds 2% of visits, it is another sign of a flu epidemic.
This season, flu watchers are keenly focusing on hospital admissions because the current vaccine may not be a good match for the most common seasonal flu strain circulating in the United States known as influenza A (H3N2) virus.
Flu seasons dominated by H3N2 tend to have higher overall hospitalization rates and more flu-related deaths, especially among older people and very young children compared with flu seasons dominated by the H1N1 virus or influenza B viruses.
In the latest CDC report, flu hospitalizations, a key measure of severity, have reached 12.6 per 100,000 for all ages. Last season, the rate was 5.8 per 100,000 and in 2012, the most recent season dominated by H2N3, the rate was 8.1%.
"We are above that now," Jhung said, suggesting this flu season "is at least as severe as 2012 was."
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