Written by:
Healthy-U Staff

Medically Reviewed by:
Matthew Sommers, PharmD

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How Long Is the Flu Contagious?

Flu season is here, and along with it, we have the opportunity to protect ourselves, our families, and our communities. In the years following the COVID-19 pandemic, it can feel particularly easy to discount the potential dangers to the community, but these dangers exist nonetheless. Aside from the uncomfortable symptoms and economic impact of missed work, numbers like 145,000 annual deaths or 9.4 million annual hospitalizations can be lost in the shuffle all too easily.

Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses. While the flu is often contagious for about five to seven days, this can vary from person to person. Understanding this variability can help you better protect yourself and others.

Understanding the Influenza Virus

The types of flu viruses and transmission methods are equally important to understand. Influenza viruses affecting humans can be categorized into types A, B, and C. Types A and B are the most common types responsible for seasonal flu outbreaks. The virus spreads through respiratory droplets when an infected person coughs, sneezes, or talks. Once the virus enters your body, it starts to multiply, eventually leading to the onset of symptoms. Unfortunately, the flu can be spread before symptoms start.

Which Is Worse: Influenza A or B?

Picture of a mask meant to avoid the spread of respiratory illness — Photo by Anna Shvets on Pexels.com

A common thought is that Influenza A is “worse” than influenza B. Influenza A does account for a majority of confirmed flu cases — about 75%. Regarding symptom severity, the two types of flu appear to be roughly equal, with similar rates of hospitalization and more dangerous outcomes.

Both influenza A and B can be treated the same way, most commonly with antiviral medications like oseltamivir (Tamiflu). Antivirals are more effective if they are started quickly — within 48 hours — after symptoms develop.

Several factors, including the original host and geographical origin, are used to characterize the different types of flu.

Influenza A viruses sometimes originate from birds or pigs, which is why the terms avian flu or swine flu have been used. Influenza A viruses can replicate and change fairly quickly, often making them a challenging target for vaccination each year. Common examples of Influenza A are H1N1 or H3N2.

Influenza B viruses are usually only found in humans. These viruses replicate and change more slowly and are a standard component of annual flu vaccines. There are two main lineages of influenza B, Yamagata and Victoria. The Victoria lineage has caused more widespread illness than Yamagata in recent years.

Other types of influenza do exist. Influenza C does affect humans but has been relatively rare in recent years. Influenza D has not been known to affect humans and has been found primarily in cattle.

How Long Does The Flu Last, And How Long Is It Contagious?

Pre-symptomatic and Asymptomatic Spread

A particularly challenging aspect of the flu is that individuals can spread it before they know they’ve been infected. This is considered pre-symptomatic transmission and can occur for about one day before symptoms appear. Sometimes, individuals have only mild or no symptoms (asymptomatic). Unfortunately, despite having no symptoms, the flu can still be spread. The flu is generally contagious as long as symptoms last or for about five to seven days after the onset of symptoms.

High-risk Groups and Longer Contagious Period

Because the flu can be spread somewhat easily (and potentially without realizing you are even infectious), it highlights the need for focusing on developing community immunity where possible and limiting the spread. Some individuals are more susceptible to severe illness or may be contagious for longer periods:

Individuals with a weakened immune system are less able to respond to infection and have a higher risk of hospitalization or death.
Elderly individuals also have a higher risk of hospitalization or death.
Children are more likely to be contagious before developing symptoms and may have a tendency to remain contagious for a longer period of time.

Taking steps to prevent the spread of flu helps you protect not only yourself but can also keep your family and members of the community safe and healthy as well.

Antiviral Medications

There are prescribed antiviral medications available that can help slow down the release of virus from infected cells or inhibit viral replication. These include:

Oseltamivir (Tamiflu).
Zanamivir (Relenza).
Baloxavir marboxil (Xofluza).

These medications work best if they are started within the first 48 hours. If you see a healthcare provider promptly they may recommend antiviral treatment, especially if you are in a high-risk group or if you are in close contact with someone at risk.

Symptomatic Relief

Woman taking medicine to help relieve her symptoms. — Photo by Polina Tankilevitch on Pexels.com

A wide variety of over-the-counter remedies for cold and flu symptoms are available. A primary consideration is taking the time to match your specific symptoms with the medications you use — many combination products may contain ingredients that you don’t necessarily need.

Zinc and vitamin C products like Emergen-C are commonly used with the intent of boosting immune response, but may not reduce the length of your illness. The B vitamin content may help improve fatigue, however.

In general it is helpful to stay hydrated and get rest to allow your body to recover. Aside from antiviral medications, the flu is often managed with supportive care. You should seek medical attention right away if you are experiencing:

Difficulty breathing.
Pain or pressure in your chest that isn’t going away.
Dizziness, confusion, or difficulty getting up.
Seizures.
Fever or cough that initially improve but then return or get worse.
Worsening of other chronic medical conditions.

Preventing the Spread of the Flu

Man coughing into the inside of his elbow. — Photo by Edward Jenner on Pexels.com

If you suspect you have the flu or are around someone who is ill:

Cover your mouth and nose with the inside of your elbow when you cough or sneeze.
Wash your hands frequently with soap and water for at least 20 seconds, and use hand sanitizer if soap and water are unavailable.
Avoid close contact with individuals who have the flu, and if you’re sick, stay home to prevent infecting others.

Getting a flu vaccine every year is one of the most effective ways to prevent getting the flu or spreading it to others.

How Long Does The Flu Vaccine Last?

Because of the speed of influenza virus replication, there is the potential for antigenic drift. The purpose of a flu vaccine is to produce antibodies that will help your body identify and fight influenza if you come in contact with a flu virus. These antibodies target very specific antigens which are present on the virus. Antigenic drift refers to the potential for the virus to slip by undetected because the antibodies were trained to detect slightly different antigens.

This potential for variability is the reason why the flu vaccine is recommended every year. It is also the reason why vaccines produced during some years have a higher percentage of efficacy compared with others. Antibody levels can fade over time, and last year’s antibody’s may no longer be effective. While not perfect, even during years when the vaccine misses some circulating strains a significant amount of medical visits, hospitalizations, and deaths are likely avoided.

Flu Vaccine Side Effects

Generally the side effects of the flu vaccine are mild, but there are some precautions. The most common side effects include:

Injection site pain
Muscle aches
Headache
Fatigue
Mild fever

These side effects generally last no longer than a day or two and are signs that your body is building immunity. In a recent study following individuals who received both vaccines, there was a slight increase of 8% to 11% incidence of side effects. Less than 1% reported requiring medical attention.

Individuals who should NOT get a flu shot include:

Infants younger than 6 months of age.
Severe allergies to vaccine components (excluding egg allergies — there are vaccines available that are safe).

It is also important to talk with your doctor prior to receiving a flu vaccine if you have a history of Guillain-Barré Syndrome.

Wrap-Up

The flu can be contagious for about one day before symptoms appear and remains contagious for approximately five to seven days after symptom onset. However, this period can vary among individuals. It’s important to be aware of pre-symptomatic and asymptomatic transmission and how certain groups can be contagious for a longer duration. By taking proper precautions, such as getting vaccinated and practicing good hygiene, we can collectively reduce the spread of the flu and protect ourselves, our families, and vulnerable members of our communities.