Influenza, commonly known as the flu, is a respiratory illness caused by influenza viruses that circulate every year. The infection typically results in a sudden onset of symptoms like fever, body aches, cough, and fatigue, lasting for about two to eight days. Contracting the exact same strain of flu twice in a single season is rare, but recurring flu-like symptoms are far more complex. This apparent re-infection is usually a result of the virus’s ability to change or the presence of other circulating respiratory pathogens.
Understanding Viral Strains and Immunity
The body’s defense against influenza is highly specific and relies on recognizing the virus’s outer coating. Influenza A viruses are categorized by two spike-like proteins on their surface: hemagglutinin (H) and neuraminidase (N). For instance, a common strain is designated H1N1, indicating the specific types of H and N proteins it possesses. These surface proteins are the antigens that the immune system learns to recognize.
When a person is infected with a particular strain, their immune system produces antibodies designed to neutralize that virus by binding directly to the H and N proteins. These antibodies offer long-lasting immunity against the specific strain encountered. This is why a person who recovered from the H3N2 strain circulating in January would likely be protected if they were exposed to that identical virus again in March.
The immune memory established after this initial exposure is the reason true reinfection with the identical pathogen is uncommon. However, this protection is narrowly focused on the surface proteins of the original invading virus. If a new virus enters the body with even slightly altered H or N proteins, the pre-existing antibodies may not recognize it well enough to prevent infection, allowing the new variant to take hold.
How Influenza Viruses Change Over Time
The influenza virus manages to evade immunity through constant mutation of its surface proteins. This mutation is why people can get the flu repeatedly and why the seasonal flu vaccine must be updated annually. This process of change primarily occurs through two distinct mechanisms: antigenic drift and antigenic shift.
Antigenic drift involves small, gradual changes that accumulate over time due to tiny errors, or point mutations, that occur when the virus replicates its genetic material. These minor genetic changes alter the shape of the H and N proteins just enough so that existing antibodies recognize the virus less effectively. This gradual accumulation of mutations creates new variants of the same subtype, which can cause yearly seasonal epidemics.
Antigenic shift, by contrast, is a sudden change that creates a new subtype of influenza A virus. This occurs when two different influenza A viruses co-infect the same cell and swap entire segments of their genetic material, a process called reassortment. This genetic mixing results in a novel virus with a combination of surface proteins that has never been seen before in the human population.
Because the human population has little to no pre-existing immunity against a virus created by antigenic shift, these events can lead to pandemics. While antigenic drift drives the need for annual vaccine updates, antigenic shift is a rare event that introduces a new subtype.
When Immunity Fails or Seems to Fail
Even when a person experiences flu-like symptoms twice, it may not signify a failure of immunity against the original virus. The second illness is often caused by a different pathogen that mimics the flu. Other respiratory viruses, including respiratory syncytial virus (RSV), adenoviruses, and common cold coronaviruses, cause symptoms nearly indistinguishable from influenza.
The initial illness may not have been the flu but one of these other circulating viruses, leading to a misdiagnosis based on symptoms alone. Also, a person’s immune response can sometimes be insufficient or decline rapidly.
For some individuals, especially the elderly or those with underlying conditions, antibody levels may decrease following vaccination or infection, increasing the risk of later infection. Another common reason for symptom recurrence is the development of a secondary bacterial infection.
The influenza virus damages the respiratory tract lining, leaving the lungs and airways vulnerable to opportunistic bacteria that can cause complications like bacterial pneumonia or sinusitis. This often presents as a worsening or return of symptoms after an initial improvement.