What to know about flu season

Flu season occurs predictably each year, typically peaking between December and March in the Northern Hemisphere.1 Influenza accounted for 140,000 to 810,000 hospitalizations and 12,000 to 61,000 deaths annually, during the flu seasons from 2010 to 2020, in the United States alone.² Understanding when flu season occurs, who faces the highest risk, how the virus spreads, and recognizing symptoms early helps you protect yourself and your family during this challenging time.

Every year, flu season arrives with predictable timing but unpredictable severity, affecting millions of people and overwhelming healthcare systems worldwide._^3,4 Understanding what makes this annual occurrence so impactful helps you prepare and respond appropriately when flu comes to your community.

When flu season occurs

For countries in the Northern Hemisphere, the typical timing of the flu epidemic peak falls during the winter season, with the vast majority occurring in February and the first half of March.¹ This seasonal pattern reflects how influenza viruses thrive in specific environmental conditions, particularly cold and dry weather that favors virus survival and transmission.³ The increased time people spend indoors during cold weather, often in close quarters with others, facilitates spread.³

Interestingly, flu seasonality varies considerably around the world. While temperate regions experience distinct winter epidemics, tropical countries show much more variability, with flu activity potentially occurring at any time of the year depending on local climate patterns, latitude, and rainfall.¹ This global variation in timing underscores how environmental factors influence when and where flu viruses circulate most actively.¹

The burden of the flu season

The numbers associated with flu season reveal its substantial impact on public health. The Centers for Disease Control and Prevention estimates that influenza accounted for 4.3 to 21 million medical visits, 140,000 to 810,000 hospitalizations, and 12,000 to 61,000 deaths annually in the United States during flu seasons from 2010-11 through 2019-20.² This wide range reflects variability in flu season severity from year to year, depending on which virus strains circulate and how well they match available preventive measures.²

Beyond the immediate health burden, influenza significantly impacts healthcare resources and economic productivity. Studies show that influenza infection is associated with prolonged hospital stays, with patients experiencing hospital stays that are twice as long or more compared to patients without flu.⁴ The ripple effects include work absences, school closures, and strain on emergency departments and hospitals during peak activity periods.

Who faces the highest risk?

While anyone can get flu, certain groups face substantially higher risk of serious complications and hospitalizations. The elderly population and those with specific chronic conditions are particularly vulnerable.² Research examining high-risk groups found that elderly flu patients with conditions like congestive heart failure, chronic obstructive pulmonary disease, coronary artery disease, or chronic kidney disease had 3 to 7 times higher 30-day hospitalization rates compared to similar patients without flu.² This elevated risk has been consistently attributed to age-related changes in immune defenses.²

Children also play a crucial role in flu season dynamics. Studies examining household transmission found that children, particularly preschool and school-age children, are more susceptible to flu infection and more likely to transmit the virus to household members.⁵ Research shows that 40% to 48% of secondary household cases are attributable to transmission from a sick child, with preschool children posing the highest transmission risk.⁵ This makes understanding flu season especially important for families with young children.

Recognizing the pattern

Flu typically begins suddenly, with symptoms such as fever, chills, severe body aches, headache, sore throat, and a dry cough. Some people – especially children – may also experience vomiting or diarrhea. This abrupt onset of symptoms helps distinguish flu from the common cold, which usually develops more gradually. While most individuals recover within 3 to 7 days, flu can lead to serious complications in some cases, particularly among children, older adults, and people with underlying chronic conditions.⁶

FAQs

Q 1: When exactly does flu season start and end each year?
A: In the Northern Hemisphere, including the United States, flu season typically begins in the fall and peaks between December and March, with the majority of peak activity occurring in February and early March. However, flu viruses can circulate year-round in tropical regions, and the period of influenza activity varies even between neighbouring countries.¹ The timing relates to environmental conditions, including cold, dry weather that favors flu virus survival and increased indoor crowding during winter months.²

Q 2: What causes the seasonality in flu infection?
A: Flu tends to be seasonal due to a combination of factors. During certain times of the year, the body’s ability to fight infections may be lower, possibly influenced by changes in sunlight and vitamin levels. The flu virus also survives and spreads more easily in cold, dry air. In addition, people, especially children, spend more time indoors or in close contact at schools, which helps the virus spread from person to person. Together, these factors explain why flu infections rise at specific times each year.^3,6

Q 3: Why are children such a concern during flu season?
A: Children, particularly preschool and school-age children, play a major role in flu transmission for several reasons. They’re more susceptible to infection, shed virus at higher levels and for longer periods, have more frequent close contact with others in school settings, and practice less effective hygiene habits. Research shows that 40% to 48% of secondary household flu cases are attributable to transmission from sick children. When a child brings flu home, household contacts face significantly elevated infection risk compared to exposure from sick adults. This makes protecting children especially important for protecting entire families.⁵

Q 4: Who should be most concerned about getting the flu this season?
A: While anyone can get flu, certain groups face a substantially higher risk of serious complications. Elderly individuals, particularly those with chronic conditions like heart failure, COPD, coronary artery disease, or kidney disease, have 3 to 7 times higher hospitalization rates when infected compared to those without flu.³ Young children, especially under age 5, face elevated risk. Pregnant women, people with weakened immune systems, and those with chronic medical conditions also experience higher complication rates. If you’re in any high-risk group, be especially vigilant about recognizing symptoms early and seeking prompt medical evaluation.⁶