The seasonal flu, or influenza, is a highly contagious infection caused by type A or B influenza viruses. According to the Centers for Disease Control and Prevention, the best way to protect yourself from contracting the flu is to get an annual flu vaccine and remember to wash your hands frequently. But unfortunately, no prevention measure is 100-percent effective.
So understanding the flu's incubation period — or, the amount of time between exposure to the virus and the appearance of symptoms — is helpful to preventing its spread. Here are four things you should know to help keep yourself (and others) healthy during the sniffles season.
The average incubation period for the flu is two days, but it can range from one to four days.
Source: Centers for Disease Control and Prevention
1. Symptoms Show Up Within a Few Days
According to the Centers for Disease Control and Prevention (CDC), the average incubation period for the flu is two days but can range from one to four days.
A study published in August 2018 in the Journal of Molecular Epidemiology and Evolutionary Genetics of Infectious Diseases found that the exact timeframe varies depending on whether the illness is due to influenza virus type A or B. The researchers reported the average incubation period to be 1.4 days with an influenza type A infection and 0.6 days with a type B infection.
2. You're Contagious During the Incubation Period...
The flu virus is present in the respiratory secretions of infected individuals — even during the incubation period. This means you can transmit the flu virus to others for about a day before symptoms of the flu develop, according to the CDC.
When an infected person coughs, sneezes or even exhales, droplets containing the flu virus enter the air, putting those in close proximity at risk of contracting the infection, according to a January 2018 study published by the Proceedings of the National Academy of Sciences of the United States of America. Direct contact with an infected person, such as kissing, is another possible way to transmit the flu.
Additionally, the virus can survive on surfaces — like countertops, doorknobs and toys — for up to nine hours, according to a November 2011 PLOS One study report. As such, flu can be transmitted by touching a surface contaminated by live flu viruses and then touching your face.
Another study, published in February 2016 by Clinical Infectious Disease, found that the levels of flu virus in respiratory secretions during the incubation period are higher with type B influenza infections compared to type A infections. This suggests that people with a type B infection might be more likely to transmit the flu to others during this timeframe.
3. ...But You're Even More Contagious When You're Sick
Though it's possible to transmit the flu virus before you develop symptoms, you're more likely to spread the illness to others once you get sick.
According to the 2016 article in Clinical Infectious Disease, virus levels with a type A influenza infection — and therefore, contagiousness — peak in the first 24 to 48 hours after you develop flu symptoms and then gradually decline as you recover.
The situation is a bit different with a type B infection. A peak in virus levels occurs in the first 24 to 48 hours of flu symptoms but a second peak occurs around the fourth day after you first get sick. This means you can be just as likely to spread the flu to others as you were initially, even though you're feeling better.
Overall, people with the flu remain contagious for about five to seven days after flu symptoms begin, according to the CDC. Children and people with a weakened immune system, however, often continue to shed the flu virus for a longer period and can spread the illness to others even after they no longer feel sick.
In fact, a study published in May 2016 by the Pediatric Infectious Disease Journal found that children 5 years old or younger can remain infectious for 20 days or longer. The researchers speculate that young children might shed the flu virus for a longer period than adults because their immune systems are less mature.
4. Antivirals Can Help Even Before Symptoms Show
If you come down with the flu, antiviral drugs — such as oseltamivir (Tamiflu), peramivir (Rapivab), zanamivir (Relenza) and baloxavir marboxil (Xofluza) — can curb the severity of your flu symptoms, speed your recovery and reduce your risk for flu-related complications.
These medicines also reduce the level of flu virus in your respiratory secretions, making it less likely that you'll spread the disease to others, even during the incubation period.
According to the CDC, oseltamivir and zanamivir are also 70 to 90 percent effective at averting development of flu symptoms if started right away (less than 48 hours) after you've been exposed to someone with the flu and either haven't gotten a flu shot or were vaccinated less than two weeks before exposure to the virus.
Although these antiviral drugs are highly effective in this setting, the CDC does not recommend their routine use except in certain situations, such as a flu outbreak in a nursing home or another long-term care facility, and in people with a weakened immune system or at high risk for flu complications.
- Centers for Disease Control and Prevention: "How Flu Spreads"
- Journal of Molecular Epidemiology and Evolutionary Genetics of Infectious Diseases: "Transmissibility and Severity of Influenza Virus by Subtype"
- Proceedings of the National Academy of Sciences of the United States of America: "Infectious Virus in Exhaled Breath of Symptomatic Seasonal Influenza Cases From a College Community"
- Clinical Infectious Diseases: "The Dynamic Relationship Between Clinical Symptomatology and Viral Shedding in Naturally Acquired Seasonal and Pandemic Influenza Virus Infections"
- PLOS One: "Survival of Influenza A(H1N1) on Materials Found in Households: Implications for Infection Control"
- Pediatric Infectious Disease Journal: "The Timeline of Influenza Virus Shedding in Children and Adults in a Household Transmission Study of Influenza in Managua, Nicaragua"
- Centers for Disease Control and Prevention: "Influenza Antiviral Medications: Summary for Clinicians"
- Centers for Disease Control and Prevention: "Key Facts About Seasonal Flu Vaccine"