Seasonal Influenza Q&A

What is seasonal influenza (flu)?

Seasonal influenza, commonly called “the flu,” is caused by influenza viruses, which infect the respiratory tract (i.e., the nose, throat, lungs). Unlike many other viral respiratory infections, such as the common cold, the flu can cause severe illness and life-threatening complications in many people. Some people, such as older people, young children, pregnant women, and people with certain health conditions, are at high risk for serious flu complications. The best way to prevent seasonal flu is by getting a flu vaccination each year.

Flu vaccines protect against the influenza viruses that research indicates will be most common during the upcoming season. Everyone 6 months and older should get vaccinated against the flu every year. Get vaccinated soon after vaccine becomes available in your community, if possible by October. Immunity sets in about two weeks after vaccination.

What are the symptoms of the flu?

Fever, chills, fatigue, body aches, sore throat, and headache are common symptoms of flu. Not everyone with flu will have a fever. The flu can cause mild to severe illness, and at times can lead to death. The flu is different from a cold. The flu usually comes on suddenly.

How does the flu spread?

The main way that influenza viruses are thought to spread is from person to person in respiratory droplets of coughs and sneezes.

If I got the flu or the flu vaccine last year, will I have immunity against the flu this year?

Not necessarily. Several studies conducted over different flu seasons and involving different influenza viruses and types of flu vaccine have shown that a person’s protective antibody against influenza viruses declines over the course of a year after vaccination and infection, particularly in the elderly. So a flu shot given during one season, or an infection acquired during one season, may not provide adequate protection through later seasons.

The decline in protective antibody against the flu that occurs after vaccination or after flu infection may be influenced by several factors, including a person’s age, the antigen used in the vaccine, and the person’s health situation (for example, chronic health conditions that weaken the immune system may have an impact).

This decline in protective antibody has the potential to leave some people more vulnerable to infection, illness and possibly serious complications from the same influenza viruses a year after being vaccinated or infected.

So, for optimal protection against influenza, annual vaccination is recommended regardless of past vaccination status or flu infection.

Does the flu have complications?

Yes. Some of the complications caused by flu include bacterial pneumonia, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes. Children may get sinus problems and ear infections as complications from the flu.

How do I find out if I have the flu?

It is very difficult to distinguish the flu from other viral or bacterial causes of respiratory illnesses on the basis of symptoms alone. There are tests available to diagnose flu.

Do other respiratory viruses circulate during flu season?

In addition to flu viruses, several other respiratory viruses also can circulate during flu season and can cause symptoms and illness similar to those seen with flu infection. These non-flu viruses include rhinovirus (one cause of the "common cold") and respiratory syncytial virus (RSV), which is the most common cause of severe respiratory illness in young children as well as a leading cause of death from respiratory illness in those aged 65 years and older.

How soon will I get sick if I am exposed to the flu?

The time from when a person is exposed to flu virus to when symptoms begin is about 1 to 4 days, with an average of about 2 days.

How long is a person with flu virus contagious?

Most healthy adults may be able to infect other people beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick. Children may pass the virus for longer than 7 days. Symptoms start 1 to 4 days after the virus enters the body.

Can the flu be treated?

Yes. There are influenza antiviral drugs that can be used to treat flu illness.

Is the “stomach flu” really the flu?

Many people use the term “stomach flu” to describe illnesses with nausea, vomiting or diarrhea. These symptoms can be caused by many different viruses, bacteria or even parasites. While vomiting, diarrhea, and being nauseous or “sick to your stomach” can sometimes be related to the flu — more commonly in children than adults — these problems are rarely the main symptoms of influenza. The flu is a respiratory disease and not a stomach or intestinal disease.