Influenza (Seasonal) (2023)

Ask the expert: Influenza Q&A

Seasonal influenza is an acute respiratory infection caused by influenza viruses which circulate in all parts of the world.

The pathogen

There are 4 types of seasonal influenza viruses, types A, B, C and D. Influenza A and B viruses circulate and cause seasonal epidemics of disease.

  • Influenza A viruses are further classified into subtypes according to the combinations of the hemagglutinin (HA) and the neuraminidase (NA), the proteins on the surface of the virus. Currently circulating in humans are subtype A(H1N1)and A(H3N2) influenza viruses. The A(H1N1) is also written as A(H1N1)pdm09 as it caused the pandemic in 2009 and subsequently replaced the seasonal influenza A(H1N1) virus which had circulated prior to 2009. Only influenza type A viruses are knownto have caused pandemics.
  • Influenza B viruses are not classified into subtypes, but can be broken down into lineages. Currently circulating influenza type B viruses belong to either B/Yamagata or B/Victoria lineage.
  • Influenza C virus is detected less frequently and usually causes mild infections, thus does not present public health importance.
  • Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.

Signs and symptoms

Seasonal influenza is characterized by a sudden onset of fever, cough (usually dry), headache, muscle and joint pain, severe malaise (feeling unwell), sore throat and a runny nose. The cough can be severe and can last 2 or more weeks. Most people recoverfrom fever and other symptoms within a week without requiring medical attention. But influenza can cause severe illness or death especially in people at high risk (see below).

Illnesses range from mild to severe and even death. Hospitalization and death occur mainly among high risk groups. Worldwide, these annual epidemics are estimated to result in about 3 to 5 million cases of severe illness, and about 290 000 to 650 000respiratory deaths.

In industrialized countries most deaths associated with influenza occur among people age 65 or older (1). Epidemics can result in high levels of worker/school absenteeism and productivity losses. Clinics and hospitals can be overwhelmed during peak illnessperiods.

(Video) Know more about seasonal influenza

The effects of seasonal influenza epidemics in developing countries are not fully known, but research estimates that 99% of deaths in children under 5 years of age with influenza related lower respiratory tract infections are found in developing countries(2).


All age groups can be affected but there are groups that are more at risk than others.

  • People at greater risk of severe disease or complications when infected are: pregnant women, children under 59 months, the elderly, individuals with chronic medical conditions (such as chronic cardiac, pulmonary, renal, metabolic, neurodevelopmental,liver or hematologic diseases) and individuals with immunosuppressive conditions (such as HIV/AIDS, receiving chemotherapy or steroids, or malignancy).
  • Health care workers are at high risk acquiring influenza virus infection due to increased exposure to the patients and risk further spread particularly to vulnerable individuals.

In terms of transmission, seasonal influenza spreads easily, with rapid transmission in crowded areas including schools and nursing homes. When an infected person coughs or sneezes, droplets containing viruses (infectious droplets) aredispersed into the air and can spread up to one meter, and infect persons in close proximity who breathe these droplets in. The virus can also be spread by hands contaminated with influenza viruses. To prevent transmission, people should cover theirmouth and nose with a tissue when coughing, and wash their hands regularly.

In temperate climates, seasonal epidemics occur mainly during winter, while in tropical regions, influenza may occur throughout the year, causing outbreaks more irregularly.

The time from infection to illness, known as the incubation period, is about 2 days, but ranges from one to four days.


The majority of cases of human influenza are clinically diagnosed. However, during periods of low influenza activity and outside of epidemics situations, the infection of other respiratory viruses e.g. rhinovirus, respiratory syncytial virus, parainfluenzaand adenovirus can also present as Influenza-like Illness (ILI) which makes the clinical differentiation of influenza from other pathogens difficult.

(Video) What is influenza?

Collection of appropriate respiratory samples and the application of a laboratory diagnostic test is required to establish a definitive diagnosis. Proper collection, storage and transport of respiratory specimens is the essential first step for laboratorydetection of influenza virus infections. Laboratory confirmation of influenza virus from throat, nasal and nasopharyngeal secretions or tracheal aspirate or washings is commonly performed using direct antigen detection, virus isolation, or detectionof influenza-specific RNA by reverse transcriptase-polymerase chain reaction (RT-PCR). Various guidance on the laboratory techniques is published and updated by WHO. (3)

Rapid influenza diagnostic tests (RIDTs) are used in clinical settings, but they have lower sensitivity compared to RT-PCR methods and their reliability depends largely on the conditions under which they are used.


Patients with uncomplicated seasonal influenza:

Patients that are not from a high risk group should be managed with symptomatic treatment and are advised, if symptomatic, to stay home in order to minimize the risk of infecting others in the community. Treatment focuses on relievingsymptoms of influenza such as fever. Patients should monitor themselves to detect if their condition deteriorates and seek medical attention Patients that are known to be in a group at high risk for developing severe or complicated illness, (see above)should be treated with antivirals in addition to symptomatic treatment as soon as possible.

Patients with severe or progressive clinical illness associated with suspected or confirmed influenza virus infection (i.e. clinical syndromes of pneumonia, sepsis or exacerbation of chronic underling diseases) should be treated withantiviral drug as soon as possible.

  • Neuraminidase inhibitors (i.e. oseltamivir) should be prescribed as soon as possible (ideally, within 48 hours following symptom onset) to maximize therapeutic benefits. Administration of the drug should also be considered in patients presenting laterin the course of illness.
  • Treatment is recommended for a minimum of 5 days, but can be extended until there is satisfactory clinical improvement.
  • Corticosteroids should not be used routinely, unless indicated for other reasons (eg: asthma and other specific conditions); as it has been associated with prolonged viral clearance, immunosuppression leading to bacterial or fungal superinfection.
  • All currently circulating influenza viruses are resistant to adamantane antiviral drugs (such as amantadine and rimantadine), and these are therefore not recommended for monotherapy.

WHO GISRS monitors resistance to antivirals among circulating influenza viruses to provide timely guidance for antiviral use in clinical management and potential chemoprophylaxis.


The most effective way to prevent the disease is vaccination. Safe and effective vaccines are available and have been used for more than 60 years. Immunity from vaccination wanes over time so annual vaccination is recommended to protectagainst influenza. Injected inactivated influenza vaccines are most commonly used throughout the world.

(Video) Why is flu season in the winter?

Among healthy adults, influenza vaccine provides protection, even when circulating viruses do not exactly match the vaccine viruses. However, among the elderly, influenza vaccination may be less effective in preventing illness but reduces severity ofdisease and incidence of complications and deaths. Vaccination is especially important for people at high risk of influenza complications, and for people who live with or care for the people at high risk.

WHO recommends annual vaccination for:

  • pregnant women at any stage of pregnancy
  • children aged between 6 months to 5 years
  • elderly individuals (aged more than 65 years)
  • individuals with chronic medical conditions
  • health-care workers.

Influenza vaccine is most effective when circulating viruses are well-matched with viruses contained in vaccines. Due to the constant evolving nature of influenza viruses, the WHO Global Influenza Surveillance and Response System (GISRS) – a systemof National Influenza Centres and WHO Collaborating Centres around the world – continuously monitors the influenza viruses circulating in humans and updates the composition of influenza vaccines twice a year.

For many years, WHO has updated its recommendation on the composition of the vaccine (trivalent) that targets the 3 most representative virus types in circulation (two subtypes of influenza A viruses and one influenza B virus). Starting with the 2013–2014northern hemisphere influenza season, a 4th component is recommended to support quadrivalent vaccine development. Quadrivalent vaccines include a 2nd influenza B virus in addition to the viruses in trivalent vaccines, and are expected to provide widerprotection against influenza B virus infections. A number of inactivated influenza vaccines and recombinant influenza vaccines are available in injectable form. Live attenuated influenza vaccine is available as a nasal spray.

Pre-exposure or post-exposure prophylaxis with antivirals is possible but depends on several factors e.g. individual factors, type of exposure, and risk associated with the exposure.

Apart from vaccination and antiviral treatment, the public health management includes personal protective measures like:

(Video) The importance of vaccinations for influenza

  • Regular hand washing with proper drying of the hands
  • Good respiratory hygiene – covering mouth and nose when coughing or sneezing, using tissues and disposing of them correctly
  • Early self-isolation of those feeling unwell, feverish and having other symptoms of influenza
  • Avoiding close contact with sick people
  • Avoiding touching one’s eyes, nose or mouth

WHO response

WHO, through the WHO GISRS system, in collaboration with other partners, monitors influenza activity globally, recommends seasonal influenza vaccine compositions twice a year for the Northern and Southern hemisphere influenza seasons, guides countriesin tropical and subtropical areas to choose vaccine formulations (Northern hemisphere vs. Southern hemisphere), to support decisions for timing of vaccination campaigns, and to support Member States to develop prevention and control strategies.

WHO works to strengthen national, regional and global influenza response capacities including diagnostics, antiviral susceptibility monitoring, disease surveillance and outbreak responses, and to increase vaccine coverage among high risk groups and preparefor the next influenza pandemic.

Estimates of US influenza-associated deaths made using four different methods.
Thompson WW, Weintraub E, Dhankhar P, Cheng OY, Brammer L, Meltzer MI, et al. Influenza Other Respi Viruses. 2009;3:37-49

(2)Global burden of respiratory infections due to seasonal influenza in young children: a systematic review and meta-analysis.
Nair H, Abdullah Brooks W, Katz M et al. Lancet 2011; 378: 1917–3


WHO recommended surveillance standards, Second edition.


Is influenza virus seasonal? ›

In the United States, flu season usually occurs in the fall and winter. While influenza viruses spread year-round, most of the time flu activity peaks between December and February. The overall health impact (e.g., infections, hospitalizations, and deaths) of flu varies from season to season.

Why is influenza seasonal? ›

Although other factors probably contribute as well, the main reason we have a flu season may simply be that the influenza virus is happier in cold, dry weather and thus better able to invade our bodies.

How long does seasonal influenza last? ›

But the truth is, the flu lasts from three to seven days, according to the CDC. If you are normally healthy, the flu can be short lived, but your cough and tiredness may stick around for two weeks.

Which type of influenza is known to cause seasonal? ›

Flu A and B viruses are responsible for seasonal flu epidemics more commonly known as the flu season. Flu viruses can change in two different ways—antigenic drift and antigenic shift.

Why is influenza common in winter? ›

Colds, flus and other respiratory illnesses are more common in colder months. People are indoors more often, allowing viruses to pass more easily from one person to another. And the cold, dry air may weaken resistance.

How long does influenza A last 2022? ›

For most healthy people, the flu is an uncomfortable but short-term illness that resolves itself as the immune system fights it off. Symptoms usually appear from one to four days after exposure to the virus, and they last five to seven days.

What's the difference between seasonal influenza and pandemic influenza? ›

People are exposed to influenza viruses throughout their lives, so most people have had exposure to the influenza strains that cause seasonal flu. Pandemic flu occurs when a new strain emerges that infects people, spreads easily from person-to-person, and to which most people do not have immunity.

What is the main cause of influenza? ›

The flu is caused by an influenza virus. Most people get the flu when they breathe in tiny airborne droplets from the coughs or sneezes of someone who has the flu. You can also catch the flu if you touch something with the virus on it, and then touch your mouth, nose, or eyes.

How do you stop seasonal flu? ›

Healthy Habits to Help Protect Against Flu
  1. Avoid close contact. Avoid close contact with people who are sick. ...
  2. Stay home when you are sick. ...
  3. Cover your mouth and nose. ...
  4. Clean your hands. ...
  5. Avoid touching your eyes, nose or mouth. ...
  6. Practice other good health habits.

How long does it take to clear influenza? ›

The symptoms of flu usually develop within 1 to 3 days of becoming infected. Most people will feel better within a week. However, you may have a lingering cough and still feel very tired for a further couple of weeks.

What are flu symptoms this year 2022? ›

Flu symptoms include:
  • Fever.
  • Cough and/or sore throat.
  • Runny or stuffy nose.
  • Headaches and/or body aches.
  • Chills.
  • Fatigue.
  • Nausea, vomiting and/or diarrhea (most common in children)
26 Oct 2022

How many days should I stay at home I have influenza? ›

Advise all employees to stay home if they are sick until at least 24 hours after their fever is gone without the use of fever-reducing medicines, or after symptoms have improved (at least 4-5 days after flu symptoms started).

When did seasonal influenza start? ›

Records show that the flu has been around for at least 1,500 years. The history of influenza begins with Hippocrates (5th century BC) who first reported that an influenza-like illness spread from Northern Greece to the islands south and elsewhere.

Which of the following are typical symptoms of seasonal influenza? ›

People who are sick with flu often feel some or all of these symptoms:
  • Fever*/feeling feverish or chills.
  • Cough.
  • Sore throat.
  • Runny or stuffy nose.
  • Muscle or body aches.
  • Headaches.
  • Fatigue (tiredness)
  • Some people may have vomiting and diarrhea, though this is more common in children than adults.

Which influenza is most common? ›

Influenza A is the most common, followed by influenza B. Both are highly contagious, and their symptoms are similar. Influenza, also known as the flu, is a viral respiratory illness that is most prevalent during fall and winter months.

Can cold weather cause influenza? ›

Cold weather may not be the only reason you get chills. Low temperatures can increase the likelihood of getting sick. The body is not as effective at fighting a virus when cold air enters the nose and upper airways, so viruses such as the common cold, the flu and COVID-19 often spread more easily in the winter.

Does flu survive better in cold? ›

Cold temps create ideal conditions

It's a myth that cold temperatures themselves cause the cold or flu. But the viruses that cause these infections thrive in dry, cold conditions.

Why do we get the flu in winter and not summer? ›

However, because indoor air is commonly heated, indoor RH is lowest during the winter. Thus, exposure to cold air outside or dry air inside during the wintertime may increase influenza virus transmission and potentially trigger a flu season.

How long is quarantine for influenza A? ›

CDC recommends that you stay home for at least 24 hours after your fever is gone except to get medical care or for other necessities. (Your fever should be gone without the use of a fever-reducing medicine.) Stay away from others as much as possible to keep from making others sick.

How many days are you contagious with influenza A? ›

People with flu are most contagious in the first three to four days after their illness begins. However, infants and people with weakened immune systems who are infected with flu viruses may be contagious for longer than seven days.

Can I have flu without fever? ›

People may be sick with flu and have respiratory symptoms without a fever. Flu viruses usually cause the most illness during the colder months of the year.

Is the Spanish flu the same as the seasonal flu? ›

Symptoms of the Spanish flu were similar to the symptoms we all watch out for during flu season. However, Spanish flu symptoms were more severe and included: A sudden, and sometimes very high, fever. Dry cough.

What version of Covid is the flu? ›

COVID-19 and the flu are caused by different viruses. COVID-19 is caused by a coronavirus called SARS-CoV-2, while flu is caused by influenza A and B viruses.

What is the name of the new flu virus? ›

The two newest infections, one an H3N2v virus and the other an H1N1v virus, occurred in children. Neither child was hospitalized, and both have recovered or are recovering from their illness.

Is influenza serious? ›

If you're young and healthy, the flu usually isn't serious. Although you may feel miserable while you have it, the flu usually goes away in a week or two with no lasting effects. But children and adults at high risk may develop complications that may include: Pneumonia.

Is influenza caused by stress? ›

Stress also suppresses the body's immune system making it more difficult for the body to ward off biological intruders, such as flu bugs (viruses) and infection (bacteria). As long as stress remains elevated, the body can struggle with health issues. These health issues can cause flu like symptoms.

How is influenza treated? ›

Currently, there are three antiviral drugs recommended for treating the flu: oseltamivir (Tamiflu®), zanamivir (Relenza®), and peramivir (Rapivab®). These drugs work by interrupting the function of neuraminidase on the virus surface and preventing the release of viral particles from infected host cells.

What is the fastest way to recover from seasonal flu? ›

12 Tips for a Speedy Flu Recovery
  1. Stay home. Your body needs time and energy to fight off the flu virus, which means that your daily routine should be put on the backburner. ...
  2. Hydrate. ...
  3. Sleep as much as possible. ...
  4. Ease your breathing. ...
  5. Eat healthy foods. ...
  6. Add moisture to the air. ...
  7. Take OTC medications. ...
  8. Try elderberry.
19 Nov 2018

Does vitamin C help with flu? ›

Myth: Vitamin C can prevent illness.

When taken before cold symptoms start, vitamin C may shorten the duration, but it doesn't keep you from getting sick.

What helps get rid of the flu fast? ›

Rest and fluids can help with your fever symptoms, but you can also take a fever reducer like acetaminophen to temporarily relieve your fever symptoms. Acetaminophen also helps to reduce pain, which will come in helpful since headache and muscle aches also often accompany fevers from the flu.

How is influenza diagnosed? ›

Diagnostic tests available for influenza include viral culture, serology, rapid antigen testing, reverse transcription polymerase chain reaction (RT-PCR), immunofluorescence assays, and rapid molecular assays.

How do I know when the flu is gone? ›

After about 4 days, you'll probably start to feel better. Your fever should be lower and your body aches should have lessened. Your cough may be worse, but this can be a sign that you're getting better. It's still important to get rest and plenty of fluid intake, and to self-isolate.

How do I get rid of flu 2022? ›

If you get sick with flu, influenza antiviral drugs may be a treatment option. Antiviral drugs work best when started early, such as one to two days after your flu symptoms begin. Check with your doctor promptly if you are at higher risk of serious flu complications and you get flu symptoms.

What month has the most flu cases? ›

The Centers for Disease Control and Prevention (CDC) has been tracking flu activity in the United States since 1982. Their data shows that over the past 36 years, February is the most common month for peak flu activity, followed by December, January, and March.

Is there a new flu strain every year? ›

A: It's a common misconception that a new strain of flu develops every year. The fact is there are already over 60 known flu viruses that have been identified and the predominance of one or any of them changes from year to year.

Should I stay in bed all day with flu? ›

Sleeping when you're sick is essential for your recovery. Sleep helps to boost your immune system, so you can fight off your illness more effectively. Your body knows what it needs, so don't worry if you find yourself sleeping a lot when you're sick, especially in the first few days.

When should you go to hospital with influenza? ›

Adults with a sustained fever of more than 102 degrees, as well as any combination of the below flu-like symptoms, should seek medical attention: Difficulty breathing. Severe chest or stomach pain. Vertigo and lightheadedness.

How did influenza spread? ›

Outbreaks of the influenza pandemic of 1918-1919 occurred in nearly every inhabited part of the world. Although it remains uncertain where the virus first emerged, it quickly spread through western Europe and around the world—first in ports, then from city to city along main transportation routes.

Where did the first influenza come from? ›

If anything, the outbreaks in Russia and North America in 1780–1781 were possible "herald waves" of the later, greater epidemic. During this true pandemic period, influenza is said to have first broken out in China and British India in the fall of 1781.

What year was the killer flu? ›

The microscopic killer circled the entire globe in four months, claiming the lives of more than 21 million people. The United States lost 675,000 people to the Spanish flu in 1918-more casualties than World War I, World War II, the Korean War and the Vietnam War combined.

How long does seasonal flu last? ›

For most healthy people, the flu is an uncomfortable but short-term illness that resolves itself as the immune system fights it off. Symptoms usually appear from one to four days after exposure to the virus, and they last five to seven days.

How does seasonal influenza affect the body? ›

Not only does having the flu impact your daily activities, it also leads to missed days from work and school. Complications can include pneumonia, ear or sinus infections, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma or diabetes.

What infection causes the seasonal flu? ›

Flu A and B viruses are responsible for seasonal flu epidemics more commonly known as the flu season. Flu viruses can change in two different ways—antigenic drift and antigenic shift.

Who is at high risk for influenza? ›

The CDC estimates that about 90 percent of flu-related deaths occur in adults 65 years and older. Residents of nursing homes and other long-term care facilities are also at higher risk. Children younger than 5, but especially children younger than 2 years old, are at higher risk of complications from flu illness.

Which influenza type is deadliest? ›

The 1918 H1N1 flu pandemic, sometimes referred to as the “Spanish flu,” killed an estimated 50 million people worldwide, including an estimated 675,000 people in the United States.

Where is influenza Most found? ›

While there are cases of it appearing all around the world, scientists found that it is far more prominent in the east than in the west, particularly in Southeast Asia. While this might suggest more people begin to contract the virus in the east than the west, it's actually the reverse.

Can you get influenza any time of the year? ›

You can catch flu all year round, but it's especially common in winter, which is why it's also known as seasonal flu. Flu isn't the same as the common cold. Flu is caused by a different group of viruses and the symptoms tend to start more suddenly, be more severe and last longer.

Does influenza change every year? ›

Flu viruses can change from year to year, so the vaccine is updated every year to protect against new flu virus strains that are expected to circulate and cause illness in the U.S. The FDA plays a key role in making sure flu vaccines are safe, effective, and of high quality.

Where does the flu go when it's not flu season? ›

"The flu virus tends to infect more people during cold and dry weather," says Dr. Dushaj. Other conditions that encourage the spread of a virus may be staying indoors with sealed windows and recirculated air—as we do in winter. This increases the risk of inhaling the same air as someone who may have the flu, he says.

Where does influenza come from each season? ›

Answer: Influenza is a virus that's spread from person to person. It originates, actually, among birds and other animals such as pigs, and new viral strains of influenza come to this country and to Europe from Southeast Asia. That's the global pattern.

Does influenza come on suddenly? ›

Influenza (flu) can cause mild to severe illness, and at times can lead to death. Flu symptoms usually come on suddenly. People who have flu often feel some or all of these symptoms: fever* or feeling feverish/chills.

Can you be immune to flu? ›

They found significant and complex immune responses in the people who got sick and the people who didn't. Scientists noticed changes in their blood 36 hours before some people actually felt sick. Although they understand that some people's immune systems resist the virus, they still don't know how or why that happens.

Where does the flu go in the summer? ›

The influenza A virus does not lie dormant during summer but migrates globally and mixes with other viral strains before returning to the Northern Hemisphere as a genetically different virus, according to biologists who say the finding settles a key debate on what the virus does during the summer off season when it is ...

How do you beat the flu season? ›

5 Ways to Fight the Flu
  1. Get the flu vaccine. It's the best way to protect yourself against the flu. ...
  2. Wash your hands well and often. ...
  3. Steer clear of someone who is sick (coughing, sneezing, etc.). ...
  4. Cough or sneeze into a tissue or your elbow — not into your hands. ...
  5. Stay home if you have the flu.

How do you fight the flu season? ›

Healthy Habits to Help Protect Against Flu
  1. Avoid close contact. Avoid close contact with people who are sick. ...
  2. Stay home when you are sick. ...
  3. Cover your mouth and nose. ...
  4. Clean your hands. ...
  5. Avoid touching your eyes, nose or mouth. ...
  6. Practice other good health habits.

Who gets influenza the most? ›

The same CID study found that children are most likely to get sick from flu and that people 65 and older are least likely to get sick from flu. Median incidence values (or attack rate) by age group were 9.3% for children 0-17 years, 8.8% for adults 18-64 years, and 3.9% for adults 65 years and older.

Where is influenza found the most? ›

While there are cases of it appearing all around the world, scientists found that it is far more prominent in the east than in the west, particularly in Southeast Asia. While this might suggest more people begin to contract the virus in the east than the west, it's actually the reverse.

What is the root cause of influenza? ›

The flu is caused by an influenza virus. Most people get the flu when they breathe in tiny airborne droplets from the coughs or sneezes of someone who has the flu. You can also catch the flu if you touch something with the virus on it, and then touch your mouth, nose, or eyes.


1. Seasonal Flu
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2. How Seasonal Influenza Viruses Evolves
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3. Seasonal Influenza Immunization 2022-2023
4. Doctor Shares Guidance To Prepare For Severe 2022 Flu Season
5. Officials Warn Of Severe 2022 Flu Season: Here's How To Prepare
6. Late Season Flu Rages Throughout U.S.
(NBC News)
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