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Delhi has recorded 1,777 H1N1 cases so far this year, including 114 new infections on August 20, as doctors across the capital report a sharp increase in patients with influenza and, in some cases, more complicated disease.
The rise is significant compared with the 229 H1N1 cases recorded during the corresponding period last year. H1N1 has also emerged as the predominant influenza subtype in Delhi this year. Of the 2,392 Influenza A cases reported till August 20, 1,777 were H1N1. The total number of Influenza A and B cases has reached 2,602.
The increase has raised concerns about whether a new or more dangerous strain is circulating. But the Indian Council of Medical Research has clarified that there is no new H1N1 strain in circulation and said there is no cause for panic. ICMR scientists said the H1N1 viruses currently circulating in India have been in circulation since 2025 and are well matched with the vaccine strains recommended for the Northern Hemisphere.
Doctors, however, say the sharp rise in infections is real and that the virus is finding a population that may have less immunity against the minor genetic changes that occur in influenza viruses.
'Virus is getting the upper hand'
At AIIMS Delhi, Professor of Medicine Dr Animesh says the current surge needs to be understood through the way influenza viruses naturally evolve.
"Influenza viruses are known to undergo variations in their genetic constitution. The virus circulating two or three years ago may have been different, and people would have developed immunity against that variant. With even minor changes in the virus, however, the present population can become susceptible again," he said.
He said the current clinical picture also appears different from what doctors were seeing a few months ago.
"There is a definite spike in cases compared with two or three months ago. I am seeing around 15 to 20 influenza cases every day in my OPD. Some patients are presenting with respiratory distress and pneumonia, while we are also seeing diarrhoea and complications such as kidney injury," Dr Animesh said.
Doctors are also seeing secondary infections in some patients after the initial influenza infection has subsided.
"For an infection to occur and spread, there has to be a combination of the agent, the host and the environment. At present, several factors are working in favour of the virus. In that sense, the virus is getting the upper hand," he said.
Dr Animesh stressed that this pattern is not unusual for influenza, which undergoes relatively small genetic changes over time.
"These subtle variations are a characteristic of influenza. Every few years, a larger number of people become susceptible, which is also why the composition of the influenza vaccine is changed every year," he said.
'Not a new or more lethal H1N1 strain'
Dr Ashish Choudhary, Professor and In-charge of Virology at AIIMS Delhi, said the virus now known as pdmH1N1 is genetically different from the H1N1 virus that circulated before the 2009 pandemic.
"The pdmH1N1 strain emerged in 2009, during the swine flu pandemic. Since then, the virus has adapted and become a seasonal influenza virus. It continues to undergo minor mutations and reappears every year," Dr Choudhary said.
He cautioned against interpreting the current rise as evidence that the virus has become more virulent.
"It is not more virulent or more lethal than what we have seen in previous years. Influenza periodically has years when the peak is larger and the number of cases increases significantly," he said.
This year's pattern is also different from 2025. While H3N2 was the predominant subtype in Delhi last year, H1N1 has accounted for the largest share of Influenza A infections this year.
Dr Choudhary said most infections continue to be mild.
"Most cases are mild influenza. There have been deaths, but the proportion is not very high," he said.
At the same time, doctors in pulmonary and critical-care units are seeing more severe patients, particularly those who already have compromised lungs or other underlying illnesses.
"The pulmonary medicine ICU has seen a higher number of patients with severe disease, particularly those whose lungs are already compromised because of co-morbidities," Dr Choudhary said.
At AIIMS, around 40 to 50 respiratory samples are being tested each day, with positivity varying between 5 and 15 per cent, he said.
Also Read: H1N1 Cases Surge Further In Delhi, Bengaluru: How Is Swine Flu Different From COVID-19?
Doctors see pneumonia, oxygen requirement
Dr Rajesh Chawla, Senior Consultant in Respiratory and Critical Care Medicine at Indraprastha Apollo Hospitals, said doctors are seeing a highly infectious pattern of seasonal influenza despite the absence of evidence of a new strain.
"Although ICMR has said there is no new strain, what we are seeing this time is a very infectious virus. Compared with last year, we are seeing more cases," Dr Chawla said.
He said the number of cases being seen in hospitals is likely to represent only a fraction of infections in the community.
"We are seeing around three to four new cases, but the actual number in the community is likely to be much higher," he said.
The more worrying feature, he said, is the number of patients developing complications.
"We are seeing patients requiring oxygen, and many are progressing to pneumonia. Some patients have also presented with diarrhoea and, in certain cases, blood in the stools," Dr Chawla said.
He attributed part of the rise to the weather, saying the current conditions are conducive to influenza transmission, and expects the current surge to settle in the coming days.
'Cough is lasting longer, GI symptoms more prominent'
Dr Neha Rastogi, Senior Consultant in Infectious Diseases at Fortis Hospital, said the rise is likely being driven by several factors rather than a single cause.
"Compared with last year, there is definitely a spike. The pre-monsoon period arrived early, and we are seeing clusters of cases. Inter-state and inter-city travel has also increased, which facilitates transmission. At the same time, increased testing means we are detecting more infections," she said.
Dr Rastogi said her OPD has seen more than 80 cases in the past week.
She also pointed to changes in the way some patients are presenting.
"We are seeing a high spike of fever during the first three to four days. The cough is becoming more persistent and difficult to manage and is taking longer to settle. Pollution may also be contributing to this prolonged respiratory illness," she said.
Another notable feature this season, according to Dr Rastogi, is the presence of gastrointestinal symptoms alongside the more familiar respiratory symptoms.
"This time, we are seeing more gastrointestinal symptoms, including diarrhoea and stomach cramps, in addition to the usual respiratory problems," she said.
Dr Rastogi expects influenza activity to remain elevated for another month and advised people to take precautions, including vaccination, masking in crowded settings and maintaining hand hygiene.
Why some patients become seriously ill
Dr GC Khilnani, Chairman of the PSRI Institute of Pulmonary, Critical Care and Sleep Medicine, explained that influenza is an RNA virus and its genetic changes can alter its characteristics without turning it into a fundamentally different virus.
"Antigenic drift means the virus remains H1N1, but its characteristics can change because of minor mutations. This is not the same strain that caused the 2009 pandemic, and there is no indication that we are dealing with another pandemic," Dr Khilnani said.
He said the speed with which influenza spreads is influenced by factors including how contagious the virus is and its incubation period.
"If the incubation period is short, the virus can spread faster. Influenza can also start spreading about a day before a person develops symptoms," he said.
Dr Khilnani said his hospital is seeing a substantial increase in patients compared with its usual daily numbers, with severe cases predominantly involving people with underlying conditions.
"We are seeing nearly 20 times the number of patients we normally see on a daily basis, and the majority of those with severe disease have co-morbidities," he said.
Children, older people and those with underlying medical conditions remain particularly vulnerable to complications. ICMR has also identified these groups as being at greater risk from seasonal influenza.
When should patients take antivirals?
Doctors say not every person with influenza requires antiviral medication.
"Antiviral medication is not required for everyone. It is primarily used in severe cases and in patients who are at higher risk of complications," Dr Animesh said.
Dr Choudhary similarly divided patients broadly into three categories: those with mild disease who generally do not need testing or antiviral medication; those with mild illness but co-morbidities who may require antivirals; and patients with severe disease who require intensive care.
Dr Khilnani said Oseltamivir is commonly used as an antiviral treatment, with the precise prescription depending on the patient's clinical condition.
"The medicines work best when started within 48 hours of the onset of illness. People should therefore seek medical advice early rather than waiting for symptoms to become severe," he said.
What should people do?
The doctors' message is one of caution rather than alarm.
The current surge is substantial, but ICMR has found no evidence of a new H1N1 strain and says the circulating viruses are matched to the recommended vaccine strains.
For most people, seasonal influenza remains self-limiting. But those who are vulnerable, particularly people with underlying illnesses, should not ignore worsening symptoms.
Dr Khilnani advised people to get tested or seek medical assessment promptly when symptoms warrant it, particularly because early treatment can be important in high-risk patients.
"People should wear masks, particularly in crowded places, and seek medical advice as soon as possible if they develop significant symptoms," he said.
The broader advice from doctors is to combine early treatment for high-risk patients with basic measures such as masking, hand hygiene and vaccination, while avoiding unnecessary panic over the current rise in H1N1 cases.
Read More: The Pandemic Before Covid: The 2009 Swine Flu Surge And Its Impact On Current Disease Response
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