This year's flu season is already the most widespread on record since health officials began keeping track 13 years ago, and has already caused the deaths of more children than what normally would be expected at this time of the year, federal health officials have said.
During the third week of January, more people sought care for flulike illnesses than at any comparable period in nearly a decade, the Centers for Disease Control and Prevention's most recent weekly report shows. Seven children died in the week ending Jan. 20, bringing the total number of pediatric deaths this flu season to 37. Nearly 12,000 people have been hospitalized with laboratory-confirmed influenza since the season started Oct. 1. The flu activity indicators are notable for the sheer volume and intensity of flu that is taking place in most of the country at the same time.
Q: Is there something particularly bad about this year's flu strains?
This season, the predominant strain is also the nastiest, H3N2, which causes the worst outbreaks of the two influenza A viruses and two types of influenza B viruses that circulate among people and are responsible for seasonal flu epidemics each year. Seasons when H3N2 strain dominates are associated with more hospitalizations, more deaths and more illnesses. Those who are particularly hard hit are the very young, the elderly and people with certain chronic health conditions, experts say. As top CDC flu expert Daniel Jernigan said: “Of the viruses we hate, we hate H3N2 more than the other ones.” This strain, which has been around for 50 years, is able to change more quickly to get around the human body’s immune system than the other viruses targeted in this year’s seasonal flu vaccine.
Q: How effective is this year's vaccine?
The CDC and clinicians recommend a yearly flu vaccine for everyone 6 months of age and older as the first and most important step in protecting against this serious disease. The body takes about two weeks to produce a full immune response. Flu vaccination can reduce illness and prevent flu-related hospitalizations. A study in Pediatrics last year was the first of its kind to show that vaccination lowered the risk of flu-associated death by half among children with underlying high-risk medical conditions and by nearly two-thirds among healthy children. But flu vaccines aren't perfect. Even in a good year, the flu vaccine isn’t as good as most other vaccines. Health officials must choose the influenza strains that vaccine makers should target for an upcoming season months in advance, when it is hard to know what strains might be circulating. When flu vaccines are well matched to circulating viruses, effectiveness is, at best, about 60 percent. (The measles vaccine, by comparison, is about 97 percent effective with two doses.) In a year when the circulating flu strains closely match the vaccine, that effectiveness rate means that about 3 in 5 people who get shots are far less likely to become so sick that they require a visit to a doctor.
In general, flu vaccines tend to work better against influenza B and influenza A (H1N1) viruses and offer lower protection against influenza A (H3N2) viruses.
Q: How long will this flu season last, and when will it peak?
Flu is very unpredictable. The exact timing and duration can vary, but influenza activity often begins to increase in October. Most of the time, flu activity peaks between December and February, although activity can last as late as May. Experts aren't sure when this season will peak. Officials were hoping the season had peaked in the first week of January, but the latest data shows activity continued to increase into the third week. Even if flu activity peaks soon, there will probably be many more weeks of flu activity based on patterns of similar seasons when this particular strain has dominated. In those seasons, flu season lasted for at least 11 to 13 more weeks after activity hit the peak. The overall hospitalization rate for the week ending Jan. 20 is now 41.9 per 100,000 people, up from 31.5 per 100,000 the week before.
Q: What does flu do to the body, and why do some otherwise healthy people die?
The virus causes damage all by itself, but the immune response it triggers, especially among young, healthy adults, also makes people sick and can lead to death. After entering the body, often inhaled through the nose or mouth, the influenza virus begins hijacking human cells in the nose and throat to make copies of itself. The virus implants in the mucous membranes at the back of the throat and upper part of the bronchial tubes. In doing so, it weakens or destroys the feather-like protective cells in the bronchial tubes that push out bacteria and other material, “like so many little oars,” said William Schaffner, an infectious disease expert at Vanderbilt University.
Without that protection, bacteria — including the kind that causes pneumonia — can work their way down the bronchial tube and into the lungs, causing complicating bacterial pneumonia, he said.
Influenza virus can also trigger such a massive immune response, especially in adolescents and younger adults with robust immune systems, that these otherwise healthy people are basically drowning in their own immune cells and proteins, said Stinchfield at Children's Minnesota. Or, as Schaffner put it, the overwhelming immune response “can make your whole body’s metabolism go awry. It’s like an army set off bombs and killed all the good stuff while targeting the virus.”