Influenza (Seasonal)

Key facts

• Influenza is an acute viral infection that spreads easily from person to person.

• Influenza circulates worldwide and can affect anybody in any age group.

• Influenza causes annual epidemics that peak during winter in temperate regions.

• Influenza is a serious public health problem that causes severe illnesses and deaths for higher risk populations.

• An epidemic can take an economic toll through lost workforce productivity, and strain health services.

• Vaccination is the most effective way to prevent infection.

Overview

Seasonal influenza is an acute viral infection caused by an influenza virus. There are three types of seasonal influenza – A, B and C. Type A influenza viruses are further typed into subtypes according to different kinds and combinations of virus surface proteins. Among many subtypes of influenza A viruses, currently influenza A(H1N1) and A(H3N2) subtypes are circulating among humans. Influenza viruses circulate in every part of the world. Type C influenza cases occur much less frequently than A and B. That is why only influenza A and B viruses are included in seasonal influenza vaccines.

Signs and Symptoms

Seasonal influenza is characterized by a sudden onset of high fever, cough (usually dry), headache, muscle and joint pain, severe malaise (feeling unwell), sore throat and runny nose. Most people recover from fever and other symptoms within a week without requiring medical attention. But influenza can cause severe illness or death in people at high risk (see below). The time from infection to illness, known as the incubation period, is about two days.

Who is at risk?

Yearly influenza epidemics can seriously affect all age groups, but the highest risk of complications occur among children younger than age two, adults age 65 or older, and people of any age with certain medical conditions, such as chronic heart, lung, kidney, liver, blood or metabolic diseases (such as diabetes), or weakened immune systems.

Transmission

Seasonal influenza spreads easily and can sweep through schools, nursing homes or businesses and towns. The main way that influenza viruses are spread is from person to person in respiratory droplets of coughs and sneezes. (This is called "droplet spread.") This can happen when droplets from a cough or sneeze of an infected person are propelled through the air and deposited on the mouth or nose of people nearby. Influenza viruses may also be spread when a person touches respiratory droplets on another person or an object and then touches their own mouth or nose (or someone else’s mouth or nose) before washing their hands.

Most people who get influenza will recover in a few days to less than 2 weeks, but some people will develop life-threatening complications (such as pneumonia) as a result of the flu. Millions of people in the United States — about 5% to 20% of U.S. residents — will get influenza each year. An average of about 36,000 people per year in the United States die from influenza-related causes, and more than 200,000 have to be admitted to the hospital as a result of influenza-related causes. Anyone can get the flu (even healthy people), and serious problems from influenza can happen at any age. People age 65 years and older, people of any age with chronic medical conditions (such as asthma, diabetes, or heart disease), pregnant women, and young children are more likely to get complications from influenza.

Disease effects

Influenza can cause serious public health and economic problems. In developed countries, epidemics can result in high levels of worker absenteeism and productivity losses. In communities, clinics and hospitals can be overwhelmed when large numbers of sick people appear for treatment during peak illness periods. While most people recover from a bout of influenza, there are large numbers of people who need hospital treatment and many who die from the disease every year. Little is known about the effects of influenza epidemics in developing countries.

Complications of Flu

Complications of flu can include bacterial pneumonia, ear infections, sinus infections, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes.

Seasonal Flu and Staph Infections

Bacterial infections can occur as co-infections with influenza or occur following influenza infection. In 2006-2007, CDC noted an increase in flu and Staphylococcus aureus (S. aureus) co-infections among children who had died or were hospitalized with influenza infection. Some of those infections were with methicillin-resistant S. aureus (MRSA). CDC is working with state and local public health authorities to monitor and investigate flu-S. aureus co-infections, including pneumonias and other types of S. aureus infections.

Treatment

Antiviral drugs for influenza are available in some countries and effectively prevent and treat the illness. There are two classes of such medicines, 1) adamantanes (amantadine and remantadine), and 2) inhibitors of influenza neuraminidase (oseltamivir and zanamivir). Some influenza viruses develop resistance to the antiviral medicines, limiting the effectiveness of treatment. WHO monitors antiviral susceptibility in the circulating influenza viruses.

Prevention

The most effective way to prevent the disease or severe outcomes from the illness is vaccination. Safe and effective vaccines have been available and used for more than 60 years. Among healthy adults, influenza vaccine can prevent 70% to 90% of influenza-specific illness. Among the elderly, the vaccine reduces severe illnesses and complications by up to 60%, and deaths by 80%.

Vaccination is especially important for people at higher risk of serious influenza complications, and for people who live with or care for high risk individuals. There are two types of vaccines:

• The "flu shot" – an inactivated vaccine (containing killed virus) that is given with a needle. The flu shot is approved for use in people 6 months of age and older, including healthy people and people with chronic medical conditions.

• The nasal-spray flu vaccine – a vaccine made with live, weakened flu viruses that do not cause the flu (sometimes called LAIV for “Live Attenuated Influenza Vaccine”). LAIV is approved for use in healthy* people 2-49 years of age who are not pregnant.

About two weeks after vaccination, antibodies develop that protect against influenza virus infection. Flu vaccines will not protect against flu-like illnesses caused by non-influenza viruses.

WHO recommends annual vaccination for (in order of priority):

• nursing-home residents (the elderly or disabled)

• elderly individuals

• people with chronic medical conditions

• other groups such as pregnant women, health care workers, those with essential functions in society, as well as children from ages six months to two years.

Influenza vaccination is most effective when circulating viruses are well-matched with vaccine viruses. Influenza viruses are constantly changing, and the WHO Global Influenza Surveillance Network (GISN), a partnership of National Influenza Centres around the world, monitors the influenza viruses circulating in humans. WHO annually recommends a vaccine composition that targets the three most representative strains in circulation.

When to Get Vaccinated

In the US, yearly flu vaccination should begin in September or as soon as vaccine is available and continue throughout the influenza season, into December, January, and beyond. This is because the timing and duration of influenza seasons vary. While influenza outbreaks can happen as early as October, most of the time influenza activity peaks in January or later.

Who Should Not Be Vaccinated

Some people should not be vaccinated without first consulting a physician. They include:

• People who have a severe allergy to chicken eggs.

• People who have had a severe reaction to an influenza vaccination in the past.

• People who developed Guillian-Barre Syndrome (GBS) within 6 weeks of getting an influenza vaccine previously.

• Children less than 6 months of age (influenza vaccine is not approved for use in this age group).

• People who have a moderate or severe illness with a fever should wait to get vaccinated until their symptoms lessen.

If you have questions about whether you should get a flu vaccine, consult your health-care provider.