We all know that time of year. As the weather begins to cool, cases of the flu begin to rise, and the flu season begins.

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 spread to 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.

Influenza A viruses are classified according to both their HA and NA subtypes. Some influenza A subtypes that you may be familiar with include H1N1 (sometimes known as swine flu) and H3N2.

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

Flu caused by H3N2 viruses predominated 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.

Data for the 2017/18 flu season indicated that there were 808,129 reported flu-related hospitalizations across the United States, with an estimated 643 pediatric deaths. There were 188 reported pediatric deaths, mostly in unvaccinated children.

In comparison, there were 404,646 reported flu-related hospitalizations during the 2019/20 flu season, when influenza B and H1N1 predominated. There were an estimated 434 pediatric deaths, with 198 reported pediatric deaths.

Additionally, the flu vaccine for the 2017/18 season was found to be 40 percent effective overall, according to the Centers for Disease Control and Prevention (CDC). If you received the vaccine, your chance of needing professional medical attention for the flu were reduced by 40 percent.

When broken down by virus, the vaccine was 65 percent effective against H1N1, 25 percent effective against H3N2, and 49 percent effective against influenza B.

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

The flu has some symptoms in common with other respiratory illnesses, such as the cold. This is one reason why it’s impossible to know whether a person has the flu just by observing their physical symptoms.

A healthcare professional can perform a lab test to confirm whether you have the flu or some other illness. However, if you experience flu-like symptoms during the traditional flu season (October to May), they may diagnose you with the flu even without the help of a lab test.

Each year, the annual flu vaccine protects against either three or four strains of flu. These are known as trivalent and quadrivalent vaccines, respectively. 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 between 40 and 60 percent 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 than flu caused by 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 undergo more genetic changes. This can lead to a poor match between the strain included in the vaccine and the strains that later circulate during flu season.

The second explanation 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. In the meantime, according to the CDC, getting the seasonal vaccine is still the best way to prevent becoming ill with flu.

Treatment of an uncomplicated case of seasonal flu, whether H3N2 or another strain, 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 for developing serious complications from the flu. These complications can include pneumonia or the worsening of a preexisting medical condition, such as asthma.

See a doctor if you suspect you have the flu and you belong to one of the following groups:

Most people that get sick with a seasonal flu 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 for complications from flu, 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:

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

  • 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 the flu, you can prevent transmitting 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.