The seasonal flu, or influenza, is a highly contagious infection caused by type A or B influenza viruses. Your first line of defense in guarding yourself against the flu is getting a yearly flu vaccine and frequently washing your hands help prevent contracting the flu. But unfortunately, no prevention measure is 100-percent effective.
The average incubation period for the flu is 2 days, but it can range from 1 to 4 days.
Source: Centers for Disease Control and Prevention
1. The Flu Incubation Period Varies
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 flu incubation period varies, depending on whether the illness is due to influenza virus type A or B. The researchers reported the average incubation period is 1.4 days with an influenza type A infection and 0.6 days with a type B infection.
2. You're Contagious During the Flu Incubation Period
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. Direct contact with an infected person, such as kissing, is another possible route of transmitting 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.
A 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 the incubation period compared to those with a type A infection.
3. You're More Contagious After the Flu Incubation Period, Though
Though it's possible to transmit the flu virus before you develop symptoms of the flu, you're more likely to spread the illness to others once you get sick.
According to the previously mentioned 2016 Clinical Infectious Diseases article, 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 influenza virus infection. A peak in virus levels occurs in the first 24 to 48 hours of flu symptoms but a second peak occurs about day 4 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 flu virus for a longer period and can spread the illness to others even after they no longer feel sick.
A study published in May 2016 by the Pediatric Infectious Disease Journal found that children 5 years old or younger can continue to shed flu virus and remain infectious for 20 days or longer. The researchers speculate that young children might shed flu virus for a longer period than adults because of an immature immune system.
4. Antivirals Can Help During the Flu Incubation Period
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 flu 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 yet 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