Flu’s Clues

Working in the pediatric ED is a job I absolutely love—well, except between November and March. Every spring, we emerge from the trenches of winter respiratory season, and I somehow forget the endless parade of coughing, sore throats, and snot that dominates our halls.

This week alone, I’ve diagnosed more Influenza A cases than I did during the entire winter of 2020. With so many respiratory viruses in play—RSV, parainfluenza, adenovirus, rhinovirus, and our old nemesis COVID—how can we clue in that influenza is the culprit? Testing every febrile, coughing child isn’t practical, so here are my tips for diagnosing flu in kids with confidence.

Symptoms/Clinical Course

The “textbook” flu presentation includes fevers, sore throat, cough, rhinorrhea, headaches, and (helpfully) myalgias. Younger kids may also have GI symptoms like vomiting and diarrhea. Fever typically lasts five days, but treatment works best if started within the first 48 hours.

For preverbal kids who can’t describe their symptoms, older sick contacts can help. If family members report flu-like myalgias and headaches, you might have your clue.

Testing

Which test should you do?  

Rapid PCR tests—either alone or as part of a panel—are my go-to for confirmation. During high flu prevalence, you can often diagnose clinically if symptoms fit the textbook profile.

Who should you test?

According to AAP guidelines, prioritize testing for:

• Kids <5 years old or belonging to disproportionately affected groups (e.g., non-Hispanic Black, Latino, American Indian, Alaska Native).
• High-risk patients, including those with asthma, heart disease, immunodeficiency, neurodevelopmental or other chronic conditions like obesity (BMI ≥40 or weight 140% of 95%ile).

Treatment

Who should be treated?

The AAP recommends antiviral treatment for:

• All kids younger than 5 years (especially < 2 years).
• Any child hospitalized or with severe/progressive symptoms.
• Kids with high-risk conditions, regardless of disease severity.

You can also consider treatment for healthy outpatients within 48 hours of symptom onset or those with high-risk siblings or household contacts.

How to treat? 

Oseltamivir remains the primary antiviral recommended by the AAP, though adherence can be tricky with its twice-daily, five-day dosing.  It shortens illness by ½ to 1 day if started early.

• Age and weight-based dosing of oseltamivir (5 day course):
  • 2 weeks to less than 1 year: 3 mg/kg orally twice a day
  • For kids age 1-12 years:
    • Weight up to 15 kg: 30 mg orally twice a day
    • Weight 15.1 to 23 kg: 45 mg orally twice a day
    • Weight 23.1 to 40 kg: 60 mg orally twice a day
    • Weight at least 40.1 kg: 75 mg orally twice a day
  • 13 years or older: 75 mg orally twice a day

Baloxavir marboxil is a single-dose option approved for kids ≥ 5 years. It reduces symptoms by up to two days and lowers viral shedding. There are some limitations on its use- it can be used in kids 5-12 years without other medical conditions, but can be used in all children/teens 12 years. It should not be used in immunocompromised patients of any age.

Baloxovir dosing by weight

• < 20kg: 2mg/kg
• 20-80 kg: 40mg
• >80 kg: 80 mg

Other less used antivirals used against influenza that I have less experience using include: 

• Zanamivir is an inhaled antiviral used twice a day for 5 days. It should be avoided in kids with asthma or other respiratory diseases.
• Peramivir is an IV medication used mostly in hospitalized kids unable to take oral medication. It is safe to use in children 6 months and older.

Complications of Influenza

• Myositis/Rhabdomyolysis: Screen kids who “refuse to walk” with a urine dip for blood. If positive, check CK, BMP, and UA.
• Myocarditis/Pericarditis: If there’s chest pain or exercise intolerance, order a CXR, EKG, troponin, and BNP.
• Bacterial Superinfection: Look for focal crackles, hypoxia, or increased distress. If suspected, get a CXR and treat with antibiotics.
• Neurologic Symptoms: Be on alert for seizures, Guillain-Barré, or encephalitis, as flu is a neurotropic virus.

If it’s not already clear how I feel about influenza, you can hear me and a colleague sing about it in this month’s Peds RAP. We even wrote a little ditty to the tune of Jingle Bells—because nothing says holiday spirit like flu education. Sing along below, and happy holidays!

Flu is Hell (Sung to the tune of “Jingle Bells”)

Constant mucus flow

Headache & body aches

To the clinic we go

Coughing all the way (cough cough cough)

Throats they really sting

Makes us want to cry

It’s awful to get the flu

It’ll last at least 5 nights...

Oh! flu is hell, flu is hell

It can be a nidus

For pneumonia, meningitis

And even myocarditis

Os-el-tam-i-vir

In the first 2 days

Not a perfect cure

Sick for one less day

If you’re over 5

Try ba-lox-o-vir

One dose and you’re done

And the symptoms disappear

Oh! flu is hell, flu is hell

Could’ve prevented it

It’s not fun, to get the flu

So vaccinate against that 💩!