What is parainfluenza?

As the second most common cause of acute respiratory illness leading to hospitalisation in children under 5, parainfluenza is of particular concern to parents. Similar to the flu (influenza), parainfluenza is caused by a viral infection. It is specifically caused by infection with one of the human parainfluenza viruses (HPIVs), which can be divided into 4 types:¹

• Types 1 and 2: manifests into croup (infection of the upper airway that causes the passage to become narrow, leading to a barking cough and rasping sound when breathing)
• Type 3: also manifests in croup, is associated with bronchiolitis and pneumonia
• Type 4: causes mild upper respiratory infection, can cause more severe illness in infants

In severe cases, HPIVs can lead to acute lower respiratory illness (ARLI), a leading cause of postneonatal mortality in children under 5 across the world.²

Symptoms of parainfluenza are similar to those of the common cold - in most cases the illness causes: 

• Fever
• Runny nose
• Cough
• Sneezing
• Irritability
• Decreased appetite
• Ear pain
• Sore throat

The infection may be more serious if the patient experiences symptoms such as a barking cough, wheezing or stridor.

What are the risk factors of parainfluenza?

As the disease is associated with young children, outbreaks often occur in childcare facilities such as nurseries or daycare centres.³ Transmission occurs in a similar way to the common cold – via contact with infectious droplets, usually dispersed by sneezing and coughing.² The HPIV types tend to circulate during different seasons. For example, HPIV-3 infection peaks in the spring, whilst HPIV-2 is more common in autumn.⁴ 

You and your children are more at risk of catching HPIV if you are:⁵

• Malnourished
• Living in a crowded environment
• Living in a heavily polluted environment
• Experiencing vitamin A deficiency
• Your child is unable to be breastfed

Is there a vaccine for parainfluenza?

Unfortunately, there is no vaccine currently available that protects against any types of HPIV. There are also no antiviral treatments available. Healthcare professionals recommend at-home symptom management, such as over-the-counter pain medication (paracetamol, ibuprofen) or use of steam/a humidifier to ease throat soreness. When an outbreak occurs, use best practise infection control measures to prevent further transmission, such as handwashing and surface disinfection . More severe symptoms, such as croup, can be managed with specific medications like corticosteroids.

Research into HPIV vaccines has been ongoing since the 1960s, and recent breakthroughs could suggest that the development of a HPIV vaccine is on the horizon. In early trials in 1995, a variant of the disease from cattle was trialled as a HPIV vaccine. In phase II, the vaccine produced antibodies that worked against HPIV-3 in infants, alongside being safe and well tolerated.⁶ More research is needed to find a safe and cost-effective way to produce this vaccine on a large scale.

More recent studies show the progress being made in parainfluenza vaccine development.

Recent vaccine developments 

Study 1 – cDNA-derived vaccine

In 2011, a genetically modified HPIV type 3 vaccine known as rHPIV3cp45 was examined in a phase I trial. Infants between 6 and 12 months were given two doses of the vaccine for up to 10 weeks apart. The initial dose produced an immune response, though this effect was not boosted by the second dose.⁷ In a subsequent study, the safety and tolerance of the vaccine were confirmed, alongside the discovery that a six-month gap between doses produced a better antibody response.⁸ These results are promising, as the vaccine shows effectiveness in young children, the group most at risk of hospitalisation from parainfluenza.

Study 2 – live-attenuated type 1 vaccine

In 2015, a live-attenuated Type 1 vaccine was trialled in adults and children. The children were separated into groups depending on prior exposure to the virus, in order to better understand the immune response in the target group. The vaccine (labelled rHPIV-1/84/del 170/942A) did not produce an adequate immune response, with only 2 patients producing antibodies. In response, the vaccine dosage was upped, which resulted in 3 patients producing antibodies.⁹ Further study into the effect of this kind of vaccine on the target population is needed.

Study 3 – live-attenuated type 2 vaccine

The latest study to be published in the pursuit of a HPIV vaccine came in 2023, in which a live-attenuated HPIV type 2 was trialled in adults and children. This vaccine (labelled rHPIV215C/948L/∆1724) was safe and attenuated in all patients, though no antibody production was observed in any of the groups. These results suggest over attenuation, where the virulence was reduced too much.¹⁰

Vaccine research – a summary 

As discussed in this section, there have been multiple attempts at creating vaccines for parainfluenza, with many focusing on one type in particular. Unfortunately, despite being safe for use, most of the vaccines were found to be over-attenuated and did not produce an adequate antibody response that would protect the recipient from subsequent infection. 

Future research should carefully adjust the attenuation of these candidates while aiming to maintain the safety profile. This is especially important when considering the target population of this vaccine – young children. Once this has been achieved, the design of a combined vaccine can begin in earnest. An ideal vaccine would be able to protect against multiple types of parainfluenza, which would require infants to receive just one vaccination and be much more manageable for parents.

Summary

Parainfluenza is a major cause of hospitalisation of children under 5, sitting above influenza in the rankings. In most cases, infection with the virus causes cold-like symptoms and a sore throat, though some cases can worsen into croup. In some instances, the infection can escalate into more serious illnesses such as bronchiolitis and pneumonia. In severe cases, parainfluenza can cause ARLI, which results in 1.6 million deaths globally each year. Currently, there are no vaccines available that protect against parainfluenza in its many forms. Research is ongoing in this area, with a focus initially on developing type-specific vaccines that produce an appropriate antibody response in the target group of young children. Once this is achieved, work can begin on engineering these separate vaccines into a combined inoculation that protects against more than one type of parainfluenza. In the meantime, the illness can be managed with over-the-counter pain medication and plenty of rest. If you find yourself in the midst of an outbreak, it is recommended that surfaces are disinfected regularly, and consistent handwashing is observed.