The new strain of influenza K is highly contagious and the hospitals are busy: a widespread vaccination campaign

The K strain was first identified in the summer of 2025, and since then has spread rapidly in many countries, mainly in Europe and East Asia. In recent months it has become dominant in North America and Israel as well. According to the Centers for Disease Control, most of the samples taken this season belong to this strain, a relatively rare phenomenon at such an early stage of the season.

The influenza virus is an enveloped RNA virus, with a spherical or elliptical structure, equipped with two central proteins that stand out from its surface. One is hemoglotinin, which is responsible for the virus adhering to the cells of the respiratory system, and the other is noramidase, which helps release the new viruses from the infected cell.

It is the combination between these types of proteins that defines the subtype of the virus, for example H3N2. In strain K there were structural changes mainly in the hemoglobin region. These changes, even if they are tiny from a molecular point of view, can dramatically affect the way the immune system recognizes the virus, and its ability to infect humans.

Similar to other flu strains, strain K also passes mainly through respiratory droplets emitted by coughing, sneezing and talking. After entering the upper respiratory tract, the virus binds to the cells, penetrates them and takes over the cell mechanisms for its replication. The result is damage to the respiratory mucosa and an acute inflammatory reaction, manifested by high fever, muscle pain, weakness, headache and cough. Among populations at risk, the disease can be complicated by pneumonia, worsening of heart and lung diseases and even death.

Why is the K strain more contagious? The main hypothesis among researchers is that the mutations in strain K improve the ability of the virus to adhere to human cells, thereby increasing the chances of infection. In addition, it is possible that the changes allow him to partially evade antibodies created by previous vaccinations or infections in past seasons.

It is still unclear whether the strain causes an inherently more severe disease, or whether the increase in the number of severe cases is simply due to the fact that the number of infected is particularly high. Experts emphasize that both options are being examined at the same time, and that the full picture will become clear only later in the season.

Mutations, evolution and the pursuit of the vaccine

Influenza viruses are constantly changing. In every replication of the genetic material, mistakes may occur, some of which are meaningless and some of which give an advantage to the virus. This process, known as antigenic drift, is why the composition of the flu vaccine changes every year.

More rarely, a deep and rapid change occurs, a sharp antigenic change, which could ignite a global epidemic. In the case of strain K, it is a drift and not such a jump, but even a gradual change may be enough to damage the suitability of the vaccine.

The current season’s vaccine was developed months before the K strain was identified, and is therefore not perfectly compatible with it. Despite this, experts emphasize that it still provides significant protection against serious illness and death, even if it does not completely prevent infection.

In Israel, influenza activity began earlier than usual. During the last three weeks, four children died from flu complications, three of them were not vaccinated. The Ministry of Health emphasizes that currently there is no unusual increase in the severity of the disease in children in relation to the extent of the morbidity, but the data certainly turn on a red light.

According to data from the National Center for Disease Control, over 500 new hospitalizations with influenza A were reported in the general hospitals in just one week. Against this background, the Ministry of Health convened the epidemic treatment team and expanded its recommendations to the public.

The guidelines include a call to get vaccinated as soon as possible and a recommendation for at-risk populations, aged 65 and over and chronically ill, to consider wearing masks in closed places and in gatherings. It was also recommended that medical staff and visitors to geriatric institutions wear masks. At the same time, the health institutions were required to increase personnel in medical centers and internal departments, as part of the preparations for a busy winter.

Despite the call, vaccination rates are still below the target. Only about 15.7% of the population has been vaccinated so far, and among those aged 65 and over the rate is about 50%. The Ministry of Health emphasizes that the vaccine, even in the season of a changing strain, remains the most effective tool for reducing severe morbidity and protecting the health system.