This post has been updated.
The CDC relies on several tracking and reporting systems to gauge the severity of seasonal flu, with one of the earliest indicators being the percentage of doctor visits for flu symptoms. That measure dropped significantly last week, the first such decline of the season. About 6.4 percent of all doctor visits were for fever, cough and other symptoms — down from 7.5 percent from the previous week.
CDC officials typically wait to see two consecutive weeks of decline before declaring that a flu season has peaked. But officials said Friday that the latest data suggest this season's peak may have already occurred.
"It has been a tough season so far this year, but this week we’re actually seeing visits to doctors’ offices, emergency departments, and outpatient clinics beginning to drop, so it looks like the peak of the season may actually be behind us," Daniel Jernigan, head of CDC's influenza division, said in a statement. Still, "we’re likely to see influenza continue to circulate until mid-April."
The number of states reporting high numbers of patients seeking treatment also fell last week. Though half of the country still has high levels of flu activity, the intensity has begun easing along the West Coast.
Data on hospitalization rates and deaths usually lag behind. Hospitalization rates are still on the rise: 74.5 hospitalizations for each 100,000 people last week. That is higher than the 51.7 rate for the same week during the especially severe 2014-2015 season, which officials use for comparisons for harsh winter flu seasons.
The past several months have been particularly harsh because the predominant flu strain, an influenza A virus known as H3N2, is the most deadly of the seasonal flu strains. It is associated with more complications, hospitalizations and deaths, especially among children, people older than 65 and those with certain chronic conditions. In a very bad flu season, the CDC estimates that influenza results in as many as 35 million illnesses, about 700,000 hospitalizations, and up to 56,000 deaths in the United States.
Officials have said the number of pediatric deaths this season could be as high or exceed the 148 deaths from the 2014-2015 season, when the same flu strain was predominant.
The number of reported children's deaths to date probably does not include all cases because of the typical lag time for reporting non-hospital deaths. States are not required to report adult deaths.
In the 2014-2015 season, the flu vaccine’s poor match resulted in an overall effectiveness of 19 percent. This season’s vaccine is almost twice as effective overall, at 36 percent, according to a midseason estimate by CDC. In children younger than 9, it actually offers much greater protection, reducing by more than half the risk of a child becoming so sick that he or she will need to see a doctor. Even with partial protection, the vaccine can reduce severity of illness, doctors say.
As in past winter flu seasons, about three-quarters of children who have died were not fully vaccinated, according to CDC data.
Experts say one reason healthy children are much more vulnerable has to do with how their immune system responds. For some who haven’t received a flu shot, infection with a flu strain they haven’t previously been exposed to can trigger their immune system to overreact. That can lead to widespread inflammation that is ultimately fatal.