The worst of the flu season is over, but increasing numbers of people are being infected by a secondary strain of the respiratory virus that could lead to a late-season bump, according to a federal health report released Friday.
The percentage of doctor visits for flu symptoms dropped for a second week in a row, according to the latest indicators from the Centers for Disease Control and Prevention. About 5 percent of all doctor visits were for fever, cough and other symptoms — down from 6.4 percent the previous week. But flu activity is still high and widespread nationwide.
“Today’s flu data show activity is down significantly for the second consecutive week, which means we peaked in early February,” said CDC spokeswoman Kristen Nordlund. “However, this is about the same level of influenza-like-illness that we saw at the peak of last season, so there is still a lot of flu out there.”
This season of misery, fevers, chills, muscle aches and overwhelming fatigue has been especially severe because of the predominant flu strain circulating. It’s an influenza A virus known as H3N2, the most deadly of the two influenza A viruses and two types of influenza B viruses that are responsible for seasonal flu epidemics each year.
H3N2 is associated with more complications, hospitalizations and deaths, especially among children, people older than 65 and those with certain chronic conditions.
The latest report shows the overall proportion of influenza A viruses is declining, but the proportion of influenza B viruses is increasing.
“Like previous seasons when H3N2 was dominant in the beginning of the season, we may see a late-season bump of influenza B,” Nordlund said in an email.
This season’s vaccine is about 36 percent effective overall, a mark that falls to 25 percent against the H3N2 strain, according to a midseason estimate by the CDC. The lower rate means that 1 in 4 people who get the shot reduce their risk of becoming sick enough to need to see a doctor.
In addition, the vaccine is 67 percent effective against the other influenza A strain, H1N1, and 42 percent effective against influenza B viruses, the CDC found. Clinicians have said it is not too late to get a flu shot, because there are several more weeks left in the season. Even partial protection can help to reduce the severity of illness and prevent hospitalizations and deaths.
In children younger than 9, the vaccine 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. As in past winter flu seasons, about three-fourths of children who have died were not fully vaccinated, according to CDC data.
Officials have said the number of pediatric deaths could be as high as or exceed the 148 deaths from the especially severe 2014-2015 season, when the same flu strain was predominant.
In a very bad flu season, such as this one, the CDC estimates that influenza results in as many as 35 million illnesses, about 700,000 hospitalizations, and as many as 56,000 deaths in the United States.
States are required to report individual flu deaths in children to the CDC and do so through a special influenza-associated pediatric mortality surveillance system. The number of reported flu-associated 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 individual seasonal flu cases or deaths of people older than 18 to the CDC for several reasons. Seasonal influenza is not often listed on death certificates of people who die of flu-related complications. Many seasonal flu-related deaths take place one or two weeks after a person’s initial infection, and most people who die of seasonal flu-related complications are not tested for flu, or they seek medical care when it’s too late in their illness for influenza to be detected from respiratory samples, according to the CDC.