0 Posted 2025-04-11Updated 2025-04-16Notes / Biology / Immunity5 minutes read (About 800 words)

Influenza

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Basic Information


Figure 9 Flint et al. 2004 ASM Press

Influenza A gets the most number of variance. They are named based on the subtype of the surface protein HA and NA. For example, H1N1, H3N2, H5N1, etc. For HA, there are 18 subtypes (H1-H18), For NA, there are 11 subtypes (N1-N11)

They are transmitted by respiratory droplets. The virus can survive on surfaces for 24-48 hours. The virus can be transmitted before the symptoms show up.

Morphalogy of the Influenza Virus


Unlike the Zika virus, it has a uniform on size and shape. The surface protein alos aligned with recognizable patterns. In the resconstructred influenza, it has heterogeneous progeny, the surface protein HA (green) and NA (yellow) distributed by random.

What’s the advantages of the heterogeneous progeny? Influenza’s enter and release is based on the 2 main surface protein, HA an NA. By changing the morphalogy of the virus, it could regulate the ratio and distribution of the HA and NA to fit the environment.

Segmented Genome

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Unlike the Zika virus, the influenza virus has a segmented genome. The segmented genome allows for reassortment of genes between different strains of the virus, which can lead to the emergence of new strains with different properties. This is one of the reasons why influenza viruses are able to evolve rapidly and evade the immune system.

Influenza causing pandemic almost every 2 years.

Increased the glycosylation of the HA protein. This glycosylation change help the virus to evade the immune system.

Treatment

Ion Channel Blockers

These drugs target the M2 ion channel protein in influenza Type A viruses. By blocking this channel, they prevent acidification inside the viral particle, which is necessary for uncoating and releasing the viral RNA.

Amantadine – Effective only against influenza A viruses.
Rimantadine – Also active against influenza A only, with a slightly better side effect profile than amantadine.

⚠️ Note: Many influenza A strains have developed resistance to these drugs, limiting their current use.

Neuraminidase Inhibitors

These medications block the neuraminidase enzyme, which is required for the virus to release new viral particles from infected cells. They work against both influenza A and B.

Oseltamivir carboxylate (Tamiflu) – An oral drug that inhibits neuraminidase in both influenza A and B.
Zanamivir (Relenza) – An inhaled version that also targets both influenza A and B.

These drugs are most effective when taken within 48 hours of symptom onset.

Vaccine

The circulating of the influenza: H3N2 (60%), H1N1 (15%), B (25%). (Paul Van Buynder; 2016)

For influenza A, technically, the antigenic drift make leads to epidemic, but the antigenic shift is the one that leads to pandemic.
Influenza B only causes epidemic, not pandemic.

For influenza vaccine, there are:

SD-IIV: Standard-dose, for most people (most common)
HD-IIV: High-dose, for elderly (more antigens)
LAIV: Live attenuated, nasal spray
aIIV: Adjuvanted, for stronger immune response
ccIIV: Cell culture-based
RIV: Recombinant Influenza Vaccine.

IIV = Inactivated Influenza Vaccine (the virus is killed, not live)

Limitation of the Vaccine

Predicted sub-strains
Unlike other vaccine, because the flu happens almost every year, getting the vaccine is suggested before each flue season.
The sub-strain of the virus are predicted based on the previous year. So, the vaccine only getting around 60% of protections.
Egg-based vaccine
95% of the vaccine are made from eggs. Birds’ sialic acid connection is slightly different from human’s. When the influenza virus infects the eggs, they would mutates to adapt to the eggs. This could lead to a mismatch between the vaccine and the circulating virus.

sialic acid difference between human and avian