Influenza (the flu) is more than just a seasonal inconvenience. It is a highly contagious viral respiratory illness that can cause serious complications, particularly in vulnerable populations. Understanding how influenza spreads, recognizing its symptoms, knowing who is at greatest risk, and staying up to date with vaccination recommendations are essential steps in reducing illness and preventing severe outcomes.
Influenza is primarily transmitted through large respiratory droplets that are released when an infected person coughs or sneezes. After exposure, symptoms typically develop within 1 to 4 days, with an average incubation period of about 2 days.
People can spread the virus even before they realize they are sick. In healthy adults, viral shedding begins approximately one day before symptoms appear and continues for about five days after symptom onset. Children are often contagious before symptoms begin and may continue shedding the virus for 10 days or longer. Individuals with weakened immune systems may remain infectious for up to three weeks, making infection control especially important in healthcare settings.
Unlike the common cold, influenza usually begins abruptly, with symptoms developing over just a few hours.
Common symptoms include:
Although the fever and body aches often improve within about a week, many people continue to experience persistent cough and fatigue for two weeks or longer.
Children often experience additional symptoms that are less common in adults. These may include:
While most people recover without difficulty, influenza can occasionally lead to severe complications involving multiple organ systems, including:
Certain groups are significantly more likely to develop severe influenza complications, including pneumonia, hospitalization, and even death.
Pregnancy increases the risk of severe influenza by approximately fivefold due to changes in the immune system and respiratory function. Vaccination is especially important during pregnancy to protect both the mother and the baby.
Children younger than 5 years, particularly those under 2 years, are at much higher risk for hospitalization and serious illness.
Adults aged 65 years and older account for the majority of influenza-related hospitalizations and deaths each year due to age-related declines in immune function.
People living with chronic diseases are also at increased risk, including those with:
Additional populations at increased risk include:
The Centers for Disease Control and Prevention (CDC) and the Advisory Committee on Immunization Practices (ACIP) recommend annual influenza vaccination for everyone aged 6 months and older, unless a contraindication exists.
Vaccination should ideally occur during the fall, at least one month before influenza begins circulating widely in the community.
Several vaccine formulations are available to meet different patient needs. Most influenza vaccines are the quadrivalent form, providing protection against two influenza type A and two influenza type B strains.
These vaccines do not contain live virus and are the preferred choice for most individuals. Available options include:
Most are administered by intramuscular injection, although some intradermal formulations are available for adults aged 18–64 years.
Older adults often have a weaker immune response to vaccination, a process known as immunosenescence. High-dose vaccines contain four times more antigen than standard vaccines, while adjuvanted vaccines include substances that enhance the body’s immune response.
The live attenuated vaccine is administered as a nasal spray and is approved only for healthy, non-pregnant individuals between 2 and 49 years of age. It should not be given to pregnant individuals or those who are immunocompromised.
Children aged 6 months through 8 years who are receiving influenza vaccination for the first time require two doses, administered at least four weeks apart, to achieve adequate immunity.
Influenza vaccination is safe during any trimester of pregnancy and while breastfeeding. Vaccination not only protects the mother but also transfers antibodies across the placenta, providing newborns with passive protection during their first six months of life, before they are eligible for vaccination themselves.
Current recommendations state that any age-appropriate influenza vaccine can be given to individuals with egg allergies, regardless of severity.
Individuals with a history of severe allergic reactions should receive their vaccine in a healthcare setting equipped to manage allergic emergencies. As with all vaccinations, patients should remain under observation for approximately 15 minutes afterward to reduce the risk of injury from fainting.
The only absolute contraindication is a previous severe allergic reaction to the influenza vaccine itself, rather than to egg protein.
Although annual vaccination remains the most effective method of influenza prevention, antiviral medications have an important role in selected patients.
Oseltamivir is a neuraminidase inhibitor used for both:
When started early, it can shorten illness duration and reduce complications in high-risk patients.
Baloxavir is the first polymerase acidic (PA) endonuclease inhibitor approved for treating uncomplicated influenza in patients 12 years of age and older.
Unlike oseltamivir, baloxavir is not approved for post-exposure prophylaxis in this context.
While antiviral medications can help treat influenza after infection, they are not a substitute for vaccination. Vaccines provide long-lasting protection, reduce disease severity, decrease community transmission, and lower the risk of hospitalization and death. Antiviral medications are generally reserved for treatment or selected high-risk exposures because they are more expensive, may cause adverse effects, and carry the potential for antiviral resistance.
