Influenza, commonly called “flu,” is a viral respiratory disease affecting about one billion people worldwide yearly. Despite being very common, its impact should not be underestimated: Influenza can occur with different forms of severity, sometimes requiring hospitalization, and in some cases, it can even have fatal consequences.

How Do You Get the Flu?

Influenza is a viral respiratory disease, meaning that the virus is passed from person to person through:

  • Droplets that are spread when coughing, sneezing, or talking (particularly in crowded, enclosed environments);
  • Direct contact with infected people (for example, through contaminated hands on the eyes, nose or mouth);
  • Direct contact with contaminated utensils and objects: the influenza virus can resist for a long time and reach the body through the mucous membranes.

Influenza: How Long Are You Contagious?

Seasonal influenza generally has an incubation period of two days but can vary from one to four. Adults can spread the flu from the day before symptoms appear to about five days after their onset, while children and people with weakened immune systems may be more contagious.

What Are the Symptoms of Influenza?

Sudden high fever, cough (usually dry), and muscle aches are the characteristic symptoms of influenza. Other common symptoms are: 

  • Headache;
  • Chills;
  • Loss of appetite;
  • Fatigue;
  • Sore throat.

Influenza may also be associated with nausea, vomiting, and diarrhea in children.

Typically, the flu lasts from one week to ten days.

Influenza can manifest with different forms of severity, and specific population groups, such as children, are at an increased risk of developing severe flu complications, such as:

  • Viral pneumonia; 
  • Secondary bacterial pneumonia;
  • Worsening of underlying medical conditions.

How to Prevent the Flu?

To prevent influenza, as well as to prevent colds, the same guidelines apply as to contain the COVID-19 pandemic:

  • Wash your hands often and well with soap and water for at least 40-60 seconds, especially after coughing or sneezing. Alcohol-based sanitizers reduce the amount of influenza virus from contaminated hands and are a viable alternative;
  • Cover your nose and mouth when sneezing or coughing with disposable tissues to be thrown away immediately;
  • Wear surgical masks in the presence of symptoms such as coughs and colds can reduce infections among close contacts;
  • Stay home in the presence of symptoms, especially in the early stages;
  • Avoid close contact with sick people, keeping a distance of at least one meter from those with flu symptoms and wearing a mask;
  • Avoid touching one’s eyes, nose, or mouth. Viruses can spread when a person touches any surface contaminated with the virus and then touches their eyes, nose, or mouth.

The Flu Vaccine

Vaccination is the most effective way to prevent influenza; a vaccine is available yearly for that specific flu season. Flu vaccines only contain inactivated viruses or parts of them, so they cannot cause flu virus infections.

The flu vaccine significantly decreases the likelihood of getting the flu, and even if you do get sick, the flu form will be less severe and generally free of complications.

Vaccination also protects others, thus reducing the burden on the health care system. Given that the COVID-19 pandemic is ongoing, extensive vaccination coverage is essential because it helps not to weaken one’s immune defenses, reduce complications in people at risk, and avoid an overload of hospital patients.

Especially in the cold season, many bacterial and viral agents circulate, causing respiratory infections and flu-like syndromes. However, since they are not influenza viruses, the flu vaccine is ineffective for these agents.

What Are the Influenza Viruses?

The main viruses responsible for influenza are types A and B in humans.

Influenza A viruses (circulating in humans and other animal species) are classified into subtypes according to two surface proteins: hemagglutinin (HA) and neuraminidase (NA). 

In recent decades, two subtypes of HA (H1 and H3) and two subtypes of NA (N1 and N2) among influenza A viruses are considered responsible for human disease. Immunity to HA and NA proteins reduces the likelihood of infection and, together with immunity to internal viral proteins, decreases the severity of illness in case of infection. To date, 16 subtypes of HA and 9 of NA have been identified.

Influenza B viruses (found only in humans) do not have distinct subtypes within their HA and NA surface proteins.

Influenza viruses undergo mutations on their surface proteins. These physiological changes allow the viruses to evade the immunity barrier present in people who have had influenza or been vaccinated against flu less than a year before, thus promoting the vast and rapid spread of infection.

For this reason, the composition of influenza vaccines must be updated each year. Fundamental to this is the surveillance activity that makes it possible to select which strains to include in the vaccine, depending on the degree of epidemiological and serological difference from what has circulated in previous influenza seasons.

Who Should Get Vaccinated?

All people 6 months of age and older can get the flu vaccine, provided they have no contraindications to vaccination. Influenza is a severe public health problem and a hefty economic burden for managing cases, complications of the disease, and implementation of control measures.

Vaccination is strongly recommended for the following categories:

  • People 60 years of age and older;
  • People who are in close contact with older people;
  • People at risk of complications who have chronic conditions (diabetes, hypertension, HIV/AIDS, asthma and other chronic heart or lung diseases);
  • Pregnant women;
  • Children aged 6 months to 6 years, given the ongoing COVID-19 pandemic, to reduce the circulation of influenza virus among adults;
  • Healthcare personnel.

When to Vaccinate Against Influenza?

Vaccination is administered from early October to late December, and the vaccine begins to be effective two weeks after the injection; the immunity obtained starts to decline within 6 to 8 months. Being vaccinated the previous year does not protect in the following year’s flu season, precisely because of mutations in flu viruses.

The vaccine is administered in a single dose intramuscularly into the deltoid of the chosen arm. At the same time, the injection is preferably given into the anterolateral thigh muscle or via the nose in young children.

After the vaccine, some symptoms may manifest, such as:

  • Pain, erythema, swelling at the injection site;
  • General discomfort;
  • Fever;
  • Muscle pain (myalgia).

Can Influenza Vaccine and Anti-COVID-19 Vaccine Be Given Together?

Yes, there is no contraindication. The flu vaccine does not interfere with the immune response to other inactivated or live attenuated vaccines, and there are no side effects beyond those already known.