The situation in Israel fits into a broader global trend. According to the World Health Organization, high influenza activity is being recorded in the Northern Hemisphere, with positive test rates exceeding 10% in large areas of North and Central America, Europe, Africa, and Asia. In several areas, including Northern and Western Europe and Central America, the positivity rate has even exceeded 30%. Similarly, in the world, the dominant strain is K, as in Israel.
Behind the alarming picture of the spread of the flu, the heavy workload in clinics and hospitals, and the high mortality rate is a relatively new player in the flu arena, the K subtype of the H3N2 influenza A virus, which has already been identified by health authorities around the world as a major cause of the outbreak.
Strain K was first identified in the summer of 2025 and has since spread rapidly across many countries, particularly in Europe and East Asia. In recent months, it has also become dominant in North America and Israel. According to the Centers for Disease Control, most of the samples taken this season belong to this subtype, a relatively rare occurrence so early in the season.
The influenza virus is an enveloped RNA virus, with a spherical or elliptical structure, equipped with two central proteins protruding from its surface. One is hemagglutinin, which is responsible for the virus's attachment to respiratory system cells, and the other is neuraminidase, which helps release the new viruses from the infected cell. The combination of these types of proteins defines the subtype of the virus, for example H3N2. In the K strain, structural changes have occurred mainly in the hemagglutinin region. These changes, even if they are tiny from a molecular perspective, can dramatically affect the way the immune system recognizes the virus, and its ability to infect humans.
Like other influenza strains, strain K is transmitted primarily through respiratory droplets emitted by coughing, sneezing, and speaking. After entering the upper respiratory tract, the virus binds to cells, invades them, and takes over the cell's mechanisms for replication. The result is damage to the respiratory mucosa and an acute inflammatory response, manifested by high fever, muscle aches, weakness, headaches, and cough. In at-risk populations, the disease may be complicated by pneumonia, worsening of heart and lung diseases, and even death.
Strain K is thought to be more contagious because its mutations improve the virus's ability to attach to human cells, increasing the chances of infection. In addition, the changes may allow it to partially evade antibodies produced by previous vaccinations or infections in past seasons.
It is not yet clear whether the strain inherently causes more severe disease, or whether the increase in severe cases is simply due to the high number of infections. Experts emphasize that both possibilities are being examined simultaneously, and that the full picture will only become clearer later in the season.
Flu viruses are constantly changing. With each replication of the genetic material, mistakes can occur, some of which are insignificant and some of which give the virus an advantage. This process, known as antigenic drift, is why the composition of the flu vaccine changes every year. More rarely, a profound and rapid change occurs, a sharp antigenic shift that could ignite a global pandemic. In the case of the K strain, this is drift rather than such a jump, but even a gradual change may be enough to impair the vaccine's suitability.
This season's vaccine was developed months before strain K was identified, so it is not a perfect match for it. However, experts emphasize that it still provides significant protection against severe illness and death, even if it does not completely prevent infection.
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