Some diseases only need to affect us once, such as measles or mumps, for us to be protected for life. On the other hand, some viruses like the flu force us to get vaccinated every year. So why do we develop lifelong immunity to some diseases, but not others?
To defend ourselves from external viral or bacterial attacks, our body develops antibodies. Basically, these little proteins deployed by our immune system wrap themselves around invading cells to prevent them from hijacking our own cells, and replicating.
Once the threat is wiped away, our antibody levels then begin to drop. Still, some of these proteins "stay put" in case a similar attack happens again. If so, these little soldiers will then initiate the production of new antibodies to attack the invading cells.
In other words, our body keeps a "memory" of past attacks in order to better counter-attack afterwards in the event of a new threat. On paper, then, this is what prevents us from getting sick a second time. This is also why an antibody test can tell us if we have been infected in the past.
Despite everything, sometimes we are still "affected" by the same disease several times. Why ? Two reasons can explain this:either the pathogen has mutated and our immune system no longer recognizes it, or our body tends to develop a weaker immune response.
Let's take the example of the flu. These viruses have a tendency to change their genes. In other words, the one who attacks us one year is generally not the same as the one who attacks us the following year. Consequence:our antibodies do not recognize the enemy and eventually let it take hold.
However, not all viruses mutate so easily. This is why immunity tends to last much longer.
A study published in 2017 in the New England Journal of Medicine pointed out, for example, that it would take more than two hundred years for even half of our antibodies to disappear after a measles or mumps infection. This same study found that it takes about fifty years to lose our antibodies against chickenpox, and about eleven years to lose half of our antibodies against tetanus.
That is why, without a booster injection, you could theoretically be infected with these last two diseases as an adult .
We still don't know why our bodies maintain a response immune system longer for some diseases compared to others. It's possible that some, such as chicken pox, re-infect us more frequently than we think, but that our previously deployed antibodies kill these new attacks "in the bud" before we notice them.
“In these cases, the immune system would be at full capacity again and again due to reinfections. It keeps our immunity vigilant “, says Marc Jenkins, an immunologist at the University of Minnesota Medical School. “In contrast, with tetanus, we are probably very rarely exposed ". This could therefore explain the shorter duration of immunity against this disease.
However, these three examples testify to the differences in the mutation capacities of viruses. However, as said above, the weak response of our immune system in the first place can also explain the fact that we can be affected several times by the same disease.
Rhinopharyngitis (or the common cold), for example, usually only infects our upper respiratory tract . And for our immune system, it's not really a problem. "Our bodies don't care about the upper airways “, underlines Mark Slifka, immunologist at the National Center for Primate Research in Oregon.
In other words, in the case of colds, they reinfect us not because the viruses responsible for them mutate rapidly, but because our bodies generally do not produce many antibodies against these pathogens in the first place.
Whether our immunity is acquired through a first infection or not a vaccine, will it last as long as our immunity to chicken pox or will we have need a new vaccine every year, like for the flu?
Unfortunately, it's hard to say right now. On the one hand, we currently have no vaccine available against this disease. And on the other hand, we still lack perspective on the production of antibodies against this disease.
Scientists from the Institute of Pathophysiology and Allergy Research at MedUni (Vienna) have nevertheless found that 60% of patients with Covid-19 seem to develop protective antibodies . On the other hand, an American study revealed a few days ago that people with mild forms of the disease saw their antibodies drop sharply during the three months following infection.
