Influenza A and B: What to Know About Symptoms and Treatments This Flu Season

Each fall, the flu emerges as a formidable force, spreading through the droplets produced when people cough, sneeze or talk, and sometimes hitching a ride to our noses by way of the infected surfaces that we touch. Influenza viruses cause tens of millions of illnesses and thousands of deaths in the United States each year.

There are two players every flu season: influenza Type A and Type B. Influenza A tends to pop up early in the season and accounts for more than 75 percent of all cases, while influenza B typically has a distinct but much smaller peak in late February or March.

“You tend to have one or the other virus predominant at any given time,” said Anice Lowen, an associate professor of microbiology and immunology at the Emory University School of Medicine. Here’s what to know about the flu viruses.

What’s the difference between Type A and Type B?

Influenza A viruses come in many varieties. Four subtypes commonly affect humans, Dr. Lowen said — you may have heard of H1N1, for example. But many more influenza A viruses have been identified in animals, which makes it a potential source of pandemics whenever a new type of influenza A jumps to humans, she said. All four flu pandemics — in 1918, 1957 and 1968 and the swine flu pandemic of 2009 — were caused by Type A viruses.

Influenza B viruses, however, only circulate in humans. They are divided into just two lineages that cause seasonal outbreaks, Dr. Lowen said. Because influenza B viruses evolve more slowly than influenza A viruses, they have shorter and more predictable flu seasons.

Still, each year experts closely monitor flu patterns all over the world and try to predict how the viruses might mutate, said Dr. Sean Liu, an assistant professor of infectious disease at the Icahn School of Medicine at Mount Sinai in New York City. “We haven’t come up with a 100 percent accurate way to do it,” Dr. Liu said. “That’s why the efficacy of flu vaccines varies from season to season.”

In the United States, flu shots are updated annually to include two types each of influenza A and B viruses; scientists pick strains of the viruses expected to be dominant that year. “Your antibodies and your immunity toward these viruses wane over time,” Dr. Liu said. “So the updated vaccine helps remind your body how to react when you face new versions of the virus.”

Public health officials recommend everyone six months and older get an annual flu shot.

What are symptoms of an influenza infection?

If you do get sick with the flu, the most common symptoms include a sudden onset of fever, chills, muscle aches, fatigue and respiratory ailments, such as a cough or sore throat.

Most otherwise healthy people are sick for three to seven days, Dr. Liu said. In some cases, the cough and fatigue can linger for two weeks.

In the past, doctors thought that Type A infections were more severe than Type B. But studies have shown that both Types A and B result in similar numbers of hospitalizations and deaths in adults. In children, however, influenza B may cause more severe disease and is associated with higher rates of mortality.

The most common complication of severe influenza is pneumonia, a lung infection that can result from the flu virus or from a secondary bacterial infection, Dr. Liu said. The flu can also trigger an extreme inflammatory response in the body and worsen existing chronic conditions like asthma or heart disease.

How do you know which type you have?

Some rapid tests available at doctors offices and urgent care centers can distinguish between influenza A and B, although they tend to have low sensitivity, a measure of how well a test performs at catching cases of flu. . Doctors can also order more sensitive P.C.R. tests to identify the specific flu virus causing infection, Dr. Liu said, “but it doesn’t change management much.”

How do you treat influenza?

The approach to treating the flu, regardless of type, revolves around relieving symptoms and supporting the body’s natural defenses, Dr. Liu said. There are four approved antiviral medications, including Tamiflu, that are commonly prescribed to shorten the duration and severity of illness. Doctors typically reserve these antivirals for people at risk of developing severe complications, Dr. Liu said, including people 65 years and older; those who are pregnant or have chronic conditions like asthma, diabetes or heart disease; and children younger than 5. Because the drugs shave off only a day or so of illness, experts believe the benefits are not worth the cost of treatment for most people.

For those recovering at home, doctors recommend staying hydrated, getting plenty of rest and using over-the-counter medications to reduce fever as needed. But if your symptoms persist, you develop a sustained fever of 102 degrees or higher, you have difficulty breathing or you experience severe chest or stomach pain, it is important to seek medical attention immediately. These may be signs that you need a stronger influenza treatment.