A mutated strain of flu has sparked an unusually early wave of infections across the UK, and health experts warn it could become the country’s worst flu season in ten years. Health officials recently issued an urgent “flu jab SOS,” encouraging people to get vaccinated as soon as possible. NHS England reports that flu cases are already three times higher than this time last year, even though the peak of the season is still ahead. Getting vaccinated remains the most effective way to protect yourself from severe flu. Still, many people are puzzled when they catch the flu despite having taken the shot. Here’s why.The Flu Is Caused by Different Types of the Influenza VirusSeasonal flu can come from several strains of the influenza virus, primarily two main types:Influenza A – This is the most widespread type and spreads easily from person to person. The subtypes most often seen in humans are A(H1N1) and A(H3N2), according to the Centers for Disease Control and Prevention (CDC).Influenza B– Though less common, Type B also spreads among humans. It can still be serious, though it tends to be milder than Type A. Influenza B viruses fall into two main lineages: B/Yamagata and B/Victoria.The flu shot is designed to protect against both major Type A strains and at least one, sometimes both, Type B lineages. Because these viruses evolve quickly, scientists adjust the vaccine each year based on which strains have been circulating in other parts of the world during the previous six months.Why Did I Catch the Flu Even After Taking the Vaccine?Your age can strongly influence how well your body responds to the vaccine. Infants and young children are at higher risk because their immune systems are still developing and may not have produced enough antibodies to fight a new virus.Older adults, on the other hand, may have weaker immune systems due to age or existing health issues like diabetes, kidney or liver disease, or cancer. For this reason, the CDC recommends a higher-dose flu shot for adults over 65 to help boost their immune response.Flu Shot Takes Two Weeks to WorkAnother common reason people get sick even after vaccination is timing. The body needs time, usually about two weeks, to develop enough antibodies after getting the shot. If you are exposed to the flu virus during this window, before your immune system has fully responded, you could still fall ill.Could I Be Mistaking Another Viral Infection for the Flu?It’s possible. Not every cough, fever, or sore throat is the flu. Viruses like RSV (respiratory syncytial virus), rhinovirus, or some types of coronavirus can cause similar symptoms. The flu vaccine only targets influenza viruses, so it won’t protect you against these other infections.The Flu Virus Keeps ChangingUnlike many viruses, influenza mutates constantly. This ongoing change, called mutation, makes it a moving target for scientists. Researchers track global patterns to predict which strains are likely to dominate, and vaccines are then formulated accordingly.If the virus shifts after the vaccine has been made, the match between the vaccine and the circulating strain may not be perfect. Even then, vaccination still helps, reducing your risk of catching the flu and making the infection less severe if you do get sick.The flu vaccine isn’t a guarantee against every illness, but it remains your best defense against serious flu, hospitalisation, and complications. Even in years when the match isn’t exact, getting vaccinated greatly lowers the chance of severe disease and saves lives.
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