H3N2 ‘Superflu’ Spreads Globally, Mexico Monitors Airports

A new winter concern looms over the Northern Hemisphere. Health authorities in the United Kingdom and the United States have issued alerts due to increasing influenza activity, specifically a subclade of the A(H3N2) virus known as the “superflu.” While Europe reports a rapid rise in infections, Mexico has activated surveillance protocols at key points to contain the possible arrival of the virus, known for its high transmissibility and mutation capacity.

What is the A(H3N2) Subclade K Virus?

The influenza A H3N2 variant virus, first identified in pigs in the U.S. in 2010, is a type A influenza variant considered among the most severe. According to the University of Guadalajara (U de G), when viruses circulating in pigs infect humans, they are called “variant viruses.”

The recent focus of concern centers on the H3N2 subclade K virus, also called genetic clade J.2.4.1. This virus possesses the M gene, a component identified in the pandemic H1N1 virus that increases human-to-human transmissibility, according to Mexican experts.

Experts from Care CHL hospitals in India explain that this virus demonstrates an “exceptional capacity to evolve through genetic reassortment,” allowing it to evade the host’s immune response. This rapid mutation rate leads to the emergence of antigenically novel viruses every two to five years.

The Crisis in Europe and the United States

On December 4, both the British and U.S. governments issued warnings about the increase in influenza activity. In the United Kingdom, it was reported that most subtyped infections belonged to the H3N2 subclade K virus. Meanwhile, the U.S. Centers for Disease Control and Prevention (CDC) detailed that A(H3N2) subclade K is the type of influenza that has circulated most in that country during the current cold season.

Data from week 48 of the year reveals the acceleration of cases:

International Epidemiological Surveillance Data (Week 48)

• United Kingdom (Gov.uk): Positivity for influenza increased slightly to 12.7% (compared to 10.9% the previous week). Of 4,154 samples, 529 were positive.
• Subtypes identified: 443 influenza A (unsubtyped), 81 A(H3N2), 5 type B, and 4 A(H1N1)pdm09.
• The majority of viruses sequenced to date belong to subclade K.
• United States (CDC): Of the 348 influenza viruses reported, 341 were type A and seven type B.
• Of the 280 subtyped type A viruses, 84.3% (236 cases) were influenza A(H3N2).
• The CDC identified subclade J.2.4.1 (known as H3N2 subclade K) in August 2025. Of 91 viruses collected since September 28, 2025, and subjected to genetic characterization, 86.8% belonged to subclade K.

In England, the fear centers on the high hospital occupancy recorded since early December. Daniel Elkeles, Chief Executive of NHS Providers, warned on December 9 that case numbers have reached a record for this time of year, increasing pressure on the NHS.

The Situation in Spain: An Early Start to the Epidemic

Spain is also on alert, reporting the early onset of the “superflu” epidemic, driven by subclade K, which has accelerated the transmission speed. However, Javier Millán, Vice President of the Spanish Society of Emergency Medicine (SEMES), indicates that more severe cases are not being generated.

As of December 5, the hospitalization rate for respiratory conditions was 15 cases per 100,000 inhabitants. 26% of these cases were caused by type A influenza (2% related to COVID-19 and 4% to respiratory syncytial virus or RSV).

The most critical situation is observed in:

• Catalonia: Incidence of 164 cases per 100,000 inhabitants, with predominance of the AH3N2 subtype and the K variant present in 58.3%.
• Navarre: Epidemic levels are at 116 cases per 100,000 inhabitants, with greater impact on children and adolescents.

Key Symptoms and Risk Groups

Influenza H3N2 causes symptoms common to other type A variants. The Pan American Health Organization (PAHO) warned in the past that H3N2 survived even during the COVID-19 pandemic, and there were patients with both viruses. Treatment can extend up to three weeks.

Characteristic Symptoms of the “Superflu”

• High fever (greater than 39° C).
• Cough (frequent, intense, and dry).
• Joint pain, body aches, and muscle pain.
• Severe fatigue and weakness.
• Headache and general malaise.
• Lack of appetite.
• Congested nose, sneezing.
• Chills and sweating.

In cases with complications, ear or chest infection may occur, and potentially progress to bacterial pneumonia.

Risk Groups

Because the disease requires “meticulous attention and proactive management,” the people at greatest risk are:

• Adults aged 65 or older and children under 5 years of age.
• Pregnant women.
• People with lung, heart, kidney, liver, or blood-related diseases.
• Those suffering from metabolic disorders or neurodevelopmental disorders.

Vaccination and Prevention Measures

The National Health Service (NHS) of England issued an urgent call for vaccination. The UK Government and the U.S. CDC agree that available vaccines continue to be effective.

Although H3N2 subclade K presents small changes in its hemagglutinin gene, influenza vaccines continue to offer crucial benefits:

• Protection against severe illness, hospitalization, and death.
• Protection against other circulating influenza viruses represented in flu vaccines.
• Helps reduce overall spread in the community.

Daniel Elkeles recommended the use of face masks at work and in public for those with symptoms, as well as staying home as much as possible. Other preventive measures include maintaining hygiene and frequent hand washing, avoiding contact with infected people, and disinfecting surfaces.

Epidemiological Surveillance at Mexico’s Airports

As of week 48, the Epidemiological Bulletin of the Ministry of Health (SSA) has not reported specific cases of subclade K in Mexico. However, Mexican authorities are already taking preventive measures.

Dr. Nadine Gasman, head of the Mexico City Ministry of Health (SEDESA), confirmed that a monitoring service is being implemented at the Mexico City International Airport (AICM) for infections such as H3N2 influenza.

Dr. Gasman detailed: “We have an international health service that monitors this type of infections and diseases at the airport.” She explained that there is a continuous operation to investigate, isolate, and care for people arriving with any type of illness or infection.

In addition to surveillance, the capital official assured that there is an active vaccination operation in terminals 1 and 2 of the AICM, as well as in bus stations, although so far material such as thermometers has not been implemented, as happened during the COVID-19 pandemic, which began in 2020. Dr. Gasman recalled that epidemiological surveillance at airports is vital, as “in the 2020 outbreak, it was at the airport where the first case was identified.”

The arrival of the H3N2 “superflu” in Europe and North America underscores the persistent threat of variant viruses. Although Mexico has not reported subclade K, the cold season and the increase in international travel demand maximum caution and raise the question: will surveillance and vaccination measures at the AICM be sufficient to contain the spread of a virus with such exceptional transmission capacity.