Why Flu Season Came Early, With H1N1 Now Leading in October

H1N1 leads early surveillance

Scientists had forecast H3N2, also called “Super-K,” as the dominant US strain. But early West Coast monitoring found H1N1 had moved ahead of H3N2 in October, while the CDC reported an increase in flu-related hospitalizations.

H1N1 is also known as swine flu and caused a pandemic in 2009. Researchers have associated early flu emergency-room visits with the G155E mutation, which changes how the virus attaches to and infects cells.

Cases rise in new places

Flu infections began increasing in August, rather than following the usual rise around fall. Early cases were concentrated on the West Coast, including Arizona, Nevada and California; Hawaii and Louisiana had reached moderate levels.

The US flu season typically starts in the South. Each year, scientists use information from Southern Hemisphere countries, where flu seasons occur earlier, to help forecast which viruses may circulate in the United States.

Some immunity may remain

Past research suggests antibodies from earlier H1N1 infections or vaccinations may not recognize the mutated virus as well, particularly among younger people. This raises the possibility that the virus could evade some existing immunity.

Vaccines and reported side effects

Adults 50 and older are eligible for mFLUSIVA, an mRNA flu vaccine. A phase 3 clinical trial found it more effective than a standard vaccine at preventing illness from influenza A and B subtypes.

The trial also found a greater chance of severe side effects. Participants more often reported injection-site pain, fatigue, headaches and muscle weakness. Vaccination remains the recommended way to prepare for flu season, and there is still time to get vaccinated.