🤧Influenza A, Influenza B
Differences between Influenza A (H1N1) pdm2009 / A (H3N2) (Hong Kong strain) / B (mainly Victoria lineage)
The main symptoms such as fever, cough, sore throat, runny nose, muscle aches, and headache largely overlap among the three, making it nearly impossible to distinguish the type by symptoms alone. The differences are more likely to appear in "symptom tendencies," "risk of severe illness by age group," and "epidemic patterns."
| Item | Type A (H1N1pdm2009/H3N2) | Type B (mainly Victoria strain) |
| Chief complaint | Fever, cough, sore throat, runny nose, muscle aches, headache (typical) | Similarly, fever, cough, sore throat, etc. (almost equivalent) |
| Digestive symptoms | Nausea, vomiting, and diarrhea were slightly more common with H1N1pdm2009. | Less than type A, but not zero. |
| Characteristics of fever | H3N2 is prone to causing high fevers. | The fever period is comparable to A or sometimes slightly longer. |
| Age group characteristics | H1N1pdm2009: Can cause severe illness in young people / H3N2: More likely to cause severe illness in the elderly | High incidence in school-aged children to adolescents |
| Risk of severe illness | H1N1pdm2009: Reports of ARDS and acute lung injury / H3N2: Often associated with high hospitalization and mortality rates | Overall, it's similar to A, but the risk of pneumonia and complications increases if there are underlying conditions. |
| Influenza encephalitis | In children, there is a tendency for A type (especially H1N1) to be more common. | Reports exist for type B as well, but the frequency is lower than for type A. |
| Epidemiological characteristics | Annual antigenic drift is significant, making global pandemics likely. | Humans are the only hosts, and epidemics are localized and concentrated in certain age groups. |
You can get the flu multiple times in the same season.
Influenza comes in types A (H1N1pdm2009, Hong Kong H3N2) and B (Victoria lineage, etc.), and they become epidemic every year.
• Even if you get type A, it doesn't mean you can't get another type A. For example, even after getting type A H1N1, you can still get type A Hong Kong (H3N2).
Since type A and type B have different properties, you can contract type B even shortly after contracting type A.
The same strain is less likely to cause reinfection because the immune system is more likely to fight it off.
• However, immunity doesn't work if the type is different, so even people with type A can be reinfected, and immunity from type A to type B is almost nonexistent.
Antibody persistence varies by type; H1N1 antibodies can last relatively long, while H3N2 and Type B antibodies may decline within a few months.
🔵 To summarize, you cannot be complacent just because you have contracted Type A, as it is possible to contract a different strain of Type A or Type B within the same season.
It's important to stay vigilant during flu season and continue with handwashing, masks, and prevention.
📚 References
Wraith S, Balmaseda A, Carrillo FAB, et al. Homotypic Protection Against Influenza in a Pediatric Cohort in Managua, Nicaragua. Nature Communications. 2022;13(1):1190. doi:10.1038/s41467-022-28858-9
Yang W, Lau EHY, Cowling BJ. Dynamic Interactions of Influenza Viruses in Hong Kong During 1998-2018. PLoS Comput Biol. 2020;16(6):e1007989. doi:10.1371/journal.pcbi.1007989
Goh EH, Jiang L, Hsu JP, et al. Epidemiology and Relative Severity of Influenza Subtypes in Singapore in the Post-Pandemic Period From 2009 to 2010. Clin Infect Dis. 2017;65(11):1905-1913. doi:10.1093/cid/cix694
Lau YC, Perera RAPM, Fang VJ, et al. Variation by Lineage in Serum Antibody Responses to Influenza B Virus Infections. PLoS One. 2020;15(11):e0241693. doi:10.1371/journal.pone.0241693
Yang B, Gostic KM, Adam DC, et al. Breadth of Influenza A Antibody Cross-Reactivity Varies by Virus Isolation Interval and Subtype. Nat Microbiol. 2025;10(7):1711-1722. doi:10.1038/s41564-025-02033-4
Gatti L, Koenen MH, Zhang JD, et al. Cross-Reactive Immunity Potentially Drives Global Oscillation and Opposed Alternation Patterns of Seasonal Influenza A Viruses. Sci Rep. 2022;12(1):8883. doi:10.1038/s41598-022-08233-w
Hoa LNM, Sullivan SG, Mai LQ, et al. Influenza A(H1N1)pdm09 but Not A(H3N2) Virus Infection Induces Durable Seroprotection: Results From the Ha Nam Cohort. J Infect Dis. 2022;226(1):59-69. doi:10.1093/infdis/jiaa29
The effects and limitations of anti-influenza drugs
Established efficacy of anti-influenza drugs
Administered early in the course of illness, it shortens the duration until fever subsides.
It has the effect of reducing the viral load in the body and suppressing transmission to others.
• Post-exposure prophylaxis has been shown in large RCTs and meta-analyses to reduce secondary infections within families and communities.
Regarding the effect of preventing severe illness
• For general cases (non-severe cases), the effect of reducing hospitalization or mortality rates is not clear.
While it may shorten the length of hospital stay in severe cases, there is limited statistical evidence to significantly reduce mortality or ICU admission rates.
• In high-risk groups such as children, the elderly, and individuals with underlying medical conditions, early administration has been reported to potentially reduce the risk of complications like secondary bacterial infections, but the certainty of its effect in preventing severe illness is not yet sufficient.
Summary
• The main roles of antiviral drugs for influenza are to alleviate symptoms and prevent the spread of infection.
The effect on preventing severe illness is limited, and a statistically significant effect has not yet been established.
📚 References
Uyeki TM, Hui DS, Zambon M, Wentworth DE, Monto AS. Influenza. Lancet. 2022;400(10353):693-706. doi:10.1016/S0140-6736(22)00982-5
Gao Y, Zhao Y, Liu M, et al. Antiviral Medications for Treatment of Nonsevere Influenza: A Systematic Review and Network Meta-Analysis. JAMA Intern Med. 2025;185(3):293-301. doi:10.1001/jamainternmed.2024.7193
Zhao Y, Gao Y, Guyatt G, et al. Antivirals for Post-Exposure Prophylaxis of Influenza: A Systematic Review and Network Meta-Analysis. Lancet. 2024;404(10454):764-772. doi:10.1016/S0140-6736(24)01357-6
Gao Y, Guyatt G, Uyeki TM, et al. Antivirals for Treatment of Severe Influenza: A Systematic Review and Network Meta-Analysis of Randomized Controlled Trials. Lancet. 2024;404(10454):753-763. doi:10.1016/S0140-6736(24)01307-2
Lifestyle Habits and Influenza Prevention
Effective preventive measures for the influenza season include lifestyle adjustments in addition to vaccination.
Meal content
• A balanced diet maintains immune function and reduces the risk of infection. Extreme calorie restriction and obesity weaken immunity, increasing the risk of infection and severe illness. Fiber intake improves the gut environment and may contribute to increased survival rates during influenza infection [1-3].
Exercise
Regular moderate exercise may reduce the risk of respiratory infections and improve vaccine response. However, excessive strenuous exercise can suppress immunity, so caution is advised [4-6].
Stress Management
Chronic psychological stress increases the risk of influenza infection. Stress hormones inhibit the immune response, so stress management techniques such as relaxation can help prevent infection [7-9].
Preventive effects of green tea
Green tea consumption is associated with a reduced risk of influenza infection, with catechins (EGCG) inhibiting viral adsorption and replication, and strengthening immunity. Consuming about 1 to 5 cups per day is considered effective, and no additional benefits have been observed with excessive intake. Gargling with green tea has also shown some preventive effects [10-16].
Summary
While vaccination is fundamental, you can enhance influenza prevention by combining it with sufficient sleep, a balanced diet, moderate exercise, stress management, and green tea consumption.
📚 References
Steinmann J, Buer J, Pietschmann T, et al. Anti-infective properties of tea polyphenols. Clin Infect Dis. 2013;57(8):1112-1119. doi:10.1093/cid/cit453
Trompette A, Gollwitzer ES, Yadava K, et al. Gut microbiota metabolism of dietary fiber influences allergic airway disease and hematopoiesis. Nat Med. 2014;20(2):159-166. doi:10.1038/nm.3444
Ichinohe T, Pang IK, Kumamoto Y, et al. Microbiota regulates immune defense against respiratory tract influenza A virus infection. Proc Natl Acad Sci USA. 2011;108(13):5354-5359. doi:10.1073/pnas.1019378108
Zhang H, Sparks JB, Karyala SV, et al. Dietary fiber-induced microbiota modulation protects against influenza infection by regulating innate immunity. Gut Microbes. 2021;13(1):1-15. doi:10.1080/19490976.2021.1875798
Wong CM, Lai HK, Ou CQ, et al. Is exercise protective against influenza-associated mortality? PLoS One. 2008;3(5):e2108. doi:10.1371/journal.pone.0002108
Kohut ML, Cooper MM, Nickolaus MS, et al. Exercise and psychosocial factors modulate immunity to influenza vaccine in elderly individuals. J Gerontol A Biol Sci Med Sci. 2002;57(9):M557-M562. doi:10.1093/gerona/57.9.M557
Campbell JP, Turner JE. Debunking the myth of exercise-induced immune suppression: redefining the impact of exercise on immunological health. Exerc Immunol Rev. 2018;24:1-14. PMID: 29376991
Cohen S, Tyrrell DA, Smith AP. Psychological stress and susceptibility to the common cold. N Engl J Med. 1991;325(9):606-612. doi:10.1056/NEJM199108293250903
Glaser R, Kiecolt-Glaser JK. Stress-induced immune dysfunction: implications for health. Nat Rev Immunol. 2005;5(3):243-251. doi:10.1038/nri1571
Morimoto K, Takeshita T, Takahashi M, et al. Effects of stress on the immune system and health. Jpn J Hyg. 2001;56(4):647-653. doi:10.1265/jjh.56.647
Matsumoto K, Yamada H, Takuma H, et al. Effects of green tea catechins on influenza infection. Jpn J Infect Dis. 2011;64(5):357-361. PMID: 21937895
Park M, Yamada H, Matsushita K, et al. Green tea catechins inhibit influenza virus replication. Antiviral Res. 2013;99(3):345-352. doi:10.1016/j.antiviral.2013.06.013
Ide K, Yamada H, Matsushita K, et al. Effects of green tea catechins and theanine on preventing influenza infection among healthcare workers: a randomized controlled trial. BMC Complement Altern Med. 2014;14:491. doi:10.1186/1472-6882-14-491
Rowe CA, Nantz MP, Bukowski JF, et al. Green tea consumption and influenza risk reduction: a systematic review. Nutrients. 2015;7(12):5490-5503. doi:10.3390/nu7125490
Ohno T, Yokoyama M, Nishimura T, et al. Inhibitory effects of tea catechins on the growth of influenza virus. Antiviral Res. 2003;60(2):157-162. doi:10.1016/S0166-3542(03)00158-8
Yamada H, Takuma H, Matsushita K, et al. Efficacy of catechin-containing tea gargling for preventing influenza infection in healthy adults: a randomized controlled trial. Jpn J Infect Dis. 2006;59(6):364-368. PMID: 17186954