This flu season has been one of the worst.  

Estimated flu cases in New York State (2025)

• The New York State Department of Health reported a record number of influenza cases during the week ending December 20, 2025: 71,123 positive flu cases in that single week alone, the highest weekly total on record since reporting began in 2004. 
• Cumulative reporting from state health data suggests about 189,000 total positive flu cases so far this season in New York State through that same mid-December period.

These figures reflect confirmed positive tests reported to the state and aren’t adjusted for estimated total illnesses (which are usually higher when accounting for mild/unreported cases).

 Flu-related deaths in New York State (2025)

• As of the latest public reporting, New York State has not yet released a consolidated statewide total for flu-associated deaths in the 2025-26 season publicly.

• National CDC data estimates ~3,100 flu deaths in the entire U.S. as of late December 2025.
  
  

• How do we prepare flu vaccines?

Global flu surveillance (year-round)

Influenza viruses circulate somewhere in the world all the time. A World Health Organization (WHO) network called GISRS (Global Influenza Surveillance and Response System) monitors this through:

• 140-plus national influenza centers in over 110 countries
• Thousands of virus samples collected from sick patients
• Continuous tracking of which flu strains are spreading, where and how fast

This global view is critical because flu seasons move geographically—what circulates in the Southern Hemisphere often reaches the Northern Hemisphere months later.

 Identifying dominant and emerging strains

Scientists analyze the collected viruses to determine:

• Which influenza A subtypes are dominant
• Usually H1N1 and H3N2
• Which influenza B lineages are circulating
• B/Victoria
• B/Yamagata (largely disappeared in recent years)

They also look for:

• Genetic mutations
• Whether current strains differ from last year’s vaccine strains
• How well antibodies from existing vaccines neutralize the new viruses

Vaccine strain selection (twice a year)

Because flu seasons differ by hemisphere, strain selection happens twice annually:

• February → for the Northern Hemisphere (U.S., Europe, etc.)
• September → for the Southern Hemisphere

WHO convenes international experts who recommend specific strains to include.

Most vaccines are:

• Trivalent (3 strains):
• 2 influenza A strains (H1N1, H3N2)
• 1 influenza B strain
• Some are quadrivalent (4 strains), adding a second B strain

Why the match isn’t always perfect

Flu viruses mutate rapidly. Problems arise when:

• A strain mutates after vaccine selection
• A less common strain suddenly becomes dominant

Even in a mismatch year, vaccination still:

• Reduces illness severity
• Lowers hospitalization risk
• Decreases deaths, especially in older adults and medically vulnerable people

The current dominant flu strain

The current dominant flu variant circulating now (winter 2025-2026) is a mutated form of the influenza A H3N2 virus known as “subclade K.” This version has several genetic changes compared with earlier H3N2 strains and is driving most of the influenza cases being reported across the United States and other parts of the world right now. This mutation occurred after the vaccines were produced.

Why annual vaccination is necessary

Unlike diseases such as polio:

• Influenza immunity wanes quickly
• The virus changes every year
• Last year’s vaccine may offer little protection against this year’s strains

That’s why flu vaccines are updated annually rather than given once for life.

Key points:

• The circulating flu virus this season is Influenza A (H3N2), subclade K—often referred to in the media as a more contagious “super-flu” variant.
• It isn’t a new type of flu species, but a mutation (a subclade) within the H3N2 category that has become the predominant strain. 
• Health experts emphasize that the current flu vaccine still offers protection against severe illness, even if it isn’t a perfect match for subclade K. 

  What the vaccine still does

• Reduces the risk of severe illness (including hospitalization and death) even if the virus has drifted slightly from the vaccine strain. 
• Preliminary data from some countries suggest good protection against severe outcomes, particularly in children. 
• Antiviral treatments (like oseltamivir or baloxavir) remain effective as well if someone gets sick. 

Yes—getting the flu vaccine now still protects you this season, especially against serious outcomes like hospitalization and complications. 

• Because the virus has drifted a bit, you might still get infected, but vaccinated individuals are less likely to have severe disease. World Health Organization

Michael Kossove writes Prof. Mike’s Health Corner for the River Reporter. He is professor emeritus and an adjunct professor of microbiology at Touro University’s School of Health Sciences. He is a polio researcher, international polio lecturer and polio survivor, and lives in Sullivan County, NY.