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What Is Parainfluenza?

Human parainfluenza virus, or simply parainfluenza, is a common virus that causes upper and lower respiratory infections. Three-quarters of children 5 and older and 90% to 100% of older people have antibodies against parainfluenza. This suggests nearly everyone will be infected with the virus at some point in their lives, usually in early childhood.

While the name sounds a lot like influenza, the scientific name for the flu virus, the two are entirely separate viruses. Parainfluenza causes symptoms that are more similar to the common cold than the flu.

The vast majority of parainfluenza infections are mild and, in general, infections become less severe between adolescence through much of adulthood. Healthy adults might never even know they’re infected. But the virus can cause severe illness in certain groups, including infants and young children, adults 65 and older as well as people who have compromised immune systems. The virus is one of the top causes of hospitalization in kids under 5. Up to 17% of hospitalizations in this age group can be attributed to parainfluenza.

Parainfluenza types

There are four main types of human parainfluenza virus (HPIV) — HPIV 1, HPIV 2, HPIV 3 and HPIV 4 — as well several subtypes, including HPIV 4A and HPIV 4B. While the types are highly similar in biological structure, each behaves differently in humans. HPIV 1, HPIV 2 and HPIV 3 are the most likely to cause potentially dangerous lower respiratory infection in young children, older adults and the immunocompromised. HPIV 1 and HPIV 2 can cause croup in children, according to the CDC, whereas HPIV 3 is more likely to cause pneumonia and bronchitis.

As with other many viruses, each type of parainfluenza follows a distinct seasonal pattern:

  • HPIV 1 outbreaks are generally seen every other year (on odd-numbered years) between September and December.
  • HPIV 2 circulates annually. It’s less common than HPIV 1 and HPIV 3, but is more frequently seen during even years when HPIV 1 isn’t as prevalent.
  • HPIV 3 makes its round annually, typically between April and June with a second wave in November or December during even-numbered years.
  • HPIV 4 is much less common overall, and thus, seasonality has yet to be established.

How is parainfluenza transmitted?

Human parainfluenza virus is transmitted in much the same way other viruses are transmitted:

  • Breathing in droplets in the air that contain the virus (when an infected person coughs or sneezes near you)
  • Close contact, such as kissing or touching the face of someone who has the parainfluenza virus
  • Touching objects or surfaces that recently were touched by someone with the virus

It’s possible to be infected with the parainfluenza virus and not know it. It takes, on average, between two and six days from infection for symptoms to appear. And considering some cases never cause any symptoms at all, it can be difficult to determine when and where the virus was transmitted.

Anyone can get parainfluenza, and reinfection throughout life is common. This is the reason symptoms of the virus generally become milder the older people get — they become more immune to the virus with subsequent infections. This is also why parainfluenza infection is much more likely to cause severe illness in children younger than 5 — they haven’t yet built up immunity against it.

Parainfluenza symptoms

The parainfluenza virus most frequently affects the upper respiratory tract, producing common cold-like symptoms:

  • Sore throat
  • Stuffy or runny nose
  • Cough
  • Sneezing
  • Fever
  • Ear pain

More-severe cases of parainfluenza can lead to lower respiratory infections, such as croup, bronchitis, bronchiolitis or pneumonia. Parainfluenza is a top cause of pneumonia and bronchiolitis in young children, second only to RSV.

The signs of severe parainfluenza include:

  • Hoarseness
  • Wheezing
  • Bark-like cough or cough that produces mucus

Severe parainfluenza can also worsen existing chronic conditions, such as asthma in children and COPD and congestive heart failure in adults.

Call your pediatrician if any signs of severe illness develop. Head straight to the ER if your child has difficulty breathing, bluish fingernails, seizures, little to no urine or a temperature over 103 degrees.

Diagnosing parainfluenza

Most cases of parainfluenza go undiagnosed, as they’re not severe enough to warrant treatment. Because symptoms mimic those of the common cold, the only way to know for sure if parainfluenza is causing your child’s illness is to have them tested. The test involves using a swab to collect mucus from your child’s nose or throat and having it analyzed in a lab. There is currently no reliable way to test for parainfluenza at home. Flu and COVID-19 tests cannot detect parainfluenza.

Depending upon your child’s symptoms, other tests may be used to detect the presence of more serious illness, such as pneumonia or bronchitis. If these are suspected, your pediatrician may order imaging, such as a chest X-ray or CT scan to evaluate your child’s lower airways.

Parainfluenza treatment

By and large, illness from the parainfluenza virus will go away on its own in about seven to 10 days. During this time, you may use a combination of at-home remedies and over-the-counter medication to treat your child’s symptoms. For example, sipping on warm broth or eating popsicles can help with throat soreness. Sitting in a hot shower can help with congestion. Acetaminophen or a nonsteroidal anti-inflammatory (NSAID) like aspirin or ibuprofen can help alleviate ear pain and bring down a fever. Be sure to talk to your pediatrician about medication dosing, especially for young kids.

Currently, there are no antiviral medications on the market designed to treat the parainfluenza virus specifically. In the event a complication arises — such as croup or pneumonia — your child’s doctor may prescribe medication to treat the secondary condition while your child recovers from the parainfluenza virus. Breathing treatments may also be recommended, particularly in children who have asthma.

Preventing parainfluenza

With no vaccination currently available to protect against the parainfluenza virus, prevention efforts are focused on good hygiene and infection control.

Disinfect shared surfaces frequently and ask about sanitization procedures in your child’s day care or classroom. Consider hosting a school drive to collect disinfectant wipes so there are plenty in the classroom. Talk to your kids about the importance of handwashing and keeping their hands away from their face as much as possible.

Breastfeeding is a great way to protect your baby from parainfluenza. Human milk contains antibodies that provide significant protection from respiratory viruses, including parainfluenza. The longer a baby is breastfed or fed expressed milk, the lower their risk for hospitalization due to respiratory illness. And the protection is long-lasting. Breastfed babies have tested positive for antibodies they acquired from their mother’s milk at 1, 2 and even 6 years of age.

Because breastmilk plays such an important role in supporting a child’s developing immune system, the American Academy of Pediatrics revised its recommendations on breastfeeding in 2022. The AAP now encourages breastfeeding until age 2 or as long as both mother and child wish to nurse.

How parainfluenza affects children and adults

The vast majority of children and adults will recover from the parainfluenza virus in about seven to 10 days. Pregnant women may experience more pronounced symptoms of the illness, but there does not appear to be any long-term risk to the mother or baby, according to 2020 research.

The group most affected by the parainfluenza virus are the immunocompromised — including children who have a rare genetic disorder called severe combined immunodeficiency disease syndrome (SCIDS) — and those who have had organ or stem-cell transplants. While these individuals also usually recover from parainfluenza infection without issue, they are at the greatest risk for hospitalization, respiratory failure and even death, particularly if the illness progresses into pneumonia. These individuals should take extra precaution to prevent contracting the parainfluenza virus and alert their doctors right away any time they do get sick with a respiratory illness.

Researching the parainfluenza virus

Scientists have been studying the parainfluenza virus since it was first discovered in the 1950s. That’s how we know about the different types of the virus and their characteristics. Researchers also have developed collaborative initiatives and surveillance systems to monitor parainfluenza outbreaks around the world, which have provided insight into the virus’s seasonality.

Scientists have been trying to develop a vaccine for the parainfluenza virus as well as an antiviral medication designed to treat parainfluenza infection. A handful have made it to clinical trials, but none have been approved yet.