H1N1 influenza is making headlines again as cases rise in Delhi and Bengaluru. Delhi has reported 1,777 H1N1 cases up to August 20, 2026, with H1N1 accounting for most of the city's reported Influenza A infections this year. Importantly, the Indian Council of Medical Research (ICMR) has said the circulating virus is not a new H1N1 strain and matches the vaccine strain recommended for the current Northern Hemisphere season. 

Bengaluru, meanwhile, has seen notable clusters of influenza-like illness, including an outbreak at a school where more than 500 students and 21 staff members reported symptoms and two students tested positive for H1N1. Karnataka Health Minister U T Khader has assured the public that there was no need for panic over H1N1 in the state, while confirming that there are currently around 300 to 400 active cases in the state, with a majority reported in Bengaluru.

So, is H1N1 actually worse in Bengaluru than Delhi?

Not necessarily. The available numbers do not support a simple city-versus-city comparison, particularly because surveillance, testing and seasonal patterns differ. But city-based experts suggest that Bengaluru may have a longer window of influenza transmission.

"Bangalore weather has always been more prone for respiratory viral infections and flu. Every year we have a lot of cases, and the weather continues to be like that through the winters also," said Dr Aishwarya R, Consultant - Infectious Diseases, Aster Whitefield Hospitals, Bengaluru.

Her explanation offers an important clue to why Bengaluru's flu season can behave differently from Delhi's.

Also Read: Delhi H1N1 Surge: Doctors Explain Why Cases Are Rising, What Is Different This Season

Why H1N1 may linger longer in Bengaluru

According to Dr Aishwarya, influenza transmission in Bengaluru can continue through the winter, while Delhi's colder conditions can alter the pattern of transmission. "What begins around June sort of lingers on till January and February. Because of that, the number of cases continue to rise," she said.

However, this should not be interpreted as a universal rule that influenza "stops" in Delhi after November. India's influenza surveillance data show that the country can experience more than one seasonal peak, and local weather, population behaviour, crowding and virus circulation all influence transmission. The Delhi government has also noted that seasonal influenza generally shows peaks during January-March and the post-monsoon period.

The World Health Organisation (WHO) notes that influenza spreads rapidly in seasonal epidemics and that its activity varies geographically and over time. So, while Bengaluru's climate and year-round population movement may contribute to prolonged transmission, case counts alone cannot establish that H1N1 is more severe there than in Delhi.

What does H1N1 look like in Bengaluru right now?

The symptoms being seen by Dr Aishwarya are broadly consistent with influenza. "Symptoms are typically a dry hacking cough, sore throat, and this may be followed by muscle pains, fever, back pains, generalized weakness," she said.

She has also noticed more patients reporting nausea this time, although gastrointestinal symptoms can occur with influenza and are not specific to H1N1. WHO describes seasonal influenza as typically involving sudden fever, aching muscles, headache, severe malaise, non-productive cough and sore throat.

Another feature Dr Aishwarya is seeing is household clustering. "You'll have one member with an infection, within five days you'll have somebody else with symptoms starting off," she said.

That is unsurprising given influenza's contagious nature. The virus spreads primarily through respiratory particles when infected people cough, sneeze or talk, making prolonged close contact particularly relevant.

H1N1 vs H3N2: Which flu is circulating?

The dominant influenza subtype can change from one season or period to another. "In the first half of the year and through last year also we had H3N2. But now, at the moment, we're having more of just H1N1 transmission everywhere," Dr Aishwarya said.

Delhi has similarly seen a shift in its dominant subtype: H3N2 was predominant in 2025, while H1N1 is predominant in 2026, according to Delhi government data reported by NDTV. This is one reason influenza surveillance matters. Influenza viruses continually evolve, and WHO updates vaccine recommendations twice a year based on global surveillance. For the 2026-27 Northern Hemisphere season, WHO has recommended an updated H1N1 component, reflecting the need to keep vaccines aligned with circulating viruses.

Who is most likely to develop complications?

For most healthy people, influenza is self-limiting. But the infection can become serious when it affects people with underlying medical conditions. "The number one [comorbidity] that tops the list is those with pre-existing lung conditions," Dr Aishwarya said. She specifically highlighted people with asthma, chronic bronchitis, previous lung damage, pulmonary fibrosis and interstitial lung disease.

Other high-risk groups include people with chronic heart, kidney and liver disease. WHO also identifies older adults, young children, pregnant women and people with certain underlying health conditions as being at higher risk of influenza-related complications.

In severe cases, influenza can progress to pneumonia, hypoxaemia and acute respiratory distress syndrome (ARDS). Dr Aishwarya said her hospital has seen a small number of patients with influenza-related ARDS, although she stressed that the numbers are low.

When does H1N1 require hospitalisation?

Persistent or high-grade fever is one reason to seek medical assessment, particularly when it continues beyond several days. "Quite often [patients come in] with continuous fevers, high-grade fevers every four to six hours, fevers beyond three to four days sometimes, and oxygen requirements," Dr Aishwarya said.

Increasing oxygen requirements, significant breathing difficulty, chest pain, confusion or deterioration in an individual with underlying disease should not be managed as an ordinary cold or flu at home. Most H1N1 patients recover with supportive care, but pneumonia and falling oxygen levels can signal severe disease and require urgent medical evaluation.

Why early antiviral treatment matters

One of the important differences between influenza and many other viral respiratory infections is that specific antiviral treatment is available. Dr Aishwarya said patients who test positive for influenza can benefit from starting antiviral therapy early. "The earlier you start the antiviral is when it works better," she said.

WHO's 2024 clinical practice guideline for influenza covers antiviral treatment for both severe and non-severe influenza and includes guidance on preventive antiviral use after recent exposure.

The commonly used influenza antiviral oseltamivir should be taken only under medical advice. Treatment decisions depend on the patient's age, risk factors, illness severity and clinical assessment.

Dr Aishwarya also cautioned against an extremely common response to sore throat and fever: reaching for an antibiotic. "Azithromycin is an antibiotic. It will only work on bacteria, not on viruses," she said. Unnecessary antibiotic use does not treat influenza and contributes to antimicrobial resistance. Antibiotics should therefore not be self-started simply because someone develops a sore throat or fever.

Can vaccination help?

Yes. Influenza vaccination remains one of the key tools for preventing severe influenza and reducing the risk of influenza illness. "Vaccination definitely decreases the complications that can come out of influenza, like requiring ICU admissions and ARDS and oxygen," Dr Aishwarya said.

However, vaccine effectiveness is not a fixed number. It varies by season, age group, vaccine type and how closely the vaccine viruses match those circulating. Studies have found moderate protection against laboratory-confirmed influenza, including H1N1, while recent prospective data also demonstrate reduced symptomatic influenza among vaccinated people.

And because influenza viruses evolve, vaccination needs to be updated regularly. WHO's current 2026-27 Northern Hemisphere recommendation includes an updated H1N1 component.

Also Read: H1N1 Outbreak At Bengaluru School, 500+ Students Fall Sick: What Parents Should Know

Bengaluru vs Delhi: What should you actually worry about?

The evidence does not show that Bengaluru's H1N1 infections are inherently more dangerous than Delhi's. What may be different is when and how long transmission occurs.

For Bengaluru, prolonged influenza activity can mean a longer period of exposure. For Delhi, the current surge is significant but ICMR has found no evidence of a new H1N1 strain driving it. The sensible response in both cities is the same: recognise symptoms early, avoid unnecessary antibiotic use, protect people at high risk, consider annual influenza vaccination where medically appropriate and seek care promptly if symptoms worsen.

H1N1 may be an old virus, but influenza has a habit of refusing to become yesterday's news. Bengaluru's current situation should not be framed as a competition with Delhi over which city has "worse" swine flu. The more useful question is who is at risk, how severe are the infections and how quickly are patients receiving appropriate care?

For most people, H1N1 will remain a self-limiting respiratory infection. But for someone with asthma, chronic lung disease, heart or kidney problems, or for an older adult, young child or pregnant woman, influenza deserves more respect.

And if influenza is suspected, the message from infectious-disease specialists is clear: don't wait several days hoping it will disappear before seeking medical advice. Early assessment can make a difference.



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.