H1N1 vs H3N2

H1N1 and H3N2 are both circulating variants of the Influenza A virus. In India, the reporting, surveillance and awareness is highly skewed towards H1N1. Both H1N1 and H3N2 have a lot of overlapping features. However, studies have shown that some features are statistically more likely in one variant more than the other.

H3N2 was responsible for one of the three major influenza pandemics that occurred in the last century. In 1968, a novel strain of H3N2 influenza virus emerged in Hong Kong that quickly led to a pandemic that was
associated with >1 million deaths world-wide. (4)

According to the data from the Center for Disease Control in the USA, during seasons with prominent circulation of H3N2 influenza A viruses, 2.7 times more deaths occurred than during seasons when H3N2 influenza A viruses were not predominant. Similarly, a greater number of hospitalizations occur during years that influenza A(H3N2) is predominant. (5)



1. Yap J, Tan CH, Cook AR, Loh JP, Tambyah PA, Tan BH, et al. Differing clinical characteristics between influenza strains among young healthy adults in the tropics. BMC Infect Dis. 2012 Dec;12(1):12.

2. Kaji M, Watanabe A, Aizawa H. Differences in clinical features between influenza A H1N1, A H3N2, and B in adult patients. Respirology. 2003 Jun;8(2):231–3.

3. Dangi T, Jain B, Singh AK, Mohan M, Dwivedi M, Singh JV, et al. Influenza virus genotypes circulating in and around Lucknow, Uttar Pradesh, India, during post pandemic period, August 2010–September 2012. Indian J Med Res. 2014 Mar;139(3):418–26.

4. Allen JD, Ross TM. H3N2 influenza viruses in humans: Viral mechanisms, evolution, and evaluation. Hum Vaccines Immunother. 2018;14(8):1840–7.
5. Pinkbook | Influenza | Epidemiology of Vaccine Preventable Diseases | CDC [Internet]. 2019 [cited 2019 Dec 26].

Available from: https://www.cdc.gov/vaccines/pubs/pinkbook/flu.html