The 2026-27 U.S. flu season is off to an unusually early and geographically atypical start, with cases rising in early September 2026 across Western states like California and Washington rather than the South. The early surge is dominated by an H1N1 subtype carrying a G155E mutation that may evade existing immunity. Health officials are urging immediate October vaccinations, highlighting the availability of Moderna's newly approved mRNA-based vaccine, mFlusiva, for adults aged 50 and older.
Early rise in Western flu cases
- ▪The 2026-27 influenza season in the United States is showing an unusually early rise in cases, with activity picking up in early September 2026.
- ▪In Los Angeles County, California, local flu activity officially began rising on September 28, 2026, which is ahead of its typical early-November start.
- ▪The 2026-27 U.S. flu season began atypically in Western states, including California, Washington, Hawaii, and Alaska, whereas the virus traditionally starts in the South and Southeast.
- ▪The percentage of positive influenza tests and emergency department visits has been rising steadily since early September 2026, particularly on the U.S. West Coast.
Dominance of mutated H1N1 subtype
- ▪A specific H1N1 mutation designated G155E has been detected by researchers, which appears to make the virus better at evading existing immunity, particularly among teenagers and young adults.
- ▪The early 2026-27 U.S. influenza season is dominated by the H1N1 subtype, which accounted for 96% of positive cases subtyped during the week ending September 26, 2026.
Recommendations for annual flu vaccination
- ▪The U.S. Centers for Disease Control and Prevention continues to recommend annual flu vaccination for everyone aged 6 months and older, ideally by the end of October.
- ▪Health officials and epidemiologists are urging people to get vaccinated against the flu in October 2026, noting that it takes about two weeks for the vaccine to become fully effective.
Approval and efficacy of mFlusiva
- ▪A clinical study published in May 2026 indicated that the mRNA-based flu vaccine mFlusiva was up to 27% more effective in preventing flu than four currently available traditional flu shots.
- ▪The U.S. Food and Drug Administration approved mFlusiva, the first mRNA-based influenza vaccine produced by Moderna, in August 2026 for adults aged 50 and older.
Debatable claims
- ▪The Trump administration's efforts to overhaul the childhood immunization schedule undermine public health
- ▪Waiting until late October to get a flu shot is the most effective strategy for winter protection
- ▪mRNA-based flu vaccines are a superior alternative to traditional flu shots
- ▪The CDC's quietness on early flu vaccine promotion is a failure of public health communication
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