It is possible to contract the influenza virus, commonly known as the flu, more than once, even within the same season. Influenza is a highly contagious respiratory illness caused by viruses that infect the nose, throat, and lungs. While a single infection typically grants immunity to the exact strain that made you sick, that protection offers little defense against other variants circulating simultaneously. The possibility of getting the flu again stems directly from the nature of the virus, which rapidly mutates and presents in multiple distinct forms.
The Role of Viral Variation
The primary reason a person can get the flu multiple times is the constant evolution and simultaneous circulation of different influenza viruses. The seasonal flu is caused by two main types, Influenza A and Influenza B, both of which are constantly changing. Within Influenza A, there are subtypes like H1N1 and H3N2, and multiple distinct strains of each can be prevalent during a single season. The virus evolves primarily through “antigenic drift,” involving small, continuous mutations in the genes that code for the virus’s surface proteins: hemagglutinin (H) and neuraminidase (N). When the virus undergoes enough minor changes, antibodies from a previous infection or vaccination may no longer effectively recognize and neutralize the new strain.
A person infected with one strain, such as a particular H3N2 variant, builds immunity against that specific version. However, this immunity provides little protection against a co-circulating strain of Influenza B or a different, antigenically drifted H3N2 strain. This continuous genetic variation necessitates the annual update of the flu vaccine, as circulating viruses shift each season.
Distinguishing Sequential Infections
Experiencing flu-like symptoms shortly after recovering from a confirmed influenza infection is a common scenario that can be misleading. The vast majority of people who are otherwise healthy will fully recover from the flu within one to two weeks, although a lingering cough or fatigue may persist longer. A second, immediate illness is frequently caused by an entirely different respiratory pathogen that simply mimics the flu.
Many other viruses circulate at the same time as influenza, including respiratory syncytial virus (RSV), common coronaviruses, adenoviruses, and parainfluenza. These can all produce overlapping symptoms like fever, body aches, and cough, making it nearly impossible to distinguish them from the flu based on symptoms alone. The only definitive way to confirm a second influenza infection is through diagnostic testing, which can identify the specific type and sometimes the strain of the virus responsible.
Strategies to Minimize Risk
The most effective strategy to reduce the risk of contracting the flu, including a subsequent infection, is to receive an annual influenza vaccination. The yearly vaccine is formulated to protect against the three or four influenza strains predicted to be the most common during that flu season. Even if the vaccine does not completely prevent infection, it offers significant protection against severe illness, hospitalization, and death.
Non-pharmaceutical interventions offer a further layer of protection against all respiratory viruses. Consistent and thorough hand hygiene, particularly washing with soap and water, disrupts the transmission cycle of the virus. Avoiding touching the face, especially the eyes, nose, and mouth, prevents virus particles from entering the body. Furthermore, staying home from work or school when sick is a measure that limits the spread of all respiratory illnesses to others.