Influenza Infection and Disease

 Influenza Transmission Time Lines

When Are People Contagious?

In studies of healthy volunteers who were infected with influenza, most people began shedding (i.e., transmitting) the influenza virus between 0.5 and 1 day after their exposure to the virus.  This means that within one day of infection, the infected people were able to infect others around them if their coughs, sneezes, and respiratory secretions were not contained.  

The amount of influenza virus shedding (for example, the amount of influenza virus capable of being spread to others by an infected individual) peaked two days after the people were infected.  The average duration of viral shedding was 4.8 days, which means that on average, people who were infected with influenza could transmit the flu to others for up to five days after being infected.

What Types of Symptoms are Common? Not everyone who was infected with influenza developed symptoms.  Fever was observed in 7.5 - 40.6% of infections, depending on what strain of influenza they had.  One in three people infected with influenza did not develop any symptoms of a clinical illness! Most symptoms began one day after infection and peaked three days after infection. The most frequently observed symptoms included nasal congestion, runny nose, sore throat, sneezing, a hoarse voice, ear pressure or earache. Symptoms like fever, muscle aches, fatigue, and headache usually peaked around 2 days after the initial infection.  Symptoms related to coughs and a congested or runny nose usually lasted longer.

To read the study, click here.