We all know that time of year. As the weather begins to cool, cases of the flu begin to rise. This is referred to as “flu season.”

The flu is a respiratory illness that’s caused by the influenza virus. There are four types of influenza virus: A, B, C, and D. Influenza A, B, and C can infect humans. However, only influenza A and B cause the seasonal epidemics of respiratory illness that occur every year.

Influenza A viruses are further divided into different subtypes based on two proteins found on the surface of the virus — hemagglutinin (HA) and neuraminidase (NA). There are 18 different subtypes of HA, which are numbered H1 through H18. Similarly, there are 11 different subtypes of NA, numbered N1 through N11.

The combinations of the different subtypes of HA and NA are used to classify influenza A viruses. Some influenza A subtypes that you may be familiar with include H1N1 and H3N2.

Let’s take a closer look at H3N2 influenza viruses.

Flu caused by H3N2 viruses during the 2017/18 flu season. Typically, flu seasons that are dominated by H3N2 activity are more severe, particularly among at-risk groups such as older adults and younger children.

for the 2017/18 flu season indicated that there were over 30,000 reported flu-related hospitalizations across the country. Almost 200 pediatric deaths occurred, mostly in unvaccinated children.

Additionally, the flu vaccine for the 2017/18 season was found to be 40 percent effective overall according to the . When broken down by virus, it was 65 percent effective against H1N1, 25 percent effective against H3N2, and 49 percent effective against influenza B.

for the 2018/19 flu season shows that H1N1 strains have been more prevalent as of January 2019. Most hospitalizations have been due to H1N1 and are occurring in older adults and young children.

The symptoms of flu caused by H3N2 are similar to other seasonal influenza viruses. Symptoms typically appear suddenly and can include:

  • cough
  • runny or congested nose
  • sore throat
  • headache
  • body aches and pains
  • fever
  • chills
  • fatigue
  • diarrhea
  • vomiting

Each year, the annual flu vaccine protects against three (trivalent) or four (quadrivalent) strains of flu. An H1N1, H3N2, and influenza B strain are included in the trivalent vaccine, while an extra influenza B strain is included in the quadrivalent vaccine.

According to the CDC, the flu vaccine reduces the risk of flu illness in the general population by during most flu seasons when the vaccine strains are a good match to circulating strains.

The flu vaccine tends to offer more protection from flu that’s caused by H1N1 viruses and influenza B viruses in comparison to H3N2 viruses. This can be explained in a couple of ways.

First, while all flu viruses mutate from year to year, H3N2 viruses tend to accrue more changes that are different from the H3N2 component of the flu vaccine. This can lead to a poor match between the strain included in the vaccine and the strains that circulate during flu season.

The second factor has to do with how flu vaccines are produced. Many flu vaccines are produced in eggs. H3N2 viruses tend to adapt to growth in eggs more readily than other types of flu viruses. These egg-adapted changes can reduce the effectiveness of the vaccine strain.

The egg adaptation problem will continue as long as flu vaccines are produced in eggs. While the H3N2 vaccine strain that was recommended for the 2018/19 flu season is different from the previous season’s H3N2 strain, it still contains the same egg-adapted mutation.

Scientists are currently working hard to further develop effective egg-free methods of vaccine production to try to avoid these changes. In the meantime, according to the CDC, getting the seasonal vaccine is still the to prevent becoming ill with flu.

Treatment of an uncomplicated case of seasonal flu, such as H3N2, involves managing symptoms while you recover. Ways to do this include:

  • getting plenty of rest
  • drinking enough fluids
  • taking over-the-counter medication to relieve symptoms such as fever, headache, and aches and pains

In some cases, doctors may prescribe an antiviral medication, such as oseltamivir (Tamiflu). When started within 48 hours after developing flu symptoms, antiviral medication can help to shorten the duration of illness and prevent complications from developing.

Some people are at an increased risk of developing serious complications from the flu. These complications can include pneumonia or worsening of a preexisting medical condition, such as asthma.

Certain individuals should see their doctor if they suspect they have flu:

Most people that get sick with a seasonal flu such as H3N2 can recover at home without the treatment of a doctor. Symptoms typically ease within a week, although cough or feelings of fatigue may linger for a couple of weeks.

If you’re in a group that’s at a higher risk of complications from flu, you should be sure to see your doctor if you come down with flu symptoms.

Symptoms that can signal an emergency and warrant prompt medical attention include:

  • feeling short of breath or having trouble breathing
  • appearance of pain or pressure in your chest or abdomen
  • dizziness that comes on suddenly
  • persistent, severe vomiting
  • feelings of confusion
  • symptoms that begin to improve but then return with a worsened cough and fever

You can take the following steps to prevent becoming ill with seasonal flu viruses, including H3N2:

  • Get the annual flu vaccine every year. Try to get it by the end of October, if possible.
  • Wash your hands frequently, particularly after using the restroom, before eating, and before touching your face, nose, or mouth.
  • Where possible, avoid crowded areas where flu can spread easily. Examples include schools, public transit, and office buildings.
  • Avoid coming into contact with people who are sick.

If you’re sick with flu, you can prevent spreading it to others by staying home until 24 hours after your fever has gone down and being sure to cover your mouth when you cough or sneeze.