Reports of “super flu” abound just in time for the holidays. Cases of respiratory illness are increasing, with the Centers for Disease Control and Prevention (CDC) reporting 11 hospitalizations per 100,000 people. Meanwhile, across the Atlantic, according to British Health Minister West Streeting StandardThe NHS is facing the worst pressure since coronavirus.
Much of this is due to the emergence of a new type: subtype K. Subtype K appeared in the summer and has since become the dominant form of influenza in many countries, including Japan, the United Kingdom, and several US states.
What do we know about subclass K?
Influenza viruses can be broadly divided into four types – A, B, C, and D, with A and B viruses responsible for the seasonal epidemics that occur every year. Subclade K is influenza A virus. Specifically, it’s a version of the H3N2 subtype.
Influenza viruses naturally evolve over time as small mutations cause subtle changes in the genetic makeup of the virus, a process known as antigenic drift. Over the past year, a particular version of the H3N2 virus has acquired several new mutations that make it genetically distinct from previous versions of the virus, giving rise to a new subclass.
According to the World Health Organization (WHO) in a press release, “These subclass K viruses contain several changes from the related A(H3N2) viruses. Current epidemiological data do not indicate an increase in disease severity, although this subclass represents a marked evolution of influenza A(H3N2) viruses.”
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Why are experts concerned?
Although there is no indication that subtype K is more dangerous than other H3N2 viruses, experts have raised a few concerns — one related to vaccines, and another related to its subtype.
Because influenza viruses are constantly changing, the influenza vaccine should be updated every year. But the timing of the K subclass was particularly difficult. The vaccine was designed before subtype K emerged, leading to concerns that the vaccines may not be a “good match” and that immunization may not necessarily result in better protection against this strain of the virus, according to the CDC.
The evidence so far suggests that this is not necessarily the case. The effectiveness of the flu vaccine varies year to year, tending to be between 30 percent and 60 percent, according to Vaccine Alliance Gavi. The World Health Organization indicates that the effectiveness of the vaccine against the disease is unknown, but early evidence indicates that it reduces the probability of hospitalization.
Data collected in the UK from Eurosurveillance suggests that the vaccine reduces the chances of an emergency room visit by 72 percent to 75 percent in children. The same data suggests the vaccine is less effective in adults but still reduces emergency room visits by 32 percent to 39 percent.
The second concern is that subcategory K is an H3N2 virus. In general, these viruses can lead to more serious illness than those caused by other influenza viruses, resulting in a particularly severe flu season, with the very young and the elderly most at risk.
Subclass K effect
While some countries — including the United Kingdom and Japan — reported an unusually early flu season, the Centers for Disease Control and Prevention notes that the timing of flu activity in the United States is comparable to previous years.
But like many other countries in the Northern Hemisphere, subcategory K appears to be dominant. Recent data from the Centers for Disease Control and Prevention’s FluView report reveal that influenza A represented 98 percent of influenza cases reported by public health laboratories, of which 89.9 percent were of the H3N2 subgroup. Nine out of 10 H3N2 virus samples studied by the CDC are subtype K.
This article does not provide medical advice and should be used for informational purposes only.
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