H1N1, commonly known as swine flu, is rising across several parts of India. Delhi has been particularly affected, reporting over 2,300 cases, nearly eight times the number recorded during the same period last year. Bengaluru, Mumbai and several other regions have also reported a surge in H1N1 cases. This respiratory illness can spread from person to person. Given the current spike in cases, one question arises: Can a person contract H1N1 twice?

"Yes, you can get H1N1 more than once. Having influenza A (H1N1) infection does not guarantee lifelong protection. The immune response generated after infection is strongest against the virus or closely related strains that caused the illness, but influenza viruses continually undergo genetic changes," says Dr Pankaj Soni, Principal Director - Internal Medicine, Max Smart Super Speciality Hospital, Saket.

How can H1N1 infect you twice?

While contracting the identical genetic strain within a single flu season is rare because your body builds immediate immunity, you can be reinfected in subsequent years as the virus undergoes antigenic drift (mutates). You can also get sick back-to-back within the same season if you are exposed to a different strain of Influenza A or B.

"One important reason for repeat infection is antigenic drift-small changes in the virus's surface proteins, particularly hemagglutinin (HA) and neuraminidase (NA). These changes can make a later strain different enough that antibodies from a previous infection do not provide complete protection. Antigenic drift is considered a major reason people can develop influenza repeatedly over their lifetime," Dr Soni further explains.

He also explains how influenza can infect an individual more than once in the same flu season. "For example, a person who has recovered from H1N1 could subsequently become infected with another influenza virus, such as H3N2 or influenza B. Immunity against one influenza subtype provides limited or no protection against other influenza types or subtypes," he adds.

However, not every second episode of fever, cough and body ache is necessarily a second H1N1 infection. "Other respiratory viruses including Cvoid-19, RSV and rhinoviruses can cause very similar symptoms. Testing may therefore be necessary when the diagnosis is clinically important."

Also read: H1N1 Is Spreading Across India: How Long Are You Actually Contagious For?

The role of vaccination

"Another common misconception is that previous H1N1 infection means vaccination is unnecessary. Influenza viruses change continuously, and protection from previous infection or vaccination can decline with time."

"Annual influenza vaccination remains the best preventive strategy, particularly for people at higher risk of complications, including older adults, pregnant women, young children and people with chronic medical conditions," he explains.

Is reinfection more dangerous?

Reinfection is generally not more dangerous for most healthy individuals. Your body retains memory B cells and T cells from the first infection. Even if the virus has mutated slightly, this "immune memory" helps your body respond faster. However, a second infection can become dangerous if you have a weakened immune system, or if you fall into a high-risk category including pregnant women, young children, older adults, or those with chronic conditions like asthma or diabetes.

"While a previous H1N1 infection may provide meaningful protection, it should not be considered permanent immunity. A new infection is possible, especially when circulating viruses have changed, or immunity has waned," Dr. Soni concludes.



Disclaimer: This content including advice provides generic information only. It is in no way a substitute for a qualified medical opinion. Always consult a specialist or your own doctor for more information. NDTV does not claim responsibility for this information.