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When the flu virus enters the human body, it infiltrates cells and hijacks their machinery to replicate. However, this process is not flawless and is prone to errors that commonly occur during the replication of its genetic material. These errors, known as mutations, allow the virus to partially or completely evade the immune system. This is why the flu vaccine must be annually updated to maintain its effectiveness and curb the virus’ spread. Here's an in-depth look.
Dry cough, frequent sneezing, high fever, muscle aches, headaches and extreme fatigue: these telltale symptoms emerge every winter, signaling the return of seasonal respiratory infections. Among them, the flu is one of the most common. As temperatures drop, the conditions become ideal for the spread of several viruses, including influenza, which impacts not only young adults but also the elderly, with potentially serious consequences for vulnerable individuals.
This flu season in France has been particularly severe, though it seems to have passed its peak. Despite a decline in flu activity, the virus continues to affect all regions of mainland France, as well as some overseas territories. Additionally, this year’s flu vaccine has proven less effective, particularly for those over 65, with its efficacy estimated at just 26%. The simultaneous circulation of three different strains of the virus has certainly contributed to the severity of the outbreak. While the situation has stabilized since early February, hospitalizations among children remain “very high,” according to a weekly report from the French public health agency, Santé Publique France.
Flu Epidemic Peak
In Lebanon, although winter arrived later than expected this year, it has now settled in, marking the start of the acute phase of respiratory infections. The delayed cold temperatures likely postponed the epidemic peak. According to several doctors interviewed by This is Beirut, the flu strains observed so far are no more severe than those in previous years. Nevertheless, preventive measures remain crucial to curb the spread of the virus.
Vaccination continues to be a key strategy to prevent or protect against severe cases of the illness during this phase. While the number of infections is steadily rising, the vaccine remains effective and is strongly recommended, particularly for vulnerable populations such as the elderly, pregnant women and individuals with chronic illnesses or weakened immune systems. These groups are at higher risk for severe complications, and vaccination can help prevent prolonged hospitalizations or even death. As with every year, the same question arises: How effective is the vaccine, and why do we see variations in its efficacy from year to year?
Vaccine Update
When the flu virus infects the host’s cells, such as human cells, it hijacks their machinery to replicate. During this process, errors can occur in the replication of its genetic material, leading to mutations. These mutations often enable the virus to partially evade the immune response, even in individuals who have previously been immunized through past infections or vaccinations. This diminished effectiveness of acquired immunity facilitates the reemergence of the infection as a seasonal epidemic each winter. Due to these continuous mutations, the flu vaccine must be annually updated to target the most prevalent strains. As a result, even those who were vaccinated the previous year need a new dose to ensure protection against emerging strains.
New Variants
The flu vaccine primarily targets two surface proteins of the virus: hemagglutinin (HA) and neuraminidase (NA). Since the pandemics of 1968 and 1977, the most prevalent human virus subtypes have been H1 and H3 for hemagglutinin, and N1 and N2 for neuraminidase. The most common circulating combinations are H1N1 and H3N2. Current vaccines typically include two type A virus strains (H1N1 and H3N2, which undergo annual specific mutations) and one type B strain for trivalent vaccines or two type B strains for quadrivalent vaccines.
Every year, the World Health Organization (WHO) releases recommendations for the flu vaccine composition for both hemispheres. These guidelines are based on the strains circulating in the opposite hemisphere during the current flu season. As a result, recommendations for the northern hemisphere are issued in late summer, while those for the southern hemisphere are released in early winter. This process enables the early detection of emerging strains and variants that could globally spread, with the virus circulating between the two hemispheres. During the summer months, when viral circulation tends to be lower, one hemisphere acts as an infectious reservoir for the other.
Vaccine Efficacy
The flu vaccine works by stimulating the immune system. By introducing either an inactivated or attenuated version of the virus, it prompts the immune system to produce specific antibodies targeting the virus' surface proteins. This trains the immune system to “recognize” and rapidly respond to these structures in case of a real infection.
Clinical and epidemiological studies generally report positive results on the flu vaccine’s effectiveness, though it can vary from season to season. The vaccine can reduce the risk of flu infection by 40-60%, depending on factors such as the circulating strains, age groups and the year. However, it doesn't always protect against all strains, especially those that have unexpectedly mutated.
A 2018 study published by the US Centers for Disease Control and Prevention (CDC) found that between 2012 and 2015, the flu vaccine reduced the risk of ICU admission due to the flu by 82% in adults. Furthermore, a 2023 study published in JAMA Pediatrics found that babies under six months, born to mothers who were vaccinated during pregnancy, had a reduced risk of flu-related emergency visits and hospitalizations.
For immunocompromised individuals who are at higher risk of severe flu complications, annual vaccination is highly recommended.
It’s important to note that immune responses can vary between individuals, depending on factors like age, medical history and the immune system's ability to respond to the vaccine. For instance, older adults may experience a weaker immune response due to age-related declines in immune function, a phenomenon known as immunosenescence.
Despite certain limitations, the flu vaccine remains the most effective tool for controlling the spread of the virus. It is particularly crucial for preventing severe illness and reducing hospitalizations during peak flu seasons.
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