Health authorities are reporting an premature onset of influenza activity in certain regions of the United States, prompting recommendations that residents prioritize getting their flu shots in October. While typical flu seasons in the U.S. begin to build momentum in late autumn and peak between December and February, this year’s viral circulation appears to be accelerating ahead of schedule.
Data from the Centers for Disease Control and Prevention (CDC) indicates that during the week ending September 26, 3.4% of respiratory specimens tested at clinical laboratories returned positive for influenza. This figure represents a sevenfold increase compared to the same period last year, when only about 0.5% of samples were positive. Although current overall activity remains low, the upward trend is significant.
“This particular year, it appears to be starting early — like right now,” Dr. William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt Health, told Live Science. He noted that patients at clinics in his area are already testing positive for the virus.
The early surge is predominantly concentrated in Western states, including Washington, California, and Arizona. In Washington, the proportion of emergency department visits diagnosed with influenza rose from 0.25% in the week ending August 29 to 1.34% by the week ending October 3, marking a more than fivefold increase over six weeks according to CDC emergency department data.
Dr. Schaffner clarified that an early start on the West Coast is not inherently alarming. “There’s nothing unusual about flu starting in the West Coast and moving east,” he explained, noting that while recent seasons have often begun in the Southeast or Midwest, the virus can initiate activity in almost any region.
H1N1 Takes the Lead
Seasonal influenza is driven by three primary virus subtypes: two strains of influenza A, designated H1N1 and H3N2, and the Victoria lineage of influenza B. Last season, a rapidly spreading variant of H3N2 known as subclade K was dominant. For the 2025-2026 season, the CDC has classified the outlook as highly severe for children, citing at least 195 pediatric deaths linked to flu-related causes.
Current data shows H1N1 leading the current wave. Of the influenza A samples subtyped by public health labs during the week ending September 26, 96% were identified as H1N1. Experts caution that this snapshot does not necessarily predict dominance throughout the entire season.
Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia, described influenza as a “moving target” due to the virus’s constant mutation, particularly in hemagglutinin, the surface protein targeted by vaccines. While influenza A strains typically spark early-season outbreaks and influenza B strains tend to persist toward the end of the season, the current predominance of H1N1 highlights the dynamic nature of viral circulation.
Vaccination Timing and Efficacy
The CDC recommends annual flu vaccination for everyone aged 6 months and older. High-risk groups, including children under 5, adults over 65, pregnant individuals, and those with chronic health conditions, face a higher risk of severe complications such as pneumonia, sepsis, and inflammation of the brain or heart.
Regarding timing, the CDC generally advises against getting vaccinated in July or August for most adults, as immunity may wane before the season peaks. However, given the early start, Dr. Schaffner emphasized that October is now the ideal window. “October is the ideal time to get vaccinated, and even more so right now,” he said.
Protection develops approximately two weeks after administration. The effectiveness of flu vaccines varies by season based on how well the formulation matches circulating strains. Over the past decade, the CDC estimates that flu shots have been 29% to 56% effective in preventing cases that require medical attention. While vaccines may be less reliable at blocking mild illness, they are highly effective at preventing severe outcomes requiring hospitalization or intensive care.
New mRNA Option Approved
In August, the FDA approved mFlusiva, the first mRNA flu vaccine, for adults aged 50 and older. Unlike traditional inactivated flu shots that contain purified virus pieces, mRNA vaccines provide genetic instructions for cells to produce hemagglutinin proteins from three flu subtypes. The mRNA breaks down in the body and does not interact with human DNA.
“With the mRNA shot, you’re less likely to get symptomatic disease,” Offit stated. The CDC’s 2026-2027 guidance continues to recommend annual vaccination for all eligible individuals, with high-dose and adjuvanted vaccines preferred for older adults.
As surveillance continues, experts note that an early start does not guarantee a severe season. The CDC’s outlook for the 2026-2027 season projects moderate severity, though confidence in this forecast remains low-to-moderate. Ongoing weekly reports will determine whether case numbers continue to climb beyond the Western states.
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