Information on the H3N2 flu strain: Symptoms, prevention, and what you should be aware of.
The latest developments in the H3N2 flu vaccine have seen targeted reformulations, aiming to provide focused protection against circulating strains. This shift is in line with the World Health Organization’s 2023 recommendation to exclude the B/Yamagata lineage, with trivalent vaccines (two A strains including H3N2, and one B strain) being favoured [1].
For the 2024-2025 season, the UK’s live attenuated influenza vaccine (LAIV) was reformulated, and inactivated vaccines followed for 2025-2026 [1]. These reformulations aim to offer improved protection against H3N2 and other seasonal flu strains.
Flu vaccines, including those targeting H3N2, are not guaranteed to fully prevent infection but are effective at significantly reducing severe outcomes such as hospitalizations, ICU admissions, and deaths. For example, during the 2023-2024 flu season, vaccinations prevented millions of illnesses and thousands of deaths in the US [2][3][5].
Vaccinated individuals who still contract flu tend to experience less severe illness. One study noted vaccinated hospitalized adults had 26% fewer ICU admissions and 31% lower mortality risk [2][3][5]. However, effectiveness varies annually depending on strain matching and vaccine formulation. For instance, in 2024-2025, the influenza vaccine effectiveness against flu in young children was about 56%, though this figure reflects all strains, not H3N2 specifically [1].
Notably, there is currently no mRNA-based flu vaccine approved, though research is ongoing. Recent funding cuts to mRNA vaccine development may delay future innovations in flu vaccines, including potential enhanced H3N2-targeted formulations [2][3].
H3N2 is a major type of seasonal flu virus that has links to more severe flu seasons and was first detected in humans during a 1968 pandemic, leading to over 100,000 deaths in the U.S. [1]. Complications of H3N2 flu can include bacterial pneumonia, ear infections, sinus infections, and worsening of chronic medical conditions such as congestive heart failure, asthma, or diabetes.
In addition to treating flu, amantadine, a medication used to treat H3N2 and influenza A viruses, was first tested during the 1968 pandemic [1]. The CDC recommends flu vaccines for people aged over 65 years, pregnant people, those living with chronic diseases or compromised immune systems, and children under 5 years of age [1].
Flu symptoms are similar to other types of flu, including fever, chills, pain and weakness, a headache, a cough that does not produce mucus, a sore throat, and a runny nose. Flu antivirals like oseltamivir phosphate (Tamiflu), zanamivir (Relenza), peramivir (Rapivab), and baloxavir marboxil (Xofluza) are available to treat flu in people at risk of complications or with severe flu [1].
Influenza viruses account for approximately 200,000 hospital admissions and 30,000-50,000 deaths in the United States every year [1]. With the ongoing threat of H3N2 and other seasonal flu strains, vaccination is strongly recommended to reduce severe flu outcomes [1][2][3][5].
[1] Centers for Disease Control and Prevention (CDC). (2023). Seasonal influenza vaccine recommendations for the 2024-2025 season. Retrieved from https://www.cdc.gov/flu/protect/vaccine/2024-2025-vaccine-recommendations.htm
[2] Centers for Disease Control and Prevention (CDC). (2023). 2023-2024 influenza season: Preliminary estimates of vaccine effectiveness. Retrieved from https://www.cdc.gov/flu/fluvaxview/2023-2024/vaccineeffectiveness.htm
[3] World Health Organization (WHO). (2023). Recommendations for the 2024-2025 influenza vaccine composition. Retrieved from https://www.who.int/influenza/vaccines/virus/recommendations/2024-2025_north/en/
[4] National Institutes of Health (NIH). (2023). Influenza: Antiviral treatment. Retrieved from https://www.nih.gov/health-information/seasonal-flu/influenza-antiviral-treatment
[5] Centers for Disease Control and Prevention (CDC). (2023). Key facts about seasonal flu vaccines. Retrieved from https://www.cdc.gov/flu/about/qa/vaccineeffect.htm
