H3N2 Flu in the US: Subclade K Variant Dominates Flu Season, Here's How to Prevent It

Liputan6.com, Jakarta - Influenza activity in the United States showed a rapid increase in December 2025, marking the start of the 2025-2026 flu season, dominated by the H3N2 strain.

The new variant, H3N2 subclade K, is of major concern due to its rapid spread and potential public health impact.

Recent data shows a significant spike in positive flu cases and hospitalizations across the country, reaching the third-highest level in 15 years.

With the United States having crossed the "epidemic threshold" in early December, it is estimated that millions of cases, tens of thousands of hospitalizations, and hundreds of deaths have already occurred due to flu this season.

In the week ending December 6, 2025, positive flu tests nationwide surged 8.1%, with the cumulative hospitalization rate reaching 6.9 per 100,000 population.

This figure reflects the aggressive spread of H3N2 flu across various regions.The majority of influenza viruses reported by public health laboratories, 86%, are H3N2.

Several states, including Colorado, New York, Louisiana, New Jersey, Connecticut, and Idaho, are reporting high rates of influenza-like illness (ILI).

In New York, for example, hospitalizations for respiratory illnesses are predominantly flu-related, with a dramatic increase in the past two weeks. This indicates that H3N2 flu is a serious health threat this winter.

A new H3N2 variant called H3N2 subclade K, formerly known as J.2.4.1, was first identified by the CDC in August 2025.

This variant has become the dominant strain in the United States and several other countries.

This virus has a small change in its hemagglutinin gene, which is antigenically "shifted" compared to the virus selected as the A(H3N2) component for the 2025-2026 US seasonal influenza vaccine.

The World Health Organization (WHO) has also noted an increase in influenza activity globally, with the proportion of influenza A(H3N2) viruses increasing.

The A(H3N2) subclade K has been spreading rapidly since August 2025.

While overall flu severity indicators remain low, some indicators are showing alarming increases.

The previous flu season (2024-2025) was a high-severity season, one of the highest ever experienced in the US.

H3N2-dominated flu seasons tend to affect older adults more severely, according to the U.S. Centers for Disease Control and Prevention.

Additionally, two influenza-related pediatric deaths have been reported through early December 2025.

The previous season recorded a record high of 288 pediatric deaths, making it the deadliest pediatric flu season outside of a pandemic.

Antigenic shifts in H3N2 subclade K suggest that vaccine effectiveness may be reduced against this variant.

However, vaccination still provides significant benefits, including protection against severe disease, hospitalization, and death from H3N2 flu.

Preliminary estimates from the UK indicate vaccine effectiveness against hospitalization is 70-75% for children and 30-40% for adults.

Therefore, the CDC recommends that everyone six months of age and older who has not been vaccinated get an annual flu shot.

Most cases of H3N2 flu can improve with supportive care such as adequate rest, hydration, and over-the-counter medications to reduce fever and pain.

Warm salt water gargles and steam inhalation can also provide relief.

For more severe cases or in high-risk groups, antiviral medications such as oseltamivir (Tamiflu), zanamivir (Relenza), and peramivir (Rapivab) may be prescribed.

These medications are most effective when taken within 48 hours of symptom onset, helping to shorten the duration of the illness and reduce the risk of complications.

Beyond treatment, prevention remains key. In addition to vaccination, maintaining hand hygiene, avoiding touching your face, and cleaning frequently touched surfaces are important steps to break the chain of H3N2 flu transmission.