Force of Infection

Influenza

While national influenza activity remains low overall, transmission is accelerating, particularly in Western states.

As regular readers know, there are multiple metrics that I use to track disease activity. For influenza, that set includes outpatient influenza-like illness (which is the proportion of visits to the doctor that are for fever and cough or sore throat), emergency department visits, wastewater concentration, and test positivity. 

Of these, test positivity shows the clearest increase. At a national level, test positivity has risen to 3.4%, up from a recent low of 0.4%. For context, this is still pretty low—peak season can exceed 30% test positivity. (That is a human-written em-dash!)

At the regional level, though, notable differences emerge. HHS Region 9, which includes California, Arizona, Hawaii, and Nevada, is reporting a test positivity of 7.1%, a marked increase over 3.4% reported just two weeks ago. HHS Region 10, which includes Alaska, Idaho, Oregon, and Washington, is at 6.0% (a slight decrease from last week's 6.5%). Both of these regions have low-but-rapidly-increasing emergency department visits for influenza, particularly in children. 

Other regions of the country are beginning to see increases as well, though the overall trends are more recent and less dramatic at present.

From these observations, I conclude that influenza is picking up steam in the West and that other regions of the country should expect activity to begin to pick up in the near term.

Outpatient influenza-like illness, which is the metric I report on most frequently here, is less dramatic. Current activity at the national level is 1.7%, which is well below the baseline of 3.1% that marks the start of flu season. The Western region is highest at 2.0%, though still low. It's not clear to me why outpatient ILI is not reflecting the increased activity evident in test positivity and ED visits. Leave a comment or send me an email if you have ideas!

As always, outpatient influenza-like illness is highest in children ages 0 to 4, currently at 5.5%, up from a recent low of 3.2%. People ages 5 to 24 are also seeing increases, now at 2.6%. Older age groups are all at or below 1.5%. Looking at more severe illness as measured by emergency department visits, the most affected age group is children ages 1 to 4, followed by children ages 5 to 17. Activity has been rising week over week for the last month or so in those age groups.

Nearly all (96.4%) of specimens subtyped were H1N1pdm09. However, only 55 samples were subtyped, so there is not a lot to go on. This season's flu vaccine formulation does include an H1N1 strain, which was selected based on viruses circulating last season. How well it matches will not be clear until later in the season.

COVID-19

National Trend

Covid-19 activity continues to decline slowly. Test positivity at the national level has declined slightly to 4.9%, down from a recent high of 5.7% in late August. Emergency department visits for Covid-19 are stable at 0.5%. And according to CDC's Center for Forecasting and Outbreak Analytics, Covid-19 is stable or declining in most Southern states (broadly defined), while states in north and New England have yet to turn the corner.

All age groups have seen improvements except people 65 and older, in whom activity is stable. Children less than one year old remain most affected, followed by children ages 1-4, as measured by visits to the emergency department for Covid-19.

South

It's that time of year again. The fourth annual regional editions of Outbreak Outlook launch October 18.

Respiratory virus activity varies a lot from place to place. Already this fall, flu activity is increasing in the West while most of the country remains low. The regional editions track and report these differences state by state, so you can make more tailored decisions based on local trends.

Here is what you need to know about opting in:

What is Outbreak Outlook? I send out a weekly report on infectious disease activity, including respiratory viruses, plus food recalls. These updates are useful for older adults, parents, people with weakened immune systems, and anyone looking to stay well.

During the winter months, the national weekly update is free for everyone. Paid subscribers have the option to receive more detailed updates tailored to the Northeast, Midwest, South, and West regions (links

A reader wrote to draw my attention to several indicators of early flu activity in the West. I took a second look at the data, and I have to revise my earlier assessment that an early flu season wasn't likely.

Pacific states are seeing a rapid rise in test positivity for influenza. This pattern is somewhat softened by unremarkable outpatient data, but the test positivity is striking. Take, for instance, this snapshot of influenza test positivity in HHS Region 9 covering California, Nevada, Arizona and Hawaii. Influenza A test positivity, shown by the yellow line, has risen sharply.

However, one important note of caution: test positivity can be misleading this early in the season, because not many specimens are tested compared to, say, December. For instance, in the region shown above, only 241 positive tests were reported during the most recent reporting week. During peak season, it'

Influenza-like illness

Still not much happening. Influenza-like illness activity (which is visits to the doctor for fever and cough or sore throat, also known as ILI) is at 1.6%, about the same as the week prior. Emergency department (ED) visits for influenza are drifting up, but remain below 0.5%.

Current activity is pretty typical for this time of year. I don’t expect a particularly early season, which means there is no immediate hurry to get vaccinated. Before Halloween should be adequate; I’ll let you know if my recommendation on timing changes.

The South has seen a slight decrease in activity, from a recent high of 1.8% to 1.7%. However, I do note that Florida has rising levels of influenza activity as measured by outpatient ILI, ED visits, and hospitalizations. I'll keep an eye on this in the weeks ahead. A few other states are also a bit high. In the Northeast, New Jersey stands out with 2.7% outpatient ILI and in the West, Alaska is reporting rising activity across several indicators.

As always, young kids have the highest levels of activity. Outpatient ILI has risen steadily in children ages 0-4 since mid-August. People ages 5-24 have seen some relief following an initial back-to-school rise. Adults are in pretty good shape.

COVID-19

National Trend

Covid-19 activity has peaked for most regions of the country and is now declining slowly. Around 0.5% of visits to the emergency department are for Covid-19, down from a Labor Day peak of 0.6%. Wastewater concentration is jumping around a bit at the regional level, but is mostly flat. Hospitalizations are holding steady and test positivity is declining.

As I shared in my recent State of the newsletter update, I want to spend more time on immunocompromised (IC) readers, who are often left out or brushed aside in public health updates. So, I put together an IC guide to seasonal vaccinations using information from reputable sources. Most of it summarizes 2025 guidance developed by the Infectious Diseases Society of America, a professional organization of infectious disease doctors. Other sources are linked within. This post is free for everyone, so please share.

Covid-19

Bottom line: Get vaccinated every year. For people who are immunocompromised, a second dose (given at least two months after the first) is likely to make protection last longer. You do not need to show documentation to prove you are eligible for a second dose; self-

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