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As flu activity continued to increase across the U.S. in December, the CDC said the proportion of deaths attributed to pneumonia and influenza had reached “epidemic threshold.”

The statement in the Centers for Disease Control and Prevention’s weekly flu report, released Monday, described flu activity as “high” in half the country.

California and Hawaii were suffering less badly than other states, with only “sporadic” flu activity, according to the CDC report for the week ending Dec. 20.

Most of the country had “widespread” flu activity, especially along the East Coast and in the Midwest.

The four pediatric deaths reported in the week ending Dec. 20, 2014, brought the total of child death associated with flu to 15 this season, the CDC said.

The percentage of all deaths in the CDC’s reporting system that was connected to pneumonia and influenza — 6.8 percent — “was at the epidemic threshold” for that week, the agency stated.

The CDC recommends everyone over 6 months of age receive a flu vaccine.