What is the flu vaccine?

The influenza virus, which causes the disease known as “influenza”, is highly transmissible and mutagenic (that is, it modifies its molecular characteristics), causing epidemics capable of killing hundreds of people a year.

Influenza epidemics are capable of infecting millions of people and causing up to690,000 deaths annually. Such as the flu pandemic of 1920, which was able to kill between 17 to 20 million people worldwide.

This high death rate prompted the creation of a vaccine in the 1930s and, beginning in the 1940s, mass manufacturing and vaccination of the population began.

Beginning in 1952, the World Health Organization (WHO) created a committee, known as theGlobal Influenza Surveillance and Response System (GISRS), capable of developing the best instruments needed to fight influenza. Among them, vaccines.

This entity, together with the Center for Disease Control and Prevention (CDC) , are some of the organizations responsible for maintaining constant monitoring of the rapid evolution of the virus.

Composition of the influenza vaccine

There are two types of flu vaccines: The vaccine that uses live, but weakened viruses, known as live vaccines. Other types of similar vaccines are the vaccine against rubella, yellow fever, rotavirus, etc.

These types of vaccines are capable of generating an immune response similar to that of the non-attenuated virus. On the other hand, because the virus is still alive, certain types of people should not be given this type of vaccine.

On the other hand, we have the inactivated influenza vaccine. As its name indicates, unlike the first, it does not use live viruses but dead ones, still capable of generating an immune response in the person.

The vaccine using killed viruses is administered as an injection, which is placed in the patient’s upper arm. The live virus vaccine is administered in the form of a nasal spray.

Characteristics of the influenza vaccine

The influenza vaccine is a testament to the ability of humans to develop powerful weapons that can help us fight these deadly pathogens.

Next, we will explain some of the characteristics that distinguish the influenza vaccine, as an effective tool to prevent the uncontrolled spread of the disease.

May prevent flu and related conditions

This is the main and biggest advantage of the vaccine. In animal and human laboratory models, the vaccine demonstrates great efficacy by inducing the production of antibodies, preventing the transmission of the virus.

Unfortunately, real life is more complicated. The prevalence and type of influenza varies from region to region, so the strain that was used to create the vaccine may not be exceptionally effective against a different strain.

However, in recent decades, the efficacy of the vaccine continues to be effective against circulating strains of influenza virus. It is even estimated that the efficacy of the 2019 season vaccine wasup to 45% , according to the Center for Disease Control and Prevention (CDC)

Develops antibodies 2 weeks after application

The influenza virus vaccine does not work immediately, but takes at least two weeks to take effect. This is because the production of antibodies in our body is not immediate, as is the case for many diseases.

Therefore, it is important to get vaccinated in advance, avoiding the days when the influenza season is estimated to start.

Provides protection against various influenza viruses

The “influenza virus” is not a single virus, but a whole family. There are four different types of influenza virus: A, B, C, and D. Type A influenza virus is the most common and has the greatest pandemic potential (as demonstrated in the 2009 pandemic).

Influenza viruses types B and C can also infect humans, although their spreadability and lethality are lower. Type D only affects animals so far, theWorld Health Organization (WHO) warns that it has the potential to be transmitted to humans.

There are also subtypes. The two most common and dangerous subtypes of group A are H1N1 and H3N2, although there are many more. Therefore, the need arises to create combined vaccines, capable of protecting the population from these variants.

Tri- and quadrivalent vaccines have been created (for example, AFLURIA Quadrivalent or FLUZONE quadrivalent), which contain particles of different subtypes of the virus (H1N1, H3N2, and group B virus subtypes 1 and 2), capable of providing greater protection. Wide.

It can be applied from 6 months of age

From the sixth month, the mother’s circulating antibodies disappear, because the child requires extra protection to increase its defenses, especially against many diseases.

Should be done every year preferably

Most vaccines only need to be given one to three times in a person’s life (for example, the chickenpox vaccine, hepatitis B, pneumococcus, etc.).

But, the flu vaccine must be given annually. Why? Because, unlike many other pathogens, respiratory viruses are constantly mutating, thanks to antigenic drift.

Antigenic drift is a self-defense mechanism of the virus, where a series of mutations, over time, give rise to different versions of its surface proteins.

The surface proteins are the “recognition signal”, which our antibodies use to detect pathogens. If the signal changes, it no longer matches what the antibodies are looking for, so the virus goes undetected.

The 2020 trivalent influenza vaccine (also used for the 2019 season) contains 2 strains of the A virus (H1N1 and H3N2) and one of the B virus, which were isolated during 2017 and 2018.

Not recommended for certain groups of people

Not all people can freely receive the flu vaccine. There are certain groups that are prone to developing adverse reactions to the contents of the vaccine.

Children under six months of age cannot receive either version of the vaccine. Neither do people who have severe allergies to vaccine components, such as gelatin or antibiotics.

Of the rest, people with chronic diseases, in serious health and even pregnant women, are free to receive the injectable vaccine.

People with an egg allergy should consult their doctor or an allergist (allergy specialist) before receiving the vaccine. This is because the antigens contained in the vaccine are extracted from fertilized eggs that are grown in laboratories.

On the other hand, the group of people who should not receive the attenuated virus vaccine is much larger, since live viruses can generate an immune reaction.

Children under 2 years of age, adults over 50 years of age, pregnant women, people taking aspirin, people with a weakened immune system, children between 2 and 4 years of age who have a history of asthma or wheezing.

Must be applied before flu seasons

Since the vaccine only takes effect two weeks later, it is important to know which seasons of the year flu outbreaks can occur. Low temperatures help the spread of the virus.

Therefore, it is important to keep abreast of local medical news, where and when you should receive the vaccine and any other recommendations from medical authorities.

Children require 2 doses

From the sixth month of life to the fourth or sex year of life, the child should receive two doses annually. From this point on, he will only need an annual dose, to strengthen his defenses against the effects of the virus.

Check with your preferred doctor for more information on the vaccination schedules in your country.

It can be purchased in doctors’ offices, health centers and pharmacies

The ease with which the vaccine can be administered in any health center facilitates vaccination, which is key to combating flu outbreaks in a region.

The mass manufacturing of the vaccine has made it possible to substantially reduce the number of comorbidities associated with influenza. In fact, according to aStudy carried out in 2007, the global production capacity was 826 million vaccines.

Does not increase risk or protect against COVID-19

The current pandemic, caused by SARS-CoV-2, has reminded the population of the importance of vaccines, especially against respiratory tract pathogens, capable of being transmitted so easily.

But, it is important to note that the vaccine does not have any palliative effect on COVID 19 or related symptoms. The flu vaccine also does not increase the risk of getting COVID 19 or any other respiratory virus.

However, it is important to receive your vaccination for the 2020-2021 season, as it reduces the number of hospitalizations and, therefore, reduces the existing overload in many hospitals, saturated by patients with COVID 19.

Benefits of the influenza vaccine

The influenza vaccine turned out to be an essential ally against the flu, which, until then, whose infection rate could even have reached 50%tag. It is important to know all its effects and benefits.

Avoid flu-related illnesses

The vaccine is not only effective against influenza, but also against symptoms or syndromes related to it. By preventing these complications, the risk of lethality from the virus is significantly reduced.

Some of these complications are otitis media and sinus infections, which are moderate, while there are other more serious ones, such as pneumonia. Inflammation ofheart , muscle, or even brain tissue can also occur .

It also avoids systemic complications. What does it mean? Conditions involving multiple organs, caused by widespread inflammation, such as occurs with sepsis or multiple organ failure.

Reduces the risk of hospitalization due to influenza

If a patient with the flu requires hospitalization, it is a serious sign that does not indicate a good prognosis if it is not properly cared for. The vaccine helps to substantially reduce hospitalization.

Over the years, the CDC has used mathematical models to estimate the number of flu cases, doctor visits, hospitalizations, and deaths to estimate how effective the vaccine is.

The most approximate Resultsfor the 2019-2020 season indicate that 38 million people became ill, but only 400,000 people were hospitalized. The number of deaths was 22,000.

It is estimated, then, that 105,000 hospitalizations were prevented during the previous season. The number and proportion of hospitalizations prevented varies substantially between age groups.

The most vaccinated age group were children between six months and 4 years of age. Thus, this was the group most protected against influenza. Next comes the group between 5 and 17 years of age and, finally, those between 18 and 49 years of age.

Reduces problems associated with chronic diseases

Those people with persistent health problems, especially respiratory problems, are more likely to become infected with influenza (since the lesions caused to the respiratory tract make the person more exposed to the virus), especially if they are not vaccinated.

Chronic obstructive pulmonary disease (COPD), personal history of pneumonia, tobacco and alcohol abuse, immunosuppressed and chronic cough, are some examples.

Not only this, but the vaccine can help reduce symptoms related to these diseases. In aStudy published in the Cochrane Database, nine trials were conducted to investigate whether the vaccine can reduce the effects of COPD.

Despite the limited number of studies performed, the inactivated vaccine appears to reduce exacerbations that occur in COPD patients two to three weeks after vaccine administration.

Protects women during and after pregnancy

During flu seasons, pregnant women are at higher risk than non-pregnant women. Not only during pregnancy, but also during the postpartum period.

Vaccination reduces 40% .

In addition, the vaccine also protects the baby, during and after pregnancy (during lactation, where the virus can be transmitted through milk), since the infant is still too young to receive a vaccine.

It is important to mention that pregnant women should only receive the vaccine injection. They cannot receive the version with attenuated viruses, since the circulating viruses can affect both the mother and the fetus.

Reduces the risk of death from influenza in children

As we mentioned, children (especially the youngest) are usually the most vaccinated group, since their defenses offer less protection against this aggressive pathogen.

A child can receive the injection starting at six months (two doses, to be given annually until eight years of age). You can also receive the attenuated virus vaccine, but only from the age of two.

Influenza Vaccine Side Effects

Not every medicine is perfect. When foreign substances enter our body, the latter can reject them, in certain cases. Some experience mild symptoms, others more severe.

Fortunately, vaccination does not cause serious symptoms in the majority of the population, regardless of age (as long as, of course, the appropriate vaccine is administered).

Below, we will describe some of the most common side effects.

Pain, redness, and swelling of the injected area

These are the most common symptoms experienced by a number of people after vaccination. In reality, pain and redness are indicators of a limited and localized inflammatory process.

By delivering foreign substances into the body, the body acts at the site where they were first recognized (ie, the injection site). Inflammation can also occur from the penetration of the needle through the skin.

It should be noted that these symptoms are usually mild and transient.

Headache and muscle

Muscle pain and headache (headache) are also short-lived symptoms and, in most cases, do not represent a danger to the patient’s health. They are also signals that our body reacts to the administered substances.


If you experience a low-grade fever (37ºC but less than 38ºC) or fever (greater than 38ºC), you should not worry. It is also a normal phenomenon, since our body is releasing certain substances from theCentral Nervous System .

These substances are called pyrogenic, they are prostaglandins, interleukins and others, released by the hypothalamus as a natural reaction to the entry of viral antigens into the bloodstream.


Nausea can be the product of two phenomena. First, your body is reacting against the antigens of the virus, generating an immune reaction (which will later create antibodies).

Second, the fear and panic that certain people have towards injections generates stress. According tostudies carried out, it is known that stress has a direct effect on the digestive system, generating sensations of stomach upset, nausea and even vomiting (in the most extreme cases).


Fear of injections (trypanophobia) a relatively common phenomenon, especially in children. People, under this state of stress, begin to produce molecules, such as cortisol, catecholamines, etc.

These substances modify blood pressure, can increase it as well as decrease it. If the blood pressure drops too quickly, the person may experience syncope (momentary loss of consciousness).

So, the injection itself does not cause fainting, it is a direct biological effect of stress and panic towards the injections or the hospital environment.

Allergic reactions

Certain people are allergic to the componentsof the vaccines: gelatin, antibiotics or ovalbumin. The latter is the main protein found in the white of eggs.

But what do eggs have to do with vaccination? The flu vaccine is created by injecting live viruses into fertilized chicken eggs. The viruses replicate inside the eggs, to then be neutralized and purified.

However, during this purification process, certain egg proteins may remain in the vaccine. If you have experienced an allergic reaction to eggs before, we recommend consulting your doctor, preferably.

As an alternative to the traditionally produced vaccine in eggs, there are variants that are created in mammalian cells. These lack ovalbumin, so the patient is safe from these allergic reactions.

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