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Flu season happens every year, beginning in the late fall and ending in the spring. Despite the repeated exposure, there is a lot of confusion about this illness. What is it? Why does it come back every year? How can it be treated, and is prevention possible?
Here: Flu 101, according to the data.
A note: people sometimes refer to vomiting illnesses as the “stomach flu,” but this is unrelated to influenza. That stomach flu is probably a norovirus.
What is the flu?
Influenza (also called the flu or the seasonal flu) is caused by a family of RNA viruses that primarily infect the respiratory tract — your nose, throat, and lungs. These viruses are classified by surface proteins (the H and N you see in names like H1N1 or H3N2), and they mutate constantly, which is why flu season is an annual event rather than a one-and-done illness. Influenza A and influenza B are different (related) viruses; influenza A is typically more severe.
Flu symptoms usually include fever, body aches, cough, and fatigue, which can last a week to 10 days (or more). Relative to the common cold, the flu makes most people feel sicker and is more likely to involve a fever and less likely to involve a stuffy nose. Children sometimes have diarrhea and vomiting.
For most healthy adults and older children, they recover without complications. However, the flu can be serious and even deadly for younger children, adults over 65, pregnant people, and those with chronic health conditions. In the 2023-2024 flu season (the most recent with complete reporting), the CDC estimates 27,000 deaths, of which 19,000 were in adults over 65 and 521 deaths among children 17 and under. There were an estimated 40 million symptomatic illnesses this year and almost 500,000 hospitalizations.
Flu in pregnancy: Because pregnancy is a mild form of immune compromise, the flu can be more severe in pregnancy. Vaccination and avoidance (see more about Tamiflu below) are recommended.
Flu in kids: Children, especially those under 5, are at higher risk than adults from the flu. They are more likely to be seriously ill, and mortality risks (while still extremely low) are higher. This is likely due to having had less prior exposure than adults would have.
How do I know it’s the flu and not something else?
Flu is characterized by fever and full-body aches and less congestion relative to a common cold. However, there is a lot of overlap in symptoms with other illnesses (including RSV, COVID-19, etc.). If you want to confirm that something is the flu, you can use an over-the-counter test strip. There are some of these that test for flu A and B, and COVID, all in the same swab.
A doctor can also confirm a flu diagnosis and what type it is, although this may be unnecessary.
How does the flu spread?
The flu spreads through respiratory droplets. The virus is living in someone, and they cough or sneeze on you, and you’re exposed. Or they cough into their hands, and you shake hands with them. Or, if you’re a parent, your child licks their hands and rubs them on your face. Regardless, the spread is due to saliva or snot from someone else getting into your nose and mouth.
Seasonal flu is reasonably contagious. For viruses, the level of contagion is measured by a value called “R-naught” (or R_0). This is an estimate of how many people a sick person will infect on average. For seasonal flu, this number is estimated at 1.28, so each person infects about 1.28 other people. That is comparable to COVID-19 and much lower than (say) the measles, which has a value of 12 to 15.
The flu is most contagious in the day before symptoms begin (which, obviously, makes it hard to prevent spread) and in the first few days of illness. By day four or five, you are becoming less contagious. A general rule of thumb is after 24 hours of no fever (without taking Advil or Tylenol), you’re safe to be around others.
The obvious corollary to this is the question of how to avoid spread. The simplest answer is to try to avoid putting someone else’s spit or snot in your nose or mouth. This seems easy, but it is surprisingly not. Washing your hands can help, as can wearing a mask, although the latter is not always feasible, especially in our houses. Hand sanitizer can help, as well.
How is the flu treated?
Most healthy children and adults will not need anything for recovery other than rest and supportive care — fluids, Advil, etc.
Many people ask about Tamiflu (the brand name for the drug oseltamivir). The evidence for the value is real, but small. A systematic review of randomized controlled trials found that oseltamivir shortens symptom duration in adults by about 16–17 hours compared to placebo. Data suggests that timing matters; a study out of Hong Kong showed starting Tamiflu within 24 hours of symptoms reduced duration by more than half.
For prevention, the picture is a bit stronger. A randomized trial published in JAMA found almost a 90% reduction in symptomatic flu cases when household contacts of people with the flu took Tamiflu as a preventative measure. A broader review also shows evidence of a reduction in infection with prophylaxis use.
Having said this, Tamiflu has some side effects, including nausea, vomiting, and headaches. As a result, use for prevention is often reserved for people who are at higher risk of complications: children, those with immune conditions, pregnant people, and, especially, the elderly. Tamiflu is considered safe in pregnancy and breastfeeding.
What are the options for preventing the flu?
There are two primary paths to flu prevention. One is mentioned above — limiting spread by washing hands and trying not to see sick people. The other is getting vaccinated. The efficacy of vaccines varies across years, depending on how close a match the vaccine is to the circulating virus. However, in most years, the efficacy is around 40 to 60%, meaning that getting vaccinated roughly halves your risk of serious illness.
The best time to be vaccinated is early to mid-October, since that is enough time for antibodies to arrive by the start of the flu season, but not so much time that they wane before the end.
The bottom line
- Influenza is a virus that causes fever, body aches, cough, and fatigue.
- It usually resolves without complications in healthy people, but can be severe or deadly, especially for young children, older adults, pregnant people, and those with chronic conditions.
- The best way to prevent getting the flu is by washing your hands and trying not to see sick people. The other is getting vaccinated, ideally in early to mid-October.
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