Introduction

Respiratory tract infections (RTIs) are infections by pathogens (germs, such as viruses and bacteria) that infect the parts of your body used for breathing.¹ Acute RTIs, ones that are not chronic, are the most common illnesses worldwide, regardless of a person’s age or gender.² RTIs can be categorised into two types: upper RTIs, which affect the sinuses and throat such as sinusitis and tonsillitis; and lower RTIs, which affect the airways and lungs such as bronchitis and chest infections. Influenza, one of the most prevalent viral RTIs, can be an upper or a lower RTI.¹ Parainfluenza is another major viral RTI, and it can also be an upper or lower RTI.

Whilst Influenza and Parainfluenza have similar names, and indeed there are some similarities in symptoms, transmission, and treatments, they are fundamentally different illnesses caused by different pathogens. In fact, in 2014 Parainfluenza was reported to be the second most common acute RTI to cause hospitalisation in children under 5 years old – more common than Influenza.³ This article will describe what Influenza and Parainfluenza actually are, and will highlight the key differences that you need to know to make informed decisions about your health.

What is Influenza?

Influenza is an RTI caused by influenza viruses, which are part of a broader scientific family of viruses called Orthomyxoviridae.⁴ Influenza viruses come in four types: A, B, C, and D. Influenza A and B viruses break out as epidemics in populations with seasonal regularity, and are typically associated with “the flu”.⁵ Influenza C infections are far milder with respect to symptoms in humans, although studies show that up to 90% of 7-10-year-olds have been exposed to Influenza C at some point.⁶ Influenza D is mainly found in agricultural animals such as cattle and swine.⁷

Influenza viruses are spread via droplets from an infected person, such as from a cough or a sneeze, which are then inhaled. The virus sequesters itself in the mucosal linings (inside epithelial cells) of the receiver’s respiratory system and multiplies rapidly.⁸ Common symptoms of influenza include a sudden fever, dry cough, aches, and exhaustion.⁹

Influenza A is the type of influenza often responsible for epidemics and pandemics. It is further categorised into subtypes based on the specific assortment of haemagglutinin (H) and neuraminidase (N) proteins on the virus’s surface. This large combination of H and N proteins, along with the virus’s ability to swap around parts of its genome (a process called “reassortment”), gives Influenza A the ability to continually change, infect new hosts, and evade host immune systems.¹⁰ Notably, a type of H1N1 Influenza A virus was responsible for the 2009 Swine Flu pandemic.¹¹

What is parainfluenza?

Parainfluenza is an RTI caused by human parainfluenza viruses (HPIVs), which are part of a broader family of viruses called Paramyxoviridae.¹² There are four types of HPIVs: HPIV 1, 2, 3, and 4. HPIV-1 and HPIV-2 are the most prevalent, in general populations, whilst HPIV-3 is less common but associated with more serious acute illness in the lower respiratory tract. HPIV-4 is not well-studied, since it often causes such mild symptoms that they go unnoticed.¹²

The different HPIV types are structurally similar to each other, with the main differences being due to genetic variation in the hemagglutinin-neuraminidase (HN) protein on their surface. The main epidemiological differences between HPIV-1, 2, 3, and 4 are the age-groups of humans they infect, the seasonality of these infections, and the types of resulting illnesses.¹³

Just as Influenza viruses spread via inhalation of infected droplets, so too do HPIVs; and just like Influenza viruses they also replicate inside mucosal epithelial cells of the respiratory tract.¹² HPIV-1 and HPIV-2 tend to favour the upper respiratory tract to infect, resulting in typical cold-like symptoms such as a fever, sore throat, sneezing, coughing, etc. HPIV-3, on the other hand, prefers the lower respiratory tract, which still often results in cold-like symptoms,¹⁴ but in some instances can cause hospitalisation with more severe symptoms.

Parainfluenza vs influenza – the differences you need to know 

General symptoms and croup

As already mentioned, both Influenza and Parainfluenza often result in cold-like symptoms. Upon infection with HPIV or Influenza you likely won’t know which illness you have, and instead will simply treat it as a “flu” that requires rest, fluids, and paracetamol/ibuprofen to aid recovery.⁹ However, the symptomatic differences are more noticeable in children (such as in Croup) and in immunocompromised individuals, such as shortness of breath and chest pains, which may be indicative of Bronchitis (often caused by HPIV-3 infection of the bronchi and bronchioles in the lungs)¹⁵ and subsequent Pneumonia.¹⁶

HPIV-1 and HPIV-2 are the leading causes of Croup; particularly HPIV-1.¹⁷ Croup is a respiratory illness that starts as an upper RTI but develops into swelling of the larynx and trachea. Croup mainly affects children under 5, and has the distinctive symptoms of a “barking cough” and a hoarse voice due to the restricted airway capacity.¹⁸ Croup is usually mild and short-lived, but the symptoms are understandably stressful to witness, which partially explains why up to 15% of emergency department visits with RTIs in children are caused by Croup.¹⁷ If your child presents symptoms of Croup, the NHS recommends keeping them sat upright, giving them plenty of fluids, and checking up on them regularly even during the night (when symptoms are worst).¹⁸

Infection seasonality

Influenza and Parainfluenza infections follow seasonal patterns. A simplified comparison between Influenza and Parainfluenza seasonality in temperate regions of the planet (i.e., not the tropics) is that Influenza infection rates peak during the winter¹⁹, whilst Parainfluenza rates peak during Summer²⁰. However, when factoring in the rest of the world, and observing the HPIV subtypes, the seasonality of both Influenza and Parainfluenza becomes more complicated.

In tropical parts of the planet across Asia, Africa, and South America, Influenza has been shown to peak during the rainy seasons,²¹ whereas there is no indication of HPIV seasonality in the tropics.²² Whilst there are subtle seasonality differences across the subtypes of Influenza viruses (particularly Type A versus Type B),¹⁹ much of the scientific focus has remained on Influenza A as it is the key instigator of epidemics. With respect to Parainfluenza, HPIV-1 and HPIV-3 epidemics in temperate regions have a seasonal ebb-and-flow, often occurring in the autumn and springtime respectively but rarely circulating simultaneously.¹² HPIV-2 has also been shown to peak Autumnally,¹² although the infection rates are lower across the board.

These steadfast cyclical rhythms for both Influenza and Parainfluenza infection rates, even in an increasingly connected world where the spread of viruses should be less impacted by seasons, suggest that the interactions between virus, host, and environment are more intertwined than first thought.²³ Clearly, there is still much for science to uncover about viral seasonality.

Vaccine availability

For Influenza, vaccination is the most effective method of protection.²⁴ Several different influenza vaccines are available, which broadly work to stimulate the B cells in your immune system to generate antibodies specific to a chosen strain of Influenza (source) that have “memory” properties to allow long-term protection.²⁵ Current Influenza vaccines are based on well-established technologies, with Inactivated Influenza Vaccines (IIVs) making up the largest proportion due to affordable production methods;²⁶ Live Attenuated Influenza Vaccines (LAIVs) are also widely produced.²⁷ However, there are some downsides to the current suite of Influenza vaccines. The key downside is that they are highly strain-specific and have low cross-reactivity, meaning that they are ineffective against different strains, or if the circulating Influenza virus undergoes genetic reassorment and evades the vaccine. It takes 6-8 months to predict the next strain of Influenza and produce a vaccine,²⁸ which is a long time considering its Winter-based seasonality. Additionally, vaccines are most effective when high proportions of a population take them – but there is relatively low uptake of Influenza vaccines due to misconceptions about vaccines, and insufficient health care policies and education globally.²⁸

In contrast, there are currently no approved, clinically available vaccines for HPIVs. “Next-generation” vaccines (developed based on viral genetic data, such as mRNA vaccines)²⁹ are currently undergoing trials for HPIVs, with recent successes found in HPIV-3 vaccine development.³ Catching HPIV does not provide you with much long-term immunity, and you will be susceptible throughout life. Ultimately preventing Parainfluenza infections relies on non-pharmaceutical methods, such as hand-washing, self-isolation, and wearing face masks.²⁰ Fortunately, these will help you stay safe from Influenza too, as well as other contagious infections circulating the population.

Summary

Knowing the key differences between respiratory tract infections Influenza and Parainfluenza empowers you to take appropriate actions to safeguard your health. Whilst they both share flu-like symptoms and are transmitted via droplets, Parainfluenza is often milder than Influenza, apart from in exceptional cases such as Croup in young children and Bronchitis and Pneumonia in immunocompromised individuals. Influenza infection rates peak in the Winter in temperate climates, whereas Parainfluenza peaks throughout the rest of the year based on its subtype. Influenza vaccines are widely available and well-established, although not entirely effective, whereas licenced Parainfluenza vaccines do not yet exist and are still in research phases. If you feel any symptoms associated with Influenza or Parainfluenza, be sure to rest, remain hydrated, and take paracetamol/ibuprofen if appropriate – and seek professional medical attention if necessary.