You can get the flu more than once, even in the same season. The influenza virus continually changes, which contributes to its potential for reinfection. This dynamic nature means that previous exposure or vaccination may not always provide complete protection against new or different circulating strains. Understanding how the virus evolves and how immunity operates helps clarify why reinfection can occur.
How Flu Viruses Evolve and Immunity Works
Influenza viruses constantly evolve through genetic changes, allowing new strains to emerge each season. One way this happens is through “antigenic drift,” which involves small, gradual mutations in the genes of the virus, particularly those coding for its surface proteins (hemagglutinin (HA) and neuraminidase (NA)). These minor alterations can modify the virus enough that existing antibodies from prior infection or vaccination may no longer effectively recognize and neutralize it. This process is a primary reason people get the flu multiple times throughout their lives.
Another, more dramatic type of change is “antigenic shift,” which involves an abrupt, major change in influenza A viruses. This shift results in entirely new HA or NA proteins, often occurring when a flu virus from an animal population, such as birds or pigs, gains the ability to infect humans. When antigenic shift happens, most people have little to no pre-existing immunity against the novel virus, potentially leading to widespread outbreaks or pandemics. While antigenic drift happens continuously, antigenic shift occurs infrequently.
The human immune system develops specific antibodies in response to an influenza infection or vaccination. This immunity provides protection against the particular strain encountered or very closely related ones. However, immunity to one type of influenza, such as influenza A, does not protect against influenza B, and vice versa. Even within the same type, different subtypes and strains can cause separate infections. The protection gained from a previous infection or vaccination can also wane over time, leaving individuals susceptible again.
Why Some People Face Higher Reinfection Risk
Several individual and environmental factors can increase a person’s vulnerability to flu reinfection. An individual’s overall immune health plays a significant role; those with weakened immune systems, such as the elderly, very young children, or individuals with chronic health conditions like asthma, diabetes, or heart disease, are more susceptible to severe flu complications and potential reinfection. Their bodies may not mount as robust an immune response to either infection or vaccination.
The severity of an initial flu infection can also influence subsequent immunity. A milder infection might not stimulate as strong or broad an immune response compared to a more severe illness, potentially leading to weaker protection against future exposures. Multiple strains of influenza viruses often circulate simultaneously within a single flu season, meaning that being infected with one strain does not protect against others present in the same period. This co-circulation can lead to consecutive infections with different strains within weeks.
The flu vaccine offers additional layers of protection, even with previous infection. While natural infection provides immunity against the specific strain encountered, the annual flu vaccine is designed to protect against the strains predicted to be most common for the upcoming season, which often include multiple types and subtypes. Vaccination can also reduce the severity of illness if an infection occurs, minimizing the risk of serious complications. Even if someone has had the flu, their immunity might not cover all circulating viruses or could have declined, making them vulnerable again.
Steps to Avoid Flu Reinfection
Receiving an annual flu vaccination is a primary strategy for preventing reinfection and reducing the risk of severe illness. Flu viruses change annually, so a new vaccine is formulated each year to target the strains expected to circulate. Even if prior infection has occurred, vaccination boosts the immune system’s ability to recognize and fight current viruses. It typically takes about two weeks after vaccination for antibodies to develop, providing protection.
Practicing good hygiene habits also helps limit the spread of influenza viruses. Regularly washing hands with soap and water, or using an alcohol-based hand sanitizer, can remove germs that cause respiratory illnesses. Avoiding touching the eyes, nose, and mouth reduces the chance of transferring the virus from contaminated surfaces into the body. These simple actions contribute significantly to personal protection.
Minimizing close contact with sick individuals and adopting respiratory etiquette are additional preventative measures. People should avoid close contact with those who are ill and cover their mouth and nose when coughing or sneezing, ideally with a tissue. Disposing of used tissues promptly and washing hands afterward helps contain the virus. Staying home from work, school, or errands when experiencing flu symptoms is also important to prevent further transmission to others.