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Delhi had recorded 1,449 H1N1 influenza cases up to 12 August 2026, against 229 in the same period a year earlier.
H1N1 is a subtype of Influenza A, named after its haemagglutinin (H1) and neuraminidase (N1) surface proteins.
Influenza viruses belong to the family Orthomyxoviridae and are grouped into types A, B, C and D; only Influenza A causes pandemics.
Small continuous mutations, called antigenic drift, drive seasonal flu; abrupt reassortment, called antigenic shift, produces novel pandemic strains.
The 2009 pandemic H1N1 virus was a reassortant carrying swine, human and avian influenza genes.
Antigenic drift is the accumulation of small mutations in the genes coding for the haemagglutinin and neuraminidase proteins as the virus replicates. Because the changes are gradual, existing immunity still offers partial protection, but enough of the surface changes each season that people can be reinfected. This is why seasonal flu recurs and why flu vaccines are reformulated. Antigenic shift is different in kind, not degree: it is an abrupt reassortment in which an Influenza A virus acquires whole gene segments from another influenza virus, producing a combination of surface proteins that humans have never met. Populations then have no pre-existing immunity, which is the precondition for a pandemic. The 2009 H1N1 virus is the standard example, carrying genetic material of North American swine, Eurasian swine, human and avian origin.
Simple Analogy: Antigenic drift is a familiar face growing a beard; you still recognise the person. Antigenic shift is a complete stranger walking in wearing the same coat.
GS Paper 3 > Science and Technology, Developments in Biotechnology; GS Paper 2 > Issues Relating to Health
General Science > Biology; General Awareness > Current Affairs
General Science and Current Affairs
General Awareness > Current Affairs
A surface protein of the influenza virus that allows it to attach to and enter host cells; the H number in a subtype name refers to its variant.
A surface enzyme that lets newly formed virus particles detach from an infected cell; the target of drugs such as oseltamivir.
The exchange of whole gene segments when two influenza viruses infect the same cell, the mechanism behind antigenic shift.
The family of segmented negative-sense RNA viruses to which all influenza viruses belong.