Flu season has arrived in the Philadelphia region before Halloween. CBS News Philadelphia health reporter Stephanie Stahl has been tracking the early activity.
The early timing lines up with what's happening across the country. According to NBC Philadelphia, the 2026-2027 flu season appears to have started earlier and more strongly than usual, with infections already climbing as early as August. As of October 21, flu infections were growing or likely growing in 44 states, per CDC estimates cited by the outlet, and early-season activity for the week ending October 17 was higher than typically seen at that point in the year.
Flu-like illnesses are continuing to surge across the Philadelphia area, according to CBS News Philadelphia. Flu can be difficult to distinguish from COVID-19, and testing is important.
H1N1 and a New Mutation Driving the Surge
Influenza A H1N1 is fueling the vast majority of flu cases reported nationwide so far this season, the NBC Philadelphia report notes. The percentage of flu tests coming back positive reached 5.7% during the week ending October 17, compared with less than 1% during the same stretch a year earlier. A mutation known as G155E, identified in September in Seattle, may be helping the H1N1 subclade evade existing immunity, though the extent of that effect has not been established.
National data paint a similar picture. Per Scientific American, the CDC's October 9 FluView report showed flu activity climbing since early September, with a national test positivity rate of 3.6% — six times higher than the same point in 2025. CDC data cited in that report found H1N1 accounted for 98.4% of positive influenza tests, with H3N2 making up just 1.6%. One CDC surveillance snapshot, however, described seasonal flu activity as increasing in many parts of the country while still characterizing overall activity as low, according to the CDC's Week 41 FluView report.
Pennsylvania Cases Spike Nearly Fourfold
The surge shows up clearly in state numbers. Pennsylvania reported 807 flu cases from October 4-10, a 386% jump from the 166 cases logged during the same week in 2025, according to Axios. State health officials told the outlet that flu activity remains low overall but is increasing rapidly, and flu-related emergency department visits were up year over year.
Axios also reported that health experts are urging anyone who hasn't gotten a flu shot yet to do so, given how much earlier the virus is spreading in Pennsylvania and nationwide. That advice echoes what's playing out region by region: as reported by CNN, the U.S. flu season started roughly 2.5 months earlier than typical, hitting West Coast states first before spreading eastward, with increases now showing up in every region of the country.
Hospitalizations at Highest Point in 16 Years
The scale of this season's early start is drawing comparisons to past outbreaks. Bloomberg reported that the percentage of positive influenza tests nationwide soared tenfold in the week ending October 10 compared with the same week a year earlier, adding that it may only be October, but flu season is already in full swing.
Hospitalization data underscore how unusual this stretch has been. The estimated U.S. flu hospitalization rate for the week ending October 17 was the highest for that point in the season since the 2010-11 flu season, according to Becker's Hospital Review. Even so, Scientific American's report noted that experts say it's too early to predict whether these early spikes will translate into a particularly severe winter flu season overall.
