After what has seemed like constant media scrutiny for months, influenza hasn’t been in the news as often in recent days. Still, CDC data indicate that flu activity remains “widespread” across the country, so it’s still too early to eliminate flu from your list of “differential diagnoses,” at work or at home.

Maybe it’s the general state of our national psyche, but this year the “flu” seems to have caused more than its usual share of anxiety. This is not a pandemic; there are no brand-new strains of flu in circulation to which no one is immune. But the H3N2 strain that has been predominant this year does tend to lead to a harsher-than-usual season. (The 2014-2015 season was also severe, but the public heard relatively little about it because media were focused on the Ebola outbreak.)

Influenza surveillance basics.

How do we know how bad a flu season really is? Since the 1997–1998 flu season, lab data and clinical reporting have facilitated real-time flu surveillance in the U.S. Public health laboratories in every state, in collaboration with National Respiratory and Enteric Virus Surveillance System laboratories, track the types (A or B) and subtypes (H3N2, H1N1, etc.) of flu in circulation.

To complement these data, the U.S. Outpatient Influenza-Like Illness Surveillance Network (ILINet) tracks the percentage of all patients each week who were diagnosed with influenza-like illness (ILI) at more than 3,000 medical offices, emergency departments, and outpatient clinics across the country. Not everyone with symptoms of ILI will be tested for flu (nor do they need to be); this information on probable cases in each region helps round out the picture provided by the data on laboratory-confirmed cases. Over time, national and regional baseline levels of influenza-like illness have been calculated, so public health officials now have a pretty good idea of just how intense (percent above baseline) and widespread influenza activity is at any given time.

The national baseline for flu activity is 2.2%; as of the week ending February 17, the percent of people visiting medical facilities for ILI was 6.4%. This is a decrease from the previous week and may indicate that we are finally past the peak of this season’s activity. This year, flu activity has peaked higher and later than in recent seasons, and this season is expected to continue for at least several more weeks. To learn about flu activity in your own state, go to http://gis.cdc.gov/grasp/fluview/fluportaldashboard.html.

What about pediatric deaths?

It has been mandatory since 2004 to report flu deaths in children to the CDC. A Pediatrics article published this month reported data on flu-related pediatric deaths in the U.S. for the flu seasons of 2011-2012 through 2015-2016. Deaths per season averaged 113, and ranged from 37 in the 2011-2012 season to 171 the following year. Half of these children had no prior medical problems. This year, as of February 17, 97 U.S. children had died from the flu. There is a lag time before pediatric flu deaths are confirmed, so this year’s number probably will be above the average of recent years.

Is vaccination worthwhile at this point?

Much has been made of the reported “10% effectiveness rate” of this year’s vaccine during the Southern Hemisphere flu season that precedes ours. This figure referred only to the vaccine’s effectiveness against the H3N2 strain; it was found to have been 50% effective against the H1N1 strain, and 57% effective against B strains.

Interim estimates of the vaccine’s effectiveness against severe influenza in the U.S. have just been published. Based on data from three months at the start of this season, those estimates are 25%, 67%, and 42% effectiveness against severe disease from H3N2, H1N1, and B strains, respectively. Among children aged six months to eight years, the risk for severe H3N2 influenza was reduced by 59%.

Right now, as at the end of most flu seasons, type B influenza is on the rise, so vaccination is still recommended for those who have not yet received it. Remember, even if you’re “healthy” and decline vaccination because you figure you’ll survive the flu, if you are infected, you will begin to shed the virus (and therefore be infectious to others) during the 24 hours before you have any symptoms. If you are in close contact with infants or others at high risk for flu complications, consider vaccination.

In general this year, the vaccine supply has been good. As needed, check https://vaccinefinder.org for sites near you where vaccine is still available.

Infection control basics

To prevent transmitting or acquiring influenza, emphasize the basics; they really do help. Cough and sneeze into a tissue or into the crook of your arm. Stay home when you’re sick, until you have been afebrile (without help from aspirin or acetaminophen!) for 24 hours. Avoid touching your eyes, nose, and mouth. Wash or alcohol-sanitize your hands often, and regularly clean and disinfect surfaces and objects.

How has this year’s flu season affected you? Leave us a comment.