Influenza is a common respiratory illness caused by a virus that infects the nose, throat, and sometimes the lungs. This illness can range from mild to severe, often leading to significant seasonal outbreaks. Due to the virus’s inherent diversity and constant ability to change, it is entirely possible to contract the flu more than once within a single flu season.
The Four Types of Influenza
A primary reason a person can contract the flu multiple times in one season is the existence of distinct viral categories. Scientists classify influenza viruses into four types: A, B, C, and D. Types A and B are the major causes of yearly seasonal epidemics in humans, while Type C typically causes only mild illness and Type D primarily affects cattle.
These distinct types are not cross-protective; the immune response generated by an infection with one type does not shield the body from the others. For example, if a person contracts Type A influenza early in the season, they develop immunity specific to that virus. If a genetically distant Type B virus circulates weeks later, the person’s existing antibodies will not recognize the new threat, allowing for a second infection.
Influenza A viruses are further divided into subtypes based on two surface proteins, hemagglutinin (H) and neuraminidase (N), such as H1N1 and H3N2. Influenza B viruses are classified into two main lineages, which can both circulate during the same season. A person could be infected by an A virus, recover, and then contract a B virus, or vice versa, leading to two bouts of illness.
Viral Mutation and Antigenic Drift
Beyond the existence of different types, the influenza virus is constantly changing, which is the second mechanism enabling repeat infections. This process is known as antigenic drift, involving small, continuous genetic mutations in the viral surface proteins, hemagglutinin (HA) and neuraminidase (NA). These proteins are what the immune system uses to recognize and neutralize the virus.
As the virus replicates, these small mutations accumulate, resulting in new viral strains that are antigenically different from earlier circulating strains. A person’s pre-existing antibodies, which matched the original strain from a prior infection, may no longer effectively recognize or neutralize the drifted variant. This reduced recognition means the immune system must mount a new defense, leaving the person susceptible to infection by the new strain.
Antigenic drift is a major factor in the seasonality of influenza and requires new vaccine formulations annually. In contrast, antigenic shift is a less frequent, abrupt change occurring only in Type A viruses, often resulting in a pandemic because the virus is completely novel to the human population. Antigenic drift is the primary driver behind contracting the flu multiple times, sometimes within a single season.
Maximizing Protection Against Multiple Strains
The most effective strategy to protect against contracting multiple strains is annual influenza vaccination. Current flu vaccines are quadrivalent, designed to protect against four different virus components. This composition includes two different influenza A viruses and two different influenza B viruses.
By targeting multiple circulating strains, the vaccine provides broader protection against the viral diversity present in a given season. This approach reduces the likelihood of infection from either a Type A or a Type B virus, decreasing the risk of dual infections. The vaccine’s formulation is updated each year based on global surveillance to match the strains predicted to undergo the most significant antigenic drift.
While vaccination is the primary defense, supplementary measures also reduce the chance of exposure to multiple strains. Practicing good respiratory hygiene, such as regular handwashing and covering coughs, helps limit the transmission of circulating respiratory viruses. Staying informed about local outbreak data can highlight which types of influenza are most prevalent, reinforcing the need for preventive action.