This flu season is on track to be one of the worst in 15 years, with nearly 12,000 people hospitalized so far, federal health officials reported Friday.
The latest weekly report from the Centers for Disease Control and Prevention shows that the season, which started early and hit many states simultaneously, continues to blanket the country.
The virus has killed at least 37 children so far, with seven deaths reported in the week ending last Saturday. By the end of this season, officials said the pediatric death toll is likely to approach, if not exceed, the 148 deaths reported during the 2014-2015 flu season. The influenza currently circulating is the same virulent strain that predominated three years ago.
It takes time for pediatric deaths to be reported because deaths outside of a hospital have to be reported to the medical examiner or coroner. As a result, the number of pediatric deaths reported so far is likely to be an underestimate. The real number may be twice as high, officials said.
As of Saturday, flu activity was reported as high, if not extreme, in 39 states plus Puerto Rico and New York City.
"It's a tough flu season," said Daniel Jernigan, who heads the CDC's influenza division.
More people are seeking care for flulike illness than at any time since the 2009 swine flu pandemic that swept the country. If that pandemic season is not included for comparison, the last time the country experienced such high levels of influenza-like illness was in 2003-2004.
The vast majority of current flu illness is from a particularly nasty strain of virus known as H3N2, which is associated with severe illness in young children and people 65 and older. But compared with previous seasons when this strain also dominated, officials are seeing two notable differences.
First, flu hit almost all the states at the same time, Jernigan said. Second, "flu activity has stayed at the same national level for three weeks in a row. We often see different parts of the country light up at different times, but there is lots of flu all at the same time."
The percentage of people visiting clinics and hospital emergency rooms and doctor's offices is at the highest level of the season and only outstripped by numbers during the pandemic of 2009-2010 and the seasonal flu season of 2003-2004, Jernigan said.
The rapid increase in cases came right after the winter holidays, he noted, and likely was triggered by children returning to school and spreading the virus.
Some parts of the country were slammed more than others.
Flu activity in California is starting to decrease, but the state has been one of the hardest hit since the beginning of the season. California's hospitalization rate this season is four times higher than it was for the same period in 2014-15, one of the most severe flu seasons where H3N2 was also the predominant strain, Jernigan said.
Even after flu activity decreases, there is a lag time for reporting on hospitalization and death rates.
Minnesota is also experiencing hospitalization rates about double what officials reported in 2014-15; hospitalization rates are also starting to increase in New York, he said.
Flu seasons are notoriously hard to predict. Officials look for clues based on patterns in past seasons when the same strains of flu have dominated. In previous H3N2 seasons, flu activity remained active for an average of about 16 weeks, and in some cases, as long as 20 weeks.
"By that measure, we are about halfway there," Jernigan said, referring to the current flu season. "But it means we have several more weeks of flu to go."
By the end of the 2014-2015 flu season, CDC officials estimate there were 34 million people sickened by the flu, including 16 million who went to the doctor or hospital emergency department, and about 710,000 people who were hospitalized for flu-like illness. During that season, officials estimate there were 56,000 flu-related deaths.
Officials expect this season's flu indicators to closely follow what happened in 2014-15.
But already, there have been surprises.
People over 65 are the group with the highest hospitalization rates. But this season, officials were surprised to see that the group with the second-highest hospitalization rate are baby boomers — those between age 50 to 64 — a change from the last several flu seasons. In most flu seasons, it's usually children under four who are the next hardest hit group.
But this season, "baby boomers have higher rates of hospitalization than their grandchildren right now," he said.
It's not clear why flu is sending so many baby boomers to the hospital. One possibility may be the mix of viruses circulating this season, and the different levels of immunity that people have developed to those viruses over time.
Besides H3N2, the two other flu strains causing illness are H1N1, an influenza A strain that caused the 2009-2010 swine flu pandemic but is now a regular human flu virus, and an influenza B strain.
H1N1 viruses don't tend to be as bad for the elderly--those over 65-- because those individuals were most likely exposed as children and have built up immunity; for the same reason, those viruses tend to be worse for non-elderly adults, such as the baby boomers.
There is twice as much H1N1 virus causing disease in the boomers this season, as compared to those over age 65, Jernigan said. Vaccination rates for baby boomers are also lower than immunization rates for those 65 and older.
"These are folks who would really benefit from higher vaccination rates," he said. "They're usually at the peak of their careers, or managing a lot of business, and them missing work because of flu would have a huge impact."
It is not too late to get a flu shot, clinicians and officials said. They note that H1N1 virus is now also active in parts of the country already hit by H3N2. Influenza B virus is also showing up, and that virus tends to become more active later in a flu seasons.
This year's vaccine protects against all of those strains; it is least effective against the H3N2 strain, but its effectiveness against the other two strains is much higher.
The CDC recommends an injectable flu vaccine for everyone 6 months or older as soon as possible because the body takes about two weeks to produce a full immune response.