Hannah Bassett, MD, a clinical associate professor of pediatrics at Stanford University School of Medicine in Palo Alto, California, began seeing children admitted with influenza in late September. “I can’t recall another year seeing more than one-off patient be admitted with influenza this early,” she said.

National data tell the same story. In the week ending October 10, 5% of flu tests run by clinical labs came back positive, up from 3.9% the week before, according to the CDC’s first weekly flu report of the season. The West Coast is seeing the most activity, and nearly all of it is H1N1, which made up 96% of the influenza A viruses the public health labs subtyped. The CDC said activity is higher than usual for this time of the year, though still within normal range of seasonal variation.

Children are feeling it more. More children are turning up in the emergency departments with flu: The share of ED visits with a flu diagnosis rose among children and teenagers but held steady among adults. Respiratory viruses often start in younger people, said Alex Greninger, MD, who heads the Infectious Disease Diagnostics lab at the University of Washington’s Department of Laboratory Medicine and Pathology in Seattle — but the return to school could have helped this wave along, he added.

Hospital admission rates, though, remain highest among adults aged 65 years or older, followed by children younger than 5 years.

The basic clinical approach hasn’t changed in view of an early flu season, two experts told Medscape Medical News. But the early start may have caught many patients before their flu shot, putting more weight on testing and antiviral decisions that are rarely clear-cut.

Testing and Antivirals

This season’s H1N1 shouldn’t make clinicians doubt their rapid flu tests, Greninger said. Rapid tests target a conserved portion of the virus, so false negatives come down mostly to swabbing techniques and at what point in the illness the patient is tested. Molecular tests are widely available, Greninger said, but how much they are billed for can be a barrier.

In terms of treatment, the CDC advises treating high-risk patients as soon as possible, ideally within 2 days of symptom onset. For everyone else, it’s the clinician’s call.

A 2023 meta-analysis of 15 randomized trials with oseltamivir that included 6166 outpatients showed that the drug did not lower patients’ risk of being hospitalized. In fact, it caused more gastrointestinal side effects, mainly nausea and vomiting.

“That doesn’t mean that the medication doesn’t work,” said Emily McDonald, MD, MSc, senior author of the oseltamivir meta-analysis paper. A professor of medicine at the McGill University Health Centre in Montréal, Quebec, Canada, McDonald said the trial mostly enrolled younger, healthier adults and probably included a smaller number of older or high-risk patients to detect a difference. Greater benefits could be seen in patients who are older, have lung disease, or are immunosuppressed — but it’s just a hypothesis and unproven, McDonald noted. With healthier and younger patients, she suggested discussing the trade-off directly: about a day less of symptoms, against a real chance of nausea and vomiting.

In children, outpatient data with oseltamivir is robust, said Bassett. For children treated with oseltamivir within 40 hours of symptom onset, illness was shortened by roughly 24 hours on average.

The American Academy of Pediatrics and the CDC consider children younger than 5 years of age at higher risk, while noting that toddlers and infants younger than the age of 2 face the most severe risk. Guidelines call for treating every hospitalized child with influenza, but the evidence is not as strong on the inpatient side as it is on the outpatient side, Bassett explained. There are no comparable robust randomized controlled trials of antivirals in hospitalized children.

In inpatient children, Bassett said, clinicians weigh how long a child has been sick, how ill the child appears and the type of symptoms they have, a pattern her survey published in 2025 has documented. Based on her estimate, only 60%-65% of hospitalized children with influenza receive an antiviral — far below what the recommendations say, which is 100%. “They don’t want to give a medication to a patient if it’s not helping, but has a real possibility of hurting,” Bassett mentioned.

Though the side effects are real, they are still manageable in children, according to Bassett. She estimated that, for every 15-20 children given oseltamivir, one will vomit who otherwise wouldn’t have. Baloxavir, however, offers a practical alternative: a single dose instead of 10 with less nausea and vomiting. But baloxavir has not been studied in hospitalized patients. Also, its demonstrated benefit is mainly in patients treated early, Bassett said.

What Could Have Led to an Early Flu Season

Usually, flu cases in Pacific Northwest peak only during December or January. This year, Greninger’s lab in Seattle saw H1N1 cases climbing in late August and early September. By his estimate, Seattle is 2-2.5 months ahead of a typical season, with H1N1 activity now rising elsewhere in the country.

The likely driver is G155E, a hemagglutinin mutation that the lab of Jesse Bloom, PhD, of Fred Hutchinson Cancer Center in Seattle, had flagged as a likely gap in population immunity, Greninger said. About 75%-80% of recent H1N1 sequences carry it, he added.

“Lots of people are sort of vulnerable to this mutation. It just got an early start here,” Greninger said.

What that means for vaccine protection is unclear. When CDC tested H1N1 viruses against antibodies raised in ferrets against the vaccine strain, about 95% reacted well. Bloom’s study, which used human sera, found reduced neutralization of H1N1 strains carrying the G155E mutation, particularly in a subset of teenagers and young adults.

What to Watch

Clinicians need to stay alert after H1N1 subsides, Greninger said. “I would not be surprised to see a second spike,” he noted, possibly within about 2 months, while also emphasizing that it is difficult to predict it.

For McDonald, the variant matters less than the treatment timing. Antivirals work best when started within about 48 hours of symptom onset, but most patients seek care only around days 5-7. “It’s more about how we work on getting high-risk patients connected with treatment earlier,” she said.

Vaccination timing matters, too. Bassett and Greninger both worried that many people, expecting a December or January peak, have not yet been vaccinated. And Greninger’s advice to anyone not yet vaccinated: get the shot now.

McDonald cautioned against assuming the worst. “Just because it’s starting early, it doesn’t mean it’s going to be worse than usual,” she said.

Bassett recommended that clinicians track severity in their own region, not just case counts, using state or CDC data. If local hospitalizations and ICU admissions rise, she said, there may be more reasons to treat a wider range of children than in past years. Greninger said test positivity is a better gauge than case counts, because who gets tested changes over the season.

Whatever patients decide regarding treatment, Bassett and McDonald urged transparency. Bassett, in her own practice, tells families the evidence for shorter hospital stays limited, but that she is willing to try treatment, and that it can be stopped if their child starts to vomit.

“Tamiflu is not a cure-all; it has its limitations,” Bassett said. “It certainly does not replace the importance of influenza vaccination.”

Greninger reported receiving central testing contracts to the University of Washington from Cepheid to do blinded influenza testing for FDA authorization studies. McDonald reported that she is the chief medical officer of Sensifai Health. Bassett reported no relevant disclosures.