The World Health Organization (WHO) has recommended that flu shots for the upcoming influenza season be updated to include strains targeting H3N2 — a virus known to cause a more aggressive form of seasonal influenza that has hit several countries hard over recent months.
Many countries in the Northern Hemisphere, including India, have reported widespread circulation of H3N2, also known as superflu.
Influenza vaccine formulations must be reviewed and updated regularly because influenza viruses are constantly evolving. Even small genetic changes can reduce the effectiveness of existing immunity, whether from previous infection or vaccination.
The WHO has advised vaccine manufacturers to develop egg-based, cell culture–based, recombinant protein, or nucleic acid–based vaccines against two influenza A viruses — H1N1 (swine flu) and H3N2 (Subclade K) — along with a specific strain of influenza B virus.
Influenza viruses are broadly classified into four types: A, B, C, and D. They are further subtyped based on two surface proteins — hemagglutinin (H) and neuraminidase (N). These proteins determine the subtype and behaviour of the virus.
Since October last year, H3N2 has repeatedly made headlines in multiple countries for triggering significant surges in infections and causing somewhat more severe symptoms compared to H1N1.
UNDERSTANDING H3N2
According to Dr Seema Dhir, an internal medicine specialist at Artemis Hospitals in Gurugram, H3N2 is a type of influenza A virus that tends to cause more serious illness than other flu strains.
First identified in 1968, it was behind the mega pandemic (also known as Hong Kong flu) that started that year, lasted till 1970 and was estimated to have killed between one and four million people globally.
In the current form of the virus, infected patients often experience high-grade fever and persistent cough, along with a slightly higher risk of complications — particularly among the elderly and children — leading to increased chances of hospitalisation, she explained.
Dr Neha Mishra, an infectious disease specialist at Manipal Hospital in Bengaluru, noted that most influenza illnesses are self-limiting in the majority of individuals.
However, in vulnerable groups such as older adults and immunocompromised patients, the infection can progress to severe disease.
“Therefore, early diagnosis and proper management of complications in these high-risk groups can significantly improve outcomes,” Dr Mishra underscored.
The WHO estimates that there are around one billion global cases of seasonal influenza annually, including 3 to 5 million cases of severe illness. These infections are responsible for approximately 290,000 to 650,000 respiratory deaths each year.
Alarmingly, 99 percent of deaths in children under five years of age with influenza-related lower respiratory tract infections occur in developing countries such as India.
YOUR PROTECTIVE SHIELD
H3N2 — unlike many other influenza viruses — tends to mutate more rapidly.
Because of these genetic changes, immunity from past infections or vaccines may not be as strong against this strain as it is against others, Dr Dhir said.
She added that this is one reason why H3N2-driven flu waves in recent months have resulted in more hospitalizations in countries such as the UK and the US.
The United Nations health agency recommends an annual flu vaccination for pregnant women, children aged six months to five years, adults over 65, individuals with chronic medical conditions, and healthcare workers.
According to the Indian Academy of Pediatrics (IAP), the country’s largest network of pediatricians, flu vaccines should be administered annually to children between six months and five years of age, as well as to those with high-risk conditions.
However, in India, uptake of the flu vaccine among children and senior adults has traditionally been low. Because influenza vaccines are not included in the government-run Universal Immunisation Programme (UIP), they are available only in the private market.
The cost of a single flu vaccine shot in India ranges between Rs 2,000 and Rs 2,500. Available influenza vaccines, according to scientific evidence, are about 50 to 60 percent effective — but even moderate effectiveness can significantly reduce severe illness, hospitalisation, and death.
Dr Pankaj Khatana, an internal medicine specialist at Marengo Asia Hospital in Gurugram, said the seasonal flu vaccine may provide protection against H3N2 along with other commonly circulating flu viruses.
India typically experiences two peaks of seasonal influenza each year — one between January and March, and another during the post-monsoon season.
Once updated vaccines targeting H3N2 become available in the country, getting vaccinated could be particularly beneficial for large sections of the population.
In addition to vaccination, general preventive measures remain essential. Wearing masks when ill, frequent handwashing, and avoiding close contact with infected individuals — especially those at higher risk of complications — continue to play an important role in limiting the spread of influenza.
