On November 25, 2021, the European Medicines Agency recommended approving the Comirnaty vaccine against covid-19 for use in children aged 5 to 11 years. This vaccine, developed by BioNTech and Pfizer, has already been approved for use in adults and children from 12 years of age.
The questions that arise as a result of this news are: why is its use not approved in people of all ages at the same time? Why is it necessary to carry out different studies for each age group?
To understand this, we must first know pharmacodynamics, which is the study of the effect that a drug has on the body. This effect varies in each person depending on their age, sex or weight, among many other factors.
It is important to understand that children are not little adults. This is why it is essential to carry out specific clinical trials in children, since the doses of the drugs or the types of therapy can change over time depending on the needs and the stage of growth.
Among all drugs, we are going to focus on vaccines. Its activity is intended to stimulate the response of the immune system so that it can defend us against future infections of viruses or bacteria.
In this case, the pharmacodynamics will study the effect that a certain vaccine causes on the immune system of people, which undergoes modifications from our birth to adulthood.
Knowing the immune system
The immune system has different components that are grouped together giving rise to two types of response.
The innate response is the first to react and is activated when it detects the presence of a foreign organism. This response is more general and contains, on the one hand, cells that will be responsible for attacking and destroying the pathogen and, on the other, cells that alert that there is an infection.
The second response is the adaptive response, which takes a few days to kick in. This response is more specific and will remember the pathogen thanks to the memory cells, fighting it the next time it appears.
It is in charge of making antibodies, which are proteins that recognize the pathogen and bind to it to mark it, speeding up its destruction. Thanks to them, the adaptive immune system can learn of a new infection or remember one that it has not seen in a long time.
This is how our immune system matures and ages
When we are born, our immune system is totally inexperienced since it has not yet been in contact with any pathogens. Let’s say you haven’t had time to train.
During the first months of life we only have the antibodies inherited from our mother, which protect us from the pathogens that she faced previously. This implies that the immune system of babies is specially prepared to react to unknown agents. But this capacity diminishes with growth.
The first infections that we suffer help us to begin to build a reserve of memory cells and antibodies that will defend us from future infections. As it matures, the adaptive part of the immune system acquires “experience” and increases its ability to remember pathogens with which it comes into contact.
However, the immune system ages at the same time as us and there are alterations that make it not work as well. Thus, our ability to produce memory cells decreases and even those that were produced years ago begin to not work well. We are losing the ability to react to infections and also to respond to the vaccines that prevent them.
Adaptation of vaccines according to age
Vaccines contain antigens, which can be a part of the pathogen, the entire dead or inactive pathogen, or instructions for making a part of the pathogen. These antigens work by promoting the manufacture of antibodies that will recognize the living pathogen and fight it when infection occurs.
Using pharmacodynamics, researchers calculate the minimum dose of antigens necessary to obtain a sufficient number of antibodies to prevent disease without producing unwanted effects. This dose will vary based on age (among other factors).
In the Vaccine Research Area of FISABIO, in Valencia, we participate in clinical trials with vaccines in all age groups: infants between 42 and 98 days of age, children between 12 and 35 months, children and adolescents between 10 and 17 years and adults in different age ranges from 18 years.
What changes in vaccines for children or adults?
The approval of the use of the Comirnaty vaccine in children aged 5 to 11 years is based on a study that shows that with a dose three times lower in this group, the immune response (measured by the level of antibodies generated against the virus) is comparable than that seen with a dose three times higher in people aged 16 to 25 years.
In addition to dose variations, another strategy used to avoid decreased protection in people over 65 years of age is to administer a third booster dose. This third dose contains the same amount of antigen as that given to adolescents and adults, but it reminds the weakened immune system to produce more antibodies.
“High-dose” vaccines are also used, such as the recommended flu vaccine in this risk group (over 65 years). In it, the dose of the standard vaccine has been quadrupled to elicit a greater response from the aging immune system.
The reaction capacity of the immune system associated with each age indicates, therefore, the composition that vaccines must have in order to obtain the necessary protective response to prevent infection. This makes different studies necessary for each age group.
Cintia Muñoz Quiles, Doctor in Biology, Researcher in the Vaccine Research Area, Fisabio, Fisabio and Vallivana Rodrigo Casares, Researcher in Clinical Trials in the Vaccine Research Area, Fisabio
This article was originally published on The Conversation. Read the original.