Differences Between Flu Strains: Types, Symptoms, And Severity 

Every year, influenza viruses circle the globe, but not all of them are the same. There are many different types and strains of the influenza family.  

The main culprits behind the wintertime illnesses we suffer from are influenza A and B. Influenza C usually causes only mild infections. Here’s the interesting part: humans don’t get sick with Influenza D at all!  

This article will cover the classification, symptoms, severity, and risk factors of these viruses to help you understand what you might be facing this flu season. 

What Is a Flu Strain? 

To understand the differences between flu strains, it helps to understand how viruses are classified. The flu family tree is broken down into types, subtypes, lineages, and then individual strains.  

A “type,” such as Influenza A, is like the general family. A “strain” is a specific, very detailed genetic version of that virus. Flu viruses are experts at disguise, changing over time in a process known as antigenic drift.  

This constant change in the genes is exactly why you can get the flu over and over again in your life. The strain doing the rounds this winter could look totally different to your immune system than the one you battled five years ago. 

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Influenza A 

Influenza A is a very adaptable virus. It is divided into subtypes according to two surface proteins, the Hemagglutinin (H) and the Neuraminidase (N). For example, you might have heard of common human subtypes H1N1 and H3N2.  

Influenza A can mutate rapidly and be transmitted between animals and humans (like birds or pigs), and it is the cause of seasonal outbreaks and global flu pandemics such as the 2009 H1N1 “swine flu” outbreak. 

Severity can vary greatly from one season to another and from one person to another. A report found that a flu season officially labeled as “high severity” season, was the worst in the U.S. since 2017.  

This surge was mainly due to the co-circulation of A(H1N1)pdm09 and A(H3N2) viruses, leading to a general increase in outpatient visits, hospitalizations, and admissions to intensive care. 

Influenza B 

Influenza B is not as genetically diverse as A, but it is also a significant contributor to seasonal flu outbreaks.  

Rather than being divided into H and N subtypes, it is classified by lineages, which have historically been the Victoria and Yamagata lineages. 

There is a common misconception that Influenza B is always a “mild” version of the flu. Actually, it’s quite hard-hitting. 

• Vulnerable populations: Children, the elderly, and immunocompromised people are particularly susceptible to serious illness from influenza B. 
• Severe complications: Similar to Influenza A, an Influenza B infection can develop into life-threatening conditions, such as pneumonia and respiratory failure. 

Influenza C and D: How They Differ 

Now let’s turn away from the big hitters and talk about Influenza C and D, which behave very differently: 

• Influenza C: This type typically causes mild respiratory illness. It doesn’t lead to major seasonal outbreaks, which is why you rarely see it in winter news reports. 
• Influenza D: This virus mainly affects cattle and other animals. It is not currently known to infect or cause disease in humans. 

Do Different Flu Strains Cause Different Symptoms?

You’d think a more aggressive strain would cause totally different symptoms, but that’s not the case. Whatever strain you’re fighting, A or B, the common symptoms overlap so much.  

You can expect sudden fever, chills, dry cough, sore throat, headaches that won’t quit, deep fatigue, and muscle aches. You cannot tell what strain of the flu someone has based on symptoms, so a lab test is needed to confirm the diagnosis.  

However, there are subtle differences in how the flu looks across age groups. For instance, children may have gastrointestinal symptoms such as vomiting or diarrhoea much more frequently than adults when infected with seasonal flu. 

What Determines Flu Severity? 

If the symptoms are so similar, how come some people end up in the hospital, and others recover at home with rest and fluids?  

How sick you get with the flu can be a complicated mix of the virus’s characteristics and the defenses your body has: 

• Personal Health: How well your body can fight the virus depends on your age, whether you’re pregnant, your immune health, and any underlying medical conditions, such as asthma, heart disease, or diabetes. 
• Immunity and Vaccination: Your past immunity and vaccination status matter. One study examined vaccine effectiveness against community infection in an intense season. It showed a total vaccine efficacy of 42%, and interestingly, the vaccine was 75% effective against Influenza B. 
• Strain Features: Sometimes, the circulating strain is just more virulent by nature. 

When to Seek Medical Care 

If you or someone you love has severe warning signs, don’t wait to see if things get better.  

Seek immediate medical advice if you have a serious or rapidly worsening illness, especially if you notice: 

• Trouble breathing, feeling short of breath, or ongoing chest pain. 
• Sudden confusion, severe dehydration and severe weakness. 
• Symptoms that seem to clear up, then come back a few days later with a worse fever or cough. 

Conclusion 

Despite sharing a name, influenza viruses differ greatly in their effects on global health, with types A, B, C, and D. Importantly, severity is not always predictable from the strain.  

Staying up to date with vaccinations, testing early when indicated, and seeking timely medical care for warning signs remain the best ways to protect yourself.