Flu season is the time of year when influenza (flu) viruses circulate more widely and cause more illness. Flu is a contagious respiratory disease that can lead to mild or severe symptoms, such as fever, cough, sore throat, muscle aches, headache, and fatigue. In some cases, flu can also cause serious complications, such as pneumonia, bronchitis, sinus infections, ear infections, and even death.
Flu season varies from year to year, depending on the types and strains of flu viruses that are circulating, the effectiveness of the flu vaccine, the weather conditions, and the level of immunity in the population. However, there are some general patterns and trends that can help you understand when flu season typically starts and ends, and what you can do to protect yourself and others from getting sick.
When does flu season start?
Flu season usually starts in the fall and lasts through the winter in each hemisphere. In the United States, flu activity often begins to increase in October and November, and peaks between December and February. However, flu outbreaks can occur as early as September or as late as May.
The exact timing and duration of flu season depends on many factors, such as:
- The types and subtypes of flu viruses that are circulating. There are four types of influenza viruses: A, B, C, and D. Only influenza A and B cause seasonal epidemics in humans. Influenza A viruses are further classified into subtypes based on two surface proteins: hemagglutinin (H) and neuraminidase (N). There are 18 different H subtypes and 11 different N subtypes. Influenza B viruses are divided into two lineages: Yamagata and Victoria. Each year, different combinations of influenza A subtypes and influenza B lineages circulate and cause illness. For example, in the 2020-2021 flu season in the United States, the predominant viruses were influenza A (H1N1)pdm09 and influenza B/Victoria.
- The genetic changes or mutations of flu viruses. Flu viruses constantly change through a process called antigenic drift, which means they can evade the immune system’s recognition and response. Sometimes, flu viruses undergo a major change called antigenic shift, which results in a new subtype or lineage that can infect humans and cause a pandemic. Antigenic drift and shift can affect how easily a virus spreads, how severe it is, and how well the vaccine matches it.
- The effectiveness of the flu vaccine. The flu vaccine is designed to protect against the flu viruses that research suggests will be most common during the upcoming season. However, predicting which viruses will circulate is not an exact science, and sometimes there may be a mismatch between the vaccine and the circulating viruses. Additionally, some people may have a reduced immune response to the vaccine due to factors such as age, health status, or previous exposure to flu viruses. The effectiveness of the flu vaccine can vary from year to year, but it is still the best way to prevent flu and its complications.
- The weather conditions. Some studies have suggested that cold and dry weather may favor the survival and transmission of flu viruses. This may explain why flu season tends to occur during the colder months in temperate regions. However, other factors such as humidity, sunlight, air pollution, and indoor crowding may also play a role.
- The level of immunity in the population. The level of immunity in the population depends on several factors, such as previous exposure to flu viruses or vaccination history. People who have been infected or vaccinated may have some degree of protection against similar or related flu viruses. However, this protection may wane over time or be overcome by new or different flu viruses. Therefore, annual vaccination is recommended for everyone 6 months of age and older.
When does flu season end?
Flu season usually ends in the spring or early summer in each hemisphere. In the United States, flu activity often declines in March or April but can last until May or June.
The exact timing and duration of flu season depends on many factors as mentioned above.
Some possible reasons why flu season ends include:
- The decrease in circulation of flu viruses due to natural cycles or interventions such as vaccination or antiviral treatment.
- The increase in temperature and humidity that may reduce the survival and transmission of flu viruses.
- The increase in exposure to sunlight that may enhance vitamin D production and immune function.
- The decrease in indoor crowding that may reduce contact between infected and susceptible individuals.
- The increase in cross-protection from previous infection or vaccination that may limit the spread of new or different flu viruses.
What can you do to prevent getting sick during flu season?
The best way to prevent getting sick during flu season is to get vaccinated every year before flu activity begins in your area. The Centers for Disease Control and Prevention (CDC) recommends that everyone 6 months of age and older get a flu vaccine by the end of October. However, getting vaccinated later can still be beneficial, as long as flu viruses are circulating.
There are different types of flu vaccines available, such as:
- Inactivated influenza vaccine (IIV), which contains killed flu viruses and is given as a shot in the arm. There are different formulations of IIV, such as standard-dose, high-dose, cell-based, recombinant, and adjuvanted.
- Live attenuated influenza vaccine (LAIV), which contains weakened live flu viruses and is given as a nasal spray. LAIV is approved for use in people 2 through 49 years of age who are not pregnant and do not have certain medical conditions.
- Jet injector influenza vaccine (JIIV), which contains inactivated flu viruses and is given as a shot in the arm using a jet injector device. JIIV is approved for use in people 18 through 64 years of age.
The flu vaccine can reduce your risk of getting sick with flu and its complications by 40% to 60% on average. The flu vaccine can also make your illness milder if you do get sick. Additionally, getting vaccinated can protect people around you who are more vulnerable to serious flu illness, such as babies, older adults, and people with chronic health conditions.
Besides getting vaccinated, you can also take everyday preventive actions to avoid getting or spreading the flu, such as:
- Avoiding close contact with people who are sick or staying home if you are sick.
- Covering your cough or sneeze with a tissue or your elbow and disposing of the tissue properly.
- Washing your hands often with soap and water or using an alcohol-based hand sanitizer.
- Avoiding touching your eyes, nose, and mouth with unwashed hands.
- Cleaning and disinfecting frequently touched surfaces and objects.
- Wearing a face mask when you are around people who are sick or when you go out in public.
If you do get sick with flu symptoms, you should seek medical attention as soon as possible, especially if you are at high risk of complications. Your healthcare provider may prescribe antiviral drugs that can shorten the duration and severity of your illness and prevent serious complications. Antiviral drugs work best when started within 48 hours of symptom onset, but they may still be beneficial when started later.
Some examples of antiviral drugs for flu include:
- Oseltamivir (Tamiflu), which is taken orally as a capsule or liquid.
- Zanamivir (Relenza), which is inhaled using a diskhaler device.
- Peramivir (Rapivab), which is given intravenously in a hospital setting.
- Baloxavir (Xofluza), which is taken orally as a single dose.
Antiviral drugs are not a substitute for vaccination and may cause some side effects, such as nausea, vomiting, headache, and dizziness. You should follow your healthcare provider’s instructions on how to use them safely and effectively.
Flu season is the time of year when flu viruses circulate more widely and cause more illness. Flu season usually starts in the fall and ends in the spring or early summer in each hemisphere. The exact timing and duration of flu season depends on many factors, such as the types and strains of flu viruses that are circulating, the effectiveness of the flu vaccine, the weather conditions, and the level of immunity in the population.
The best way to prevent getting sick during flu season is to get vaccinated every year before flu activity begins in your area. The flu vaccine can reduce your risk of getting sick with flu and its complications by 40% to 60% on average. You can also take everyday preventive actions to avoid getting or spreading the flu, such as washing your hands, covering your cough, wearing a mask, and staying away from people who are sick. If you do get sick with flu symptoms, you should seek medical attention as soon as possible and take antiviral drugs if prescribed by your healthcare provider.
Flu season can be unpredictable and challenging, but by taking these steps you can protect yourself and others from this common and potentially serious disease.