After an early start to the flu season, various public health authorities, including the US Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO), have reported the detection of a new dominant flu strain. This strain, with the very technical-sounding name of influenza AH3N2 subclade K, is a variation of the usual seasonal flu strain AH3N2 with some new mutations. Unfortunately, based on anecdotal reports that some people felt much worse than usual, the new virus was quickly dubbed by the media as the “superflu.”

This had the effect of sensationalizing the news of a new strain. More than a few social media posts claiming this could be a new pandemic went viral (no pun intended). Even when scientists countered that it doesn’t seem to be very “super” at all, fake news continues to spread. What are the facts about influenza AH3N2 subclade K, and is there any reason to worry?

The influenza virus is a shapeshifter by nature. It comes in four antigenic variants: influenza A, B, C, and D. Two of these antigenic variants, specifically influenza A and B, cause most cases of flu in humans. Influenza A is responsible for previous pandemic outbreaks, including the notorious AH1N1 Spanish flu in 1918, which affected nearly 1/3 of the world’s population, and the more recent AH1N1 Swine flu pandemic in 2009.

The influenza virus genome consists of eight segments of single-stranded RNA. RNA viruses such as influenza, HIV, and SARS-CoV-2 are more prone to mutations compared to DNA viruses. Genetic changes in influenza A can happen through spontaneous mutations from these errors and also through another mechanism known as recombination.

Spontaneous mutations are usually introduced by errors in copying when viral RNA is duplicated. These mutations occur at random, and most of the mutations that occur are silent or might even be harmful to the virus. Occasionally, these mutations can affect the proteins on the viral surface and decrease the ability of our body to recognize the virus. This process is known as antigenic drift, and immune escape in this case is gradual.

Recombination occurs when two or more virus strains infect the same cell and end up mixing their different segments of RNA together. This can cause a more drastic genetic change, and the process is known as antigenic shift. Antigenic shift can occur between influenza A viruses from different host animals and give rise to a novel pandemic virus to which humans have little or no immunity.

For example, influenza A viruses that infect birds can cause more severe pneumonia in humans. It is, however, very difficult for humans to get infected with avian flu due to the less efficient binding of the virus with proteins in the upper airways of humans. If an avian influenza virus combines with a human influenza virus, it may result in a virus that causes more severe disease than regular human influenza and is more transmissible than avian flu. The 2009 AH1N1 swine flu was an antigenic shift phenomenon where genetic material from different swine influenza viruses got mixed together and resulted in a highly transmissible virus. While the 2009 AH1N1 influenza virus did not cause more severe disease, there was almost no immunity to it at the outset due to the radical change in its genetic signature, causing it to spread quickly.

The new influenza AH3N2 subclade K is believed to be the product of antigenic drift. It is a direct descendant of influenza AH3N2 subclade J.2.4 with added mutations, so it is technically also subclade J.2.4.1. The new mutations seem to decrease the effective immune response of the body against the virus based on animal serum studies. This may have given it a survival advantage that is consistent with the increasing numbers of influenza AH3N2 subclade K being detected as a proportion of the circulating influenza viruses. Based on early studies, these genetic changes don’t seem to have significantly affected the effectiveness of the current vaccines. The current flu season vaccine is still protecting against severe disease and hospitalization. There is also no evidence that influenza AH3N2 subclade K causes more severe disease when confirmed genotyped cases are compared to other circulating strains.

Just how much the new influenza AH3N2 subclade K mutations are enabling immune escape is not yet clear. There are a lot of confounding factors. Many parts of the world had an early start to the flu season. This means that the number of flu cases is peaking earlier. Therefore, when compared to the same time frame as previous years, the case counts for this year would be higher earlier in the season. Another major confounder is that, compared to other years, there has been very low flu vaccine uptake, which, as we would expect, results in more cases of flu and more severe symptoms in unvaccinated persons.

There is currently limited flu vaccine availability since the flu season is already well underway. The Southern Hemisphere vaccine, however, should be available by around February. We get availability for both Northern and Southern Hemisphere vaccines in the Philippines six months apart. We only need to get one vaccine shot once a year, so if you haven't gotten it in the last year, you can get it soon when it becomes available.

In the meantime, you can protect yourself and others by wearing a mask, especially if you are in the vulnerable population or if you are living with someone who is elderly or immunocompromised. Stay home if you have flu symptoms and talk to your doctor if you get sick. Proceed to the ER if you have warning signs, like shortness of breath or low oxygen levels (

The bottom line is that the “superflu” does not seem to be very super at all, and there is no reason to believe it will cause another pandemic. The observation, however, that fewer people are taking the yearly influenza vaccine is very troubling, and this will result in more cases of severe influenza and deaths from the virus. It is important to treat the emergence of influenza AH3N2 subclade K as a warning that the virus continues to mutate. We should use this opportunity to educate people about the importance of the yearly flu shot in keeping everyone healthy. As a bonus, a higher vaccination rate means fewer cases and fewer chances for the virus to mutate, and it might just head off the next influenza pandemic.