The CDC's latest weekly report shows that the percentage of doctor visits for fever, cough and other flu symptoms was the same during the week of Feb. 4-10 as the week before — about 1 in every 13 visits.
The figures are based on updated data showing the percentage of Americans seeking medical care for such symptoms to be just slightly lower than peak levels during the 2009-10 swine flu pandemic. The week ending last Saturday was the first one in which that indicator — a key early flag of flu activity — did not increase. Data on hospitalization rates and deaths usually lag behind.
At the end of last week, the hospitalization rate for flu was almost 68 people per 100,000, higher than the 64.2 per 100,000 in the especially severe 2014-15 season. The same flu strain predominant then is circulating now, and this influenza A virus, known as H3N2, is the most deadly of the seasonal flu strains.
The last several months have been particularly harsh because of H3N2, which is associated with more complications, hospitalizations and deaths, especially among children, people older than 65 and those with certain chronic conditions.
It’s not clear whether this flu season, which started early and has remained intense, has hit its peak. Officials and experts say that even after the worst is over, several more weeks of flu activity should be expected, even into spring.
“I think that this year, we’re going to have flu in February, well into March and poke a foot or two into April,” said William Schaffner, an infectious disease specialist at Vanderbilt University, which is part of the CDC’s flu surveillance network.
Children’s vulnerability is of special concern this season. As in past winter flu seasons, about three-quarters of children who have died were not fully vaccinated, and about half were previously healthy, officials said. And the number of reported pediatric deaths 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.
Based on current trends, the number of child deaths could exceed the 148 reported during the 2014-15 season, when the vaccine’s poor match resulted in an overall effectiveness of 19 percent. This season’s flu vaccine is almost twice as effective overall, at 36 percent, according to a midseason estimate released Thursday by CDC. In children younger than 9, the vaccine actually offers much greater protection, reducing by more than half the risk of becoming so sick that they need to see a doctor.
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, Schaffner said.
“We’re sending in troops to fight against the viral invaders, but we send in too many troops,” he said. As a result, “you get collateral damage.”
A study published this week found that in young healthy mice with no exposure to the virus, the immune system overreacted to the influenza virus, which led to more inflammation, greater lung damage and increased mortality compared with healthy mice exposed to the virus. The authors said the evidence reverses earlier assumptions that the young are more susceptible to the flu because their immune systems don’t mount a strong enough response.
The finding underscores why flu vaccination is so important for children, health officials and experts say. Even though the vaccine is not perfect, it still provides some protection.
“I would rather use a partially effective vaccine that offers a degree of prevention than getting no vaccine — that just exposes us completely to more hospitalizations and more deaths,” Schaffner said. “I can’t imagine being the parent of a child who dies of influenza when that child was not vaccinated,” he added. “You’re going to live forever with the thought, ‘Could it have made a difference?’ ”