The answer to whether a person can contract the influenza virus more than once is yes. Influenza is a contagious respiratory illness caused by influenza viruses that circulate globally every year. The possibility of repeat infections stems from the complex biology of the virus, which constantly changes and exists in multiple forms. This continuous evolution means that the immune protection gained from one infection or vaccination may not recognize a subsequent viral exposure.
The Different Types of Influenza Viruses
The influenza virus family is categorized into four main types: A, B, C, and D. Types A and B are the primary causes of the seasonal epidemics that occur almost every winter in the United States and other temperate regions of the world. Influenza A viruses are known to infect various animals and are classified into subtypes based on two surface proteins, hemagglutinin (H) and neuraminidase (N), such as A(H1N1) and A(H3N2).
Influenza B viruses primarily circulate among humans and are classified into lineages, historically B/Victoria and B/Yamagata. Infection with one type, such as Influenza A, provides no protection against the other, Influenza B, because they are genetically distinct. A person can also be infected by different subtypes of Influenza A or different lineages of Influenza B in the same season, leading to multiple episodes of the flu.
How Viral Mutation Causes Repeat Infections
The main reason individuals can contract the flu multiple times is the constant, rapid genetic change, known as antigenic evolution, that influenza viruses undergo. This process forces the immune system to continuously play catch-up, as the antibodies produced against an older strain may not effectively neutralize a newer one. The most common form of change is called antigenic drift, which involves small, frequent mutations in the genes that code for the H and N surface proteins.
These gradual genetic changes cause the virus to look slightly different to the immune system each year, allowing it to evade pre-existing antibody protection. Antigenic drift is the reason why new seasonal flu vaccines must be formulated annually to match the circulating strains. Both Influenza A and B viruses undergo this gradual evolution, which ensures the virus can cause infections in successive seasons.
A more dramatic change, called antigenic shift, occurs only in Influenza A viruses and involves an abrupt, major change in the virus’s surface proteins. This shift happens when a new subtype of Influenza A emerges in humans, often through the exchange of gene segments between human and animal viruses. Since most people have little to no immunity against the drastically altered virus, an antigenic shift can potentially lead to a worldwide pandemic.
Mistaking Other Infections for the Flu
A person who believes they have contracted the flu twice in a short period may have experienced a second infection caused by a completely different pathogen. Many other viruses and bacteria can cause symptoms that closely mimic the flu, including fever, body aches, cough, and fatigue. This phenomenon is known as having a flu-like illness without having influenza.
Common culprits that circulate during the same time of year as the flu include respiratory syncytial virus (RSV), adenoviruses, and rhinovirus, which is the most frequent cause of the common cold. Even common coronaviruses, which are distinct from the virus that causes COVID-19, can produce similar respiratory symptoms. Because the symptoms are often clinically indistinguishable from influenza, it is easy to mistake one of these non-flu infections for a second case of the flu.
How Long Does Flu Immunity Last
Immunity acquired from a flu infection or a vaccination is generally temporary and declines over time, a process known as waning immunity. For most healthy individuals, the protective level of antibodies generated by a vaccine or infection is strongest during the first three to six months. This temporary nature is a primary factor necessitating annual vaccination, even without considering the virus’s ability to mutate.
The effectiveness of the immune response is also highly strain-specific, meaning that protection is best against the exact virus strain encountered and less effective against antigenically different strains. While some antibody memory can persist for years, the level of protection against the current season’s circulating strains typically decreases significantly within six to eight months. This decline, combined with antigenic drift, ensures that the body’s defenses must be regularly updated to maintain optimal protection.