Why vaccine dosages are different in children and adults

At birth, our species is particularly vulnerable and the development that began in the womb continues during our first years of life. At all levels. Just as we must learn to walk, our immune system must learn to defend itself against infections. Over time, it will go through different stages of maturation, in the same way that we go from “four legs” to standing and then walking and running.

This maturation process is one of the reasons why scientists are studying the immune response to a vaccine in different age groups and why, for example, Covid-19 vaccines need to be tested separately in children aged 5 to 11. and among those aged 12 to 16. Doctors want to use the dose of vaccine that provides the best protection with the fewest side effects. And that will depend on the functioning of the immune system – which itself derives from its level of development. So many points that cannot really be assessed from the outside.

I am an immunologist, and this is how I explain to my patients, children and adults, how, each in their own way, they will react to vaccines.

The two faces of our immune system

The immune maturation process begins shortly after birth.

During our early days, our main protection comes from the antibodies transmitted by our mother through the placenta and breast milk. They provide what is called “passive immunity”. The adaptive immune system of the newborn (or acquired immunity) – the part of the immune system that will make antibodies – is not yet fully operational: if it is indeed already active, it may take years to reach maturity and become fully effective.

A newborn baby heads its mother
Immediately after birth, a child’s immune system depends largely on the antibodies received from its mother through the placenta and breastfeeding. Jonathan Borba/Pexels, CC BY

Fortunately, we are also born with what is called a innate immune system, which remains active throughout our life. Unlike the adaptive immune system, it does not need to be trained to be able to fight infections. Without his good offices, people would get sick much faster and more often.

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Skin and mucous membranes are among the first lines of defense of this innate immune system. And if germs do manage to break through its physical barriers, it has fight-ready enzymes – to attack and literally break down foreign organisms. In addition, some specialized cells search for anything that is not “us” in order to destroy it, while others, called “phagocytes”, swallow invaders identified as such.

The innate immune system is therefore the first to answer when the safety of our body is engaged. It saves us a little time. Then our adaptive immune system kicks in and joins the fight.

When mature, our adaptive system can generate immunity by actively producing suitable antibodies following infection or vaccination. Antibodies are proteins that act like suction cups that stick to viruses or bacteria to help the body identify them in order to get rid of them more quickly and thus prevent the infection from spreading. Antibodies are specialized: each one recognizes and eliminates a specific germ.

This system is said to be adaptive because it adjusts to each new infection and learns from it to respond specifically to it. This work of recognition is not lost, because, subsequently, if our body is again exposed to similar germs, this system will be able to remember them and will be able to react quickly.

Vaccines take into account immune development

Just as a child will learn to walk even if you don’t secure the stairs and swimming pools for him or her, your immune system is quite capable of learning how to fend off an invading virus without a vaccine. But, in some cases, the risk of injury is much greater.

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Vaccines work by triggering the creation of antibodies that will recognize a specific germ and work to fight it in a safer way than if the infection were first acquired without a vaccine. The effectiveness of a vaccine depends on the amount of antibodies that our body will produce in response to the injection, their effectiveness… and the safety of the vaccine itself. Hence, in particular, a work of dosage of the latter.

When researchers want to assess the concentration of a vaccine to adopt for different age groups, they need to know which parts of the immune system will be involved in the expected response and may not yet be fully active depending on the age of the target audience. It is partly for this reason that some vaccines – like the one against Covid-19are tested and approved on different schedules for adults, teens, children and babies. (In France, the Academy of Medicine has only just approved the extension of vaccination to children aged 5 to 11 in certain cases: co-morbidities, proximity to vulnerable people, etc. Editor’s note)

For Pfizer’s Covid-19 vaccine, children aged 5 to 11 have been shown to have a similar immune response and safety with a third of the dose used for children aged 12 and over.

A number of vaccines for infants are given serially, that is, they are given the same type of vaccine several times in a few months. Not because the vaccine is ineffective, but because a baby’s adaptive immune system is likely to be forgetful or not “listening” well at this age – much like a baby falters when trying. to stand and walk. With every exposure, every part of the immune system strengthens and improves to defend against potential infection.

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Childhood and old age, when our natural protection is less

After the age of 4, and throughout adulthood, our immune systems tend to be more responsive and less prone to forgetfulness. It is no coincidence that this is the time when people are most likely to get most of their allergies.

When studying vaccines, scientists tend to already be interested in patients between the ages of 18 and 55. As the immune system is mature for this age group, they can rely on its results to identify any adverse reactions. The observations also give doctors the first clues to predict what might happen when the vaccine is given to others; this also allows them to be immediately attentive in younger groups to the possible occurrence of side effects already identified.

Around the age of 55, the adaptive immune system begins to weaken again and lose memory, recalling in a way what is observed in the infant (where conversely he has not yet acquired all the necessary experience). Fortunately, vaccine boosters can provide a quick refresher for these patients. (As is now the case against Covid-19 in France. Editor’s note)

Ultimately, vaccines are the safest environment for learning the immune system, and tailoring doses to different age groups helps ensure that each patient gets just what they want. necessary to make his

The original version of this article was posted on The conversation, a non-profit news site dedicated to sharing ideas between academic experts and the general public.

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