Daijiworld Media Network - New Delhi
New Delhi, Aug 23: India is witnessing a rise in H1N1 cases, with Delhi emerging as one of the key hotspots this season, prompting health authorities to strengthen surveillance and issue an advisory. Doctors are also stressing the importance of annual influenza vaccination, particularly for people at higher risk of developing complications.
Commonly known as swine flu, H1N1 emerged in 2009 and was initially associated with influenza viruses found in pigs, which led to its popular name. The virus subsequently established sustained transmission among humans and now circulates as seasonal influenza A(H1N1)pdm09.
According to Delhi Health Department data, the national capital had reported 1,777 H1N1 cases as of August 20, including 114 new cases in a single day. Delhi had recorded 2,602 influenza cases overall, of which 2,392 were Influenza-A and 210 were Influenza-B. The increase prompted the Delhi government to issue a health advisory and step up surveillance, while Union Health Minister JP Nadda reviewed the situation and the preparedness of hospitals.
Dr Rajeev Chowdry, Director of Internal Medicine at Yatharth Hospital, Faridabad, said modern seasonal flu vaccines are designed to protect against the influenza viruses that surveillance indicates are most likely to circulate during the upcoming season. The H1N1 component is included alongside H3N2 and influenza B components in current trivalent influenza vaccines.
H1N1 is a subtype of influenza A. According to the US Centers for Disease Control and Prevention, the influenza A(H1N1)pdm09 viruses currently circulating among humans are descendants of the virus that emerged in 2009. Like other influenza viruses, they have continued to undergo genetic and antigenic changes.
This continuous evolution is one of the main reasons influenza vaccination is recommended every year. The virus circulating during one season may not be antigenically identical to strains that circulated in previous years. The World Health Organization therefore conducts global influenza surveillance and recommends vaccine compositions twice a year, separately for the northern and southern hemispheres, to ensure the best possible match with circulating strains.
For the 2026-27 northern hemisphere influenza season, the WHO recommendation includes an H1N1pdm09-like strain in both egg-based and cell-culture or recombinant vaccines.
The flu vaccine works by exposing the immune system to influenza antigens, allowing the body to develop antibodies without causing influenza infection from the vaccine itself. Following vaccination, the body needs time to develop protection, with the CDC stating that it generally takes around two weeks for protective antibodies to develop.
Getting vaccinated before influenza activity becomes widespread is therefore preferable. Dr Chowdry said vaccination can reduce the risk of developing influenza and can also reduce the likelihood of severe disease if infection occurs.
However, vaccination does not guarantee that a person will never contract influenza. Vaccine effectiveness varies from season to season depending on factors such as the match between vaccine strains and circulating viruses, the person's age and immune response and the particular influenza strain involved.
“But vaccination can still reduce the risk of severe outcomes,” Dr Chowdry said.
H1N1 symptoms are broadly similar to those caused by other seasonal influenza infections. People may develop fever, sore throat, cough, body aches, headache, chills and fatigue. In some cases, influenza can progress to serious complications such as pneumonia and respiratory failure.
WHO India describes influenza as a contagious respiratory illness that can range from mild to severe. Severe infection can result in hospitalisation or death, with young children, pregnant women, older adults and people with certain underlying health conditions facing a higher risk of complications.
The WHO estimates that seasonal influenza causes around one billion cases globally every year, including three to five million cases of severe illness and between 290,000 and 650,000 respiratory deaths annually.
H1N1 spreads in much the same way as other seasonal influenza viruses. An infected person can release respiratory particles while coughing, sneezing or talking, which may then be inhaled by people nearby. Close contact with an infected person and contaminated hands or surfaces can also contribute to transmission.
Dr Chowdry said the virus is highly contagious and can survive in the environment for a period of time. He also stressed that H1N1 spreads from person to person and is not acquired by eating properly cooked pork products.
Annual vaccination is recommended because immunity can decline over time and influenza viruses continue to evolve. The WHO considers health workers, older adults, pregnant women and people with certain chronic medical conditions among the priority groups for seasonal influenza vaccination.
People with conditions affecting the lungs, heart, kidneys, liver, nervous system or metabolism may face a greater risk of complications. Those with weakened immune systems are also more vulnerable, while children and older adults remain important groups because severe influenza can disproportionately affect them.
Health authorities generally recommend getting vaccinated before influenza activity rises in a particular area, while allowing around two weeks for the immune response to develop. CDC guidance says September and October are generally suitable months for vaccination in the northern hemisphere, although vaccination can continue later while influenza viruses are circulating.
Dr Chowdry advised people, especially those at higher risk of complications, to discuss the appropriate timing and vaccine formulation with their doctor.
Along with vaccination, maintaining basic respiratory hygiene can help reduce transmission. Regular handwashing with soap and water or an alcohol-based hand sanitiser, along with avoiding close contact with people who are coughing or showing flu-like symptoms, can reduce the risk of infection.
People who develop significant flu symptoms, particularly those at higher risk of complications, should not automatically assume they have a routine cold. Early medical assessment can be important in severe, progressive or high-risk cases, as antiviral medicines may be useful when influenza infection is suspected or confirmed and treatment is medically indicated.