'Why did I catch the flu even after taking the vaccine?' Doctor explains

You got the flu shot but still caught the flu? A doctor explains why vaccines don't always offer full protection, yet remain your best defence against severe illness and hospitalisation

You read you should get the annual flu shot, and this year, you decided to get one, ticked all the right health boxes, but still ended up sniffling under a blanket. It sounds unfair. But before you dismiss the vaccine as useless, we asked a doctor to explain what’s really going on inside the immune system, and whether catching the flu post-jab means the vaccine failed.

Why do some people still get the flu after being vaccinated?

According to Dr Arvind K Minz, Clinical Director–Internal Medicine, Marengo Asia Hospitals, Faridabad, the flu vaccine is like a best-guess security system. Every year, scientists predict which flu strains are most likely to show up during the upcoming season and build the vaccine accordingly. But the flu virus mutates rapidly.

 

So, if you are exposed to a strain that is slightly different from what the vaccine was designed for, your immune system might not have the perfect match of antibodies ready.

 

“Even then,” says Dr Minz, “the vaccine ensures the illness is much milder and protects against serious complications, hospitalisation, or death.”

 

“Scientists make their best predictions based on global surveillance, but if the virus mutates or shifts patterns, protection levels can dip,” he explains.

 

Still, even partial protection helps. Vaccinated people usually have shorter, milder illnesses and are less likely to spread the virus to colleagues or family.

Could I be mistaking another viral infection for the flu?

Possible. Not every fever and cough equals influenza. Other viruses like RSV (respiratory syncytial virus), rhinovirus, or even some coronaviruses can cause flu-like symptoms.

 

“The flu vaccine only protects against influenza viruses,” says Dr Minz. “So, if you have caught something else, the vaccine won’t help with that.”

How do scientists decide which flu strains go into the vaccine each year?

Dr Minz says twice a year, global experts track circulating flu strains across hemispheres. They study which ones are spreading, how they are mutating, and which are most likely to dominate the next season.

 

Based on this data, the World Health Organization (WHO) recommends three or four strains to include in the seasonal vaccine.

What factors influence how well the vaccine works?

According to Dr Minz, age, health conditions, and immunity levels all play a role. Older adults, infants, or people with chronic illnesses often generate fewer antibodies after vaccination.

 

“Healthy adults and children tend to develop more robust protection,” says Dr Minz. “Even your past exposure to flu viruses can affect how your body responds to a new vaccine.”

When should you get the flu shot?

Getting vaccinated too early might mean your immunity wanes by the end of flu season. Getting it too late, and you might be unprotected when the virus strikes.

 

Dr Minz recommends the sweet spot: “Ideally, get your flu shot two to four weeks before flu season starts — usually between September and November in India.”

 

But if you have missed that window, late protection is still better than none.

 

People who have been vaccinated often recover faster, have fewer complications, and are less likely to land in hospital or intensive care.

Is natural immunity better than vaccine protection?

Natural infection might sound like a shortcut to immunity, but it is not a safer one.

 

“While natural infection provides strong immunity against that particular strain, the flu virus mutates so quickly that protection fades by the next season,” says Dr Minz.

 

The vaccine, meanwhile, offers broader and safer immunity, without the fever, fatigue, and missed work days.

What should you do if you get the flu despite being vaccinated?

“Stay hydrated, get plenty of sleep, and manage fever with paracetamol or ibuprofen if needed,” advises Dr Minz.

 

If symptoms hit hard, antiviral medication prescribed within 48 hours can shorten your illness. “And yes, isolate yourself to protect others, even if your version is mild,” he says. 

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This content is for informational purposes only and is not a substitute for professional medical advice.