Some immunizers only need one dose to provide lifelong protection. Why might covid vaccines need four or more?
Many countries are considering whether to offer more booster doses of the vaccine that protects against Covid-19 to anticipate the risk of new waves of infections.
The World Health Organization (WHO) has recommended that, for the time being, doses be prioritized for the most vulnerable people.
In Brazil, the Ministry of Health already recommends three doses for the entire population aged 12 to 49 years. For those over 50 or who have an immunity problem, a fourth dose is indicated.
But why does this immunizer seem to need repeated doses, when protection from other vaccines can last a lifetime?
How often you should be vaccinated depends in part on how quickly the virus or bacteria you are fighting changes and mutates.
For example, we all need to get doses of the measles vaccine in childhood, which should protect us against this pathogen for life.
The measles virus doesn’t change much. So once the body has seen it for what it is, it can continue to recognize it for decades. After all, he will continue to be more or less the same.
Influenza viruses, on the other hand, evolve very quickly.
A vaccine given this year will train your immune system to recognize three or four strains that are currently in circulation.
But next winter, the infectious agent will mutate so much and become so different that your body can no longer recognize it well.
Therefore, the flu vaccine is offered every year to those who need it most, such as the elderly, children and pregnant women.
Both laboratory studies and infection rates suggest that the virus that causes Covid-19 has mutated enough to escape some of the protection provided by the first round of vaccines, which began to be applied in 2021.
However, available immunizations remain about 90% effective against hospitalization after a third dose is applied. That rate drops to around 75% after three months, according to the UK Health Safety Agency.
The National Institute of Communicable Diseases in South Africa says that “booster vaccination increases antibody levels”.
The ability to ‘remember’ threats
There is evidence that our body’s ability to block the coronavirus decreases relatively quickly after vaccination or infection.
But the ability to avoid more serious illnesses lasts longer. The problem is to determine exactly how long this protection will remain valid, a topic that is still being studied by experts.
Even if a pathogen hasn’t changed much, the immune system’s memory can fade as antibodies and other forms of protection begin to wear off.
And defense cells seem to remember some infections better than others, for reasons not yet fully understood.
Part of this probably has to do with the different types of immunity we develop, according to microbiologist Simon Clarke of the University of Reading in the UK.
Antibodies produced by the immune system to some viruses after an infection or vaccination disappear relatively quickly. But this process often leaves T cells behind, which offer slower, longer-lasting protection. These defense units won’t stop you from catching the infection, but they can keep you from getting very sick and needing to be hospitalized or at risk of death.
Where in the body these immune responses occur also plays a role, says Clarke.
The virus that causes covid-19 affects the nose and respiratory tract. While there are immune responses that take place in this part of the body, most of the antibodies produced after vaccination are found in the blood.
So you can still get the infection, but antibodies and other immune strategies will prevent it from ‘deepening’ in your body, protecting you against the more serious complications of the disease.
the virus is new
Another thing to keep in mind is how often you are exposed to an infection.
You may never come into contact with tetanus, which means the vaccine is your body’s only chance to learn what the disease-causing bacteria look like and the best way to fight it.
After a few years, this immune memory tends to disappear.
On the other hand, a common respiratory pathogen called respiratory syncytial virus (RSV), which can make children very sick, is often extremely mild or asymptomatic in adults.
You’ve probably been exposed to it so many times that your immune system becomes very efficient at fighting it.
Before the end of 2019, no one had come into contact with the Covid-causing coronavirus and therefore there was no immunity against it.
More than two years after the emergence of this infectious agent, the data show that people had a lot of contact with it. According to research done in Brazil, Sweden and the United Kingdom, the combination of vaccines and infection provides strong protection.
However, some scientists have raised concerns that this will lead to more people developing the so-called long-term covid, with symptoms that linger for months (or even years).
Will we still need reinforcements?
WHO officials said in January that “repeated booster doses of the original vaccine composition are unlikely to be appropriate or sustainable.”
Many higher-income countries have offered everyone a third dose of the vaccine.
However, when it comes to a fourth dose, most booster strategies have so far been targeted at groups most vulnerable to Covid complications.