Influenza has officially reached epidemic proportions in the United States.
According to the US Centers for Disease Control and Prevention, 7.3% of deaths last week were caused by pneumonia and the flu.
That is just above the epidemic threshold of 7.2%.
Nine of the ten US regions had "elevated" flu activity, confirming that seasonal flu has spread across the country and reached high levels several weeks before the usual time of late January or February.
Compared to last year's mild flu season, doctors' offices and hospitals have been overwhelmed with flu cases.
Patients have been waiting through the night to be seen in emergency departments.
In Illinois, 24 hospitals struggling to cope with the flood of flu cases had to turn away people arriving in the emergency department.
In Pennsylvania, the Lehigh Valley Hospital outside Allentown has set up a tent for people who arrive with less-severe flu.
In Boston, flu cases are ten times higher than they were last year, causing Mayor Thomas Menino to declare a public health emergency on Wednesday.
This year's flu vaccine is 62% effective, scientists reported in the CDC's weekly publication.
That is considered "moderate" effectiveness and means that almost four in ten people who receive the vaccine and are exposed to the virus will nevertheless become infected.
It is line with the effectiveness of previous years' flu vaccines, which typically range from 50% to 70%.
Tens of thousands of Americans die every year from flu, even in non-epidemic years.
Experts recommend the vaccine for everyone over six months of age.
Even if it does not prevent flu, immunisation can reduce the severity of the illness, preventing pneumonia and other life-threatening results of flu.