Covid: how long immunity lasts after infection

Gone are the days when scientists thought catching Covid was a once-in-a-lifetime experience each of us would have.

With the spread of new variants, especially the omicron-derived sublines such as BA.2 and BA.5, cases of reinfection have become much more common—in some individuals, the second episode of the disease occurs within a very short interval, a few weeks after the first frame.

And, although the immunity window can vary from a few weeks to even a few years from person to person, the latest scientific evidence allows us to understand a little better how our defense cells work, how long this protection usually lasts and what are the factors that facilitate contact with the coronavirus over and over again.

coordinated counterattack

But first of all, how does our immune system work during a viral infection?

It all starts when a virus invades the body and starts using our own cells to create new copies of itself.

At one time or another, this abnormal process draws the attention of defense units, which begin a counterattack to contain the pathogen’s expansion.

This work involves a veritable battalion of cells, of which it is possible to highlight two among the most important: T and B lymphocytes.

T lymphocytes are responsible for coordinating the immune response. They identify infected cells and kill them.

B lymphocytes, on the other hand, are responsible for producing specific antibodies, a kind of “personalized antidote” that sticks and inactivates the viruses.

“It is as if the T lymphocyte fires a missile that destroys the diseased structure. Hence, the remaining viruses are neutralized by the B lymphocyte antibodies”, summarizes Antonio Condino Neto, a senior professor of immunology at the Institute of Biomedical Sciences at USP (University of Sao Paulo).

If all this work is successful, eventually the infection is controlled and the viruses are completely eliminated from the body.

This, in turn, generates a type of learning for the immune system. For a while, the circulating defense cells know what to do if the virus in question decides to try a new invasion.

A similar protective mechanism happens during vaccination — with the advantage that the immune units are “trained” without the body suffering from the action of a real pathogen.

But then comes an important question: how long does this immunity last?

The answer to this question varies considerably depending on the virus and the characteristics of each one.

“On the one hand, there are diseases such as measles or rubella, which we usually only have at most once in a lifetime and that’s it”, says Condino Neto.

“On the other hand, we have flu, Covid and colds, which we can catch several times”, he compares.

But what differentiates one group from another?

A very effective dribble

There are several reasons that help to understand why in some cases immunity lasts for many years (or even forever) and in others, it goes away quickly.

One of the main factors has to do with the characteristics of the virus and the interaction it has with our organism.

Let’s start with the portion of these pathogens that is stable and stays pretty much the same over decades or centuries.

This feature represents good news for the immune system, which is able to recognize the infectious agent and retrieve instructions on how to fight it, thanks to previous infection or vaccination.

Now, imagine the opposite scenario, what happens when viruses circulate very quickly and are a true walking metamorphosis?

This is the case with Sars-CoV-2, the coronavirus responsible for the current pandemic: it undergoes genetic mutations all the time as it is transmitted from person to person,

If these changes bring advantages to the pathogen — such as an easier way to infect cells or the ability to evade the immune response, for example — they will thrive.

This is how worry variants arise. These new versions of the virus are gaining ground and are behind reissues in the waves of cases, hospitalizations and deaths.

Over the past two and a half years, we’ve seen this phenomenon happen at least five times, with the arrival of alpha, beta, gamma, delta, and omicron variants.

More recently, the appearance of omicron-derived subvariants, such as BA.2 and BA.5, has accelerated and deepened this process even further.

In short, all these strains carry changes in genes that appeared in the “original” virus, first detected in January 2020 in Wuhan, China.

From the point of view of our defenses, this fact represents very bad news. This is because the immune response obtained through a previous infection or vaccination becomes increasingly outdated.

With the passage of time — and the emergence of new variants with more diverse genetic mutations — the result of the work of B lymphocytes becomes less and less effective.

That’s because the antibodies they make are specifically assembled to neutralize the cause of the first infection — or, preferably, are aligned with the original vaccine formulation, which carries instructions to fight older versions of the virus.

In other words, if a variant that tries to invade the body presents significant changes in structure, those antibodies can no longer act as expected.

What is happening now?

This viral enhancement process appears to be far from over: since late 2021 and early 2022, several subvariants of the omicron have been detected.

Lines such as BA.2 and BA.5 have an even greater ability to infect our cells and escape previous immunity.

And that, in turn, makes reinfection frames increasingly frequent and with short windows compared to the first Covid episode — after all, there is an incompatibility between the virus that our immune system recognizes and the versions of it that are currently circulating.

“The virus finds a way to pierce our defenses”, observes Condino Neto.

Research by the Danish Institute of Serology, which has not been published in any specialized journal, became one of the first to draw attention to the fact that a reinfection with the BA.2 variant could happen shortly after a person has been affected by BA.1 (the “original” omicron).

In the article, the researchers describe rare cases of people who have had a second case of Covid from BA.2 about 20 days after testing positive for the first time with BA.1.

Let it be clear: from what is known so far, cases like these, in which Covid recurs in a few days, are atypical and the tendency is for immunity to last at least a few months.

Research carried out at Yale and Temple universities, both in the United States, and published in December 2021 in The Lancet estimated that, in an endemic situation, reinfection can occur in a time window that varies from three months to up to five years.

It is worth noting that the study was carried out before the spread of the omicron and its subvariants, which may have interfered with this window of immunity.

“In addition, in situations where the person has the same disease twice in less than 45 days, we need to assess whether the virus was not ‘hidden’ for a while and the infection was reactivated later”, differentiates immunologist Luiz Vicente Rizzo, director -research superintendent at Hospital Israelita Albert Einstein, in São Paulo.

“We have evidence that the coronavirus is able to hide in the central nervous system and testicles for a period, places where the action of the immune system is limited”, he adds.

While science is still trying to put all the pieces of this puzzle together, there is greater consensus that the coronavirus is always modifying itself in ways that go unnoticed by the immune system.

A study released in June 2022 by Imperial College London, United Kingdom, revealed that people infected with the omicron have a weaker immune response and little ability to prevent new cases of Covid, even among those who had already caught Covid in the first place. past and were vaccinated with three doses.

Experts have even classified the subvariants that are in circulation as “stealthy”, due to their ability to act hidden from defense cells.

Research has shown that these new viral versions do not leave a “mark” on the defense cells, so they remember very well how to fight the virus from then on.

“Being infected with the omicron does not represent a potent boost to immunity against future reinfections,” explains Professor Rosemary Boyton, lead author of the work in a press release.

Great news with an expiration date?

Despite so many changes in the relationship between the coronavirus and our immunity, at least one thing continues to work relatively well most of the time: the protection of vaccines against Covid-related complications, hospitalizations and deaths.

This protection happens through the activation of the immune system’s memory and the action of cells such as T lymphocytes.

In practical terms, the coronavirus even manages to enter the body and evade antibodies at a first stage.

But soon comes the battalion of immune cells commanded by T lymphocytes, which control the situation and prevent the virus from gaining ground and causing more serious symptoms.

“The virus even enters and causes discomfort, but, thanks to the immune memory, it soon takes ‘a few inverts’ and leaves”, says Condino Neto.

That’s exactly what we’ve witnessed over the past few months: even though the omicron is behind record Covid cases, hospitalizations and deaths have not risen at the same rate.

And even in those people who are testing positive for the second or even the third time, the tendency is for the condition to be much milder, marked by symptoms that usually resemble a common cold, such as a runny nose, cough and sore throat.

“Therefore, it is very important that people are up to date with their vaccination schedule. Those who are late in their doses should rush to take theirs”, suggests Condino Neto.

According to the CoronavirusBra1 portal, only 51% of the Brazilian population took the third dose of vaccine, which is considered essential to expand protection against the most serious forms of Covid.

The USP researcher reinforces that the immunizer helps to “wake up” the immune system and “increases the chances that, if you catch Covid again, you will have a milder form of the disease”.

Rizzo adds that, in addition to being up-to-date with vaccinations, we cannot “let our guard down” with other preventive measures.

“The coronavirus didn’t come from another planet. We just have to wear a mask and take some precautions that it doesn’t pass”, he explains.

“We don’t have to wait for the government to say we need to take care of ourselves. Everyone should think about their own responsibility in this story.”

The immunologist warns that, given the recent history of mutations and variants, the hypothesis that an aggressive version of the virus may appear, which manages to completely escape the protection provided by the available immunizers, is not ruled out.

“Every time someone gets infected, we enter a kind of lottery. Will there be a coronavirus full of mutations that escapes the vaccines?”, he asks.

According to the information panel of the National Council of Health Secretaries (Conass), the daily moving average of new cases of Covid-19 in Brazil exceeds 57 thousand.

“Our current behavior is only encouraging this scenario to become reality”, he laments.

Boyton, from Imperial College, agrees with his Brazilian colleague’s point of view. “The concern is that the omicron could potentially undergo further mutations and become an even more pathogenic variant, or capable of overcoming vaccine protection.”

“In this scenario, people who previously caught this variant would be less protected against the micron, depending on their immunological profile”, he concludes.