How the WHO Selects Influenza Vaccine Strains: What the Latest Recommendations Mean for You

What Did the WHO Recommend for the Upcoming Flu Season?

Each year, the WHO convenes expert consultations to review influenza surveillance data collected from its Global Influenza Surveillance and Response System (GISRS), a network spanning over 140 countries. Based on which influenza A and B virus strains are circulating most widely and causing the most illness, the WHO recommends specific strains to include in seasonal vaccines. These recommendations are issued twice annually — once for the northern hemisphere (typically in February or March) and once for the southern hemisphere (typically in September).

The latest announcement addresses the composition for vaccines to be manufactured and distributed ahead of the northern hemisphere's flu season, which generally peaks between October and March. Vaccine manufacturers rely on these recommendations to begin production months in advance, making the timing of the WHO's guidance critical to ensuring adequate global supply. The recommendations cover both egg-based and cell- or recombinant-based vaccine production platforms, as viral characteristics can differ depending on the manufacturing method.

How Does the WHO Decide Which Flu Strains to Include in Vaccines?

The strain selection process is one of the most complex exercises in preventive medicine. Scientists at WHO Collaborating Centres — including major laboratories in London, Atlanta, Melbourne, Tokyo, and Beijing — continuously collect and analyze influenza virus samples from around the world. They assess the antigenic properties of circulating viruses, meaning how well the viruses are recognized by antibodies generated from previous vaccines or infections. When surveillance data shows significant antigenic drift — mutations that allow the virus to partially evade existing immunity — the WHO may recommend updating one or more vaccine components.

The process also considers epidemiological data on disease burden, the geographic spread of different virus subtypes, and the performance of the current season's vaccine. Despite this rigorous process, influenza's rapid mutation rate means vaccine effectiveness can vary from season to season, typically ranging from approximately 40% to 60% in well-matched years according to CDC estimates. Nonetheless, vaccination remains the single most effective public health intervention for reducing flu-related hospitalizations and deaths.

Why Is Annual Flu Vaccination Still Important?

Unlike some vaccines that confer long-lasting immunity, influenza vaccines require annual updates because the virus undergoes frequent genetic changes. This phenomenon, known as antigenic drift, can render last year's vaccine less effective against this year's dominant strains. The WHO estimates that seasonal influenza causes approximately 3 to 5 million cases of severe illness worldwide each year, with up to 650,000 respiratory deaths annually. Older adults, young children, pregnant individuals, and people with chronic medical conditions face the highest risk of serious complications.

Public health authorities including the CDC and the European Centre for Disease Prevention and Control (ECDC) recommend annual vaccination for all individuals aged six months and older, with particular emphasis on high-risk groups and healthcare workers. Research consistently shows that even in seasons where the vaccine is imperfectly matched to circulating strains, vaccination reduces the severity of illness and the risk of hospitalization. Efforts are also underway to develop universal influenza vaccines that would provide broader, longer-lasting protection against multiple strains, though these remain in clinical development.