When it was revealed at the beginning of the year that the causative agent of a mysterious disease in China is a new coronavirus, scientists’ concern about the growing epidemic in the country almost gave way to despair. As a rule, representatives of this family do not form strong immunity in those who have been ill: already six months or a year after recovery, the same infection can easily be picked up again.
However, the more we learn about the new virus, the more we are convinced that Sars-Cov-2 is very different from its predecessors. This means that there is hope that the immune protection in recovered patients will be much longer-term – perhaps even lifelong.
However, so far this is only an assumption. The main questions on which the strategy to combat the epidemic depends remain unanswered.
The BBC Russian service says that scientists have learned about immunity to Covid-19 in the six months since the beginning of the pandemic, and why most researchers of the virus and vaccine developers are rather optimistic about it.
What is immunity in general?
Immunity is the body’s ability to resist infection, that is, to protect against a particular disease by neutralizing its pathogen (virus, bacterium or fungus).
Immunity is divided into congenital (it allows us to fight diseases in general) and acquired, that is, it is formed after birth – as a result of a meeting with a specific infection (and protects only from it).
Having met for the first time with the causative agent of any disease, our body develops a strategy to combat it – and at the next meeting it immediately provides effective resistance. In fact, this means that our body can remember and, after some time, recognize the past illnesses.
Often, the immune memory – as scientists call this phenomenon – stores memories of events that have long been erased from ordinary memory.
For example, you yourself hardly remember how in early childhood you had chickenpox or were vaccinated with BCG, but your immunity remembers this – and does not allow you to get sick again, neutralizing particles of the chickenpox virus (or Koch’s bacillus) that got into your body.
How long does immunity last?
The duration of immune memory depends on the disease. Chickenpox, mumps, or measles most often leave behind lifelong immunity – but this is the exception rather than the rule.
Vaccinations that protect against hepatitis A or yellow fever are recommended to be repeated every 10 years.
Contrary to popular belief, seasonal flu also forms a stable immunity in a patient – albeit only to the same type (strain) of the virus that he has had.
The flu quickly mutates, so that within a few months a person can get infected again. However, in this case, the disease is often mild, since antibodies to the previous infection provide partial protection to the patient (more on this below).
What are antibodies and how do they work?
The body selects the key to any new infection by trial and error.
Imagine that each viral particle is a dagger with a bizarre blade, and your body is a scabbard factory. Each conveyor in this plant produces cases of the same shape, but together they provide an almost endless variety of sizes and configurations.
As soon as one of the cases approached the dagger in shape and the blade was hidden, all the free conveyors of the plant immediately begin to stamp the scabbard of the same shape on an industrial scale – and sooner or later all the daggers can be secured.
In this case, each conveyor is a specific blood cell (the so-called B-lymphocyte), and the scabbard is the protein molecules (antibodies) produced by this cell. When one of the molecules fits the shape of the “blade” (a spike on the surface of the coronavirus), it seems to cover it with itself, changing its shape – and thus prevents the virus from entering the cell.
But outside the cell, the virus cannot multiply: in order to stamp its own copies, it needs its building materials. Therefore, the growth in the number of viral particles slows down.
At the same time, the body receives a signal that some of the antibodies managed to neutralize the invader – and launches molecules of a similar shape “into mass production.” When the thorns on all viral particles can be blocked, the person recovers.
Will all patients have the same antibodies?
It is not necessary at all, because the same dagger can be put in different sheaths.
For example, the case cannot be much shorter than the blade or have a narrower opening than its thickness. But at the same time, the scabbard may well be slightly longer, and the hole – slightly wider than the blade itself.
In the blood of Covid-19 patients, scientists have found a total of more than 250 types of antibodies produced by the patients’ bodies in an attempt to stop the infection. 28 of them fought the disease (that is, neutralized the virus) most effectively.
What happens after you recover?
Unclaimed cases “go to the warehouse”: antibodies remain in the blood and continue to patrol the body in case of re-infection. That is why they can be found in recovered patients after weeks, months and even years (see above).
Some of the B-lymphocytes are converted into so-called memory cells. When they re-encounter an infection, they are activated and re-start the already-debugged pipeline, providing a quick immune response.
However, this does not mean that such a person is protected from re-infection. The strength of the immune response depends on many factors, first of all – on the severity of the illness and the time elapsed after recovery.
Typically, the more severe the disease progresses, the stronger the defense against re-infection will be. And the more time has passed since recovery, the less effective the immune response will be.
Is it true that people who have not been sick with Covid-19 can be immune to the virus?
In theory, this is possible. As mentioned above, antibodies to coronavirus (as well as to other diseases) can be of different forms. Some of them may be effective against more than one infection.
For example, in blood samples donated by healthy people long before the start of the epidemic (2015-2018), antibodies were found that can effectively neutralize the new Sars-Cov-2 coronavirus. Although they were originally produced to protect against some other disease.
The mere presence of such antibodies does not mean that people have effective immune defenses. However, this increases the likelihood that such people will have a milder infection of Covid-19.
However, the opposite is also possible. The fact is that the new virus itself is not too dangerous for the body – the main problems for the health of patients are created by their own immune system.
The most dangerous symptoms in seriously ill patients begin to appear just after the development of antibodies to Covid-19. And it is possible that the presence of similar antibodies to previous diseases in a person’s blood can only activate infection, increasing an aggressive immune response (the so-called cytokine storm) and complicating the course of the disease.
Only antibodies provide immunity from coronavirus?
The simple answer is no. In addition to antibodies, other agents are involved in the fight against infection, including the so-called T cells (or T lymphocytes), which are also involved in the formation of immune memory.
Moreover, judging by the latest data, T cells play no less, and maybe even a greater role in the immune defense against Covid-19 than antibodies to coronavirus, since the lifespan of T-lymphocytes is much longer.
As in the case of antibodies, which can provide partial protection against a new virus, despite the fact that they remain in the body after fighting another infection, T cells can exhibit similar properties and effectively neutralize the Covid-19 pathogen even in people who have not been met with him.
This may be one of the explanations for why some patients experience the disease in such a severe form, while in others, Covid-19 may not cause any symptoms at all: some of us may be partially protected by the immunity leftover from previous diseases.
The fact is that the common cold is caused by four different viruses belonging to the same “crowned” family (coronaviridae). So in the body of many people who have had a cold over the past couple of years, it is highly likely that there are either antibodies to one of these coronaviruses, or T cells that remember a previous infection.
This phenomenon is called cross-immunity and, according to some reports, almost every third person has such nonspecific T cells. And together with “cross” antibodies, up to half of the world’s population can have partial protection against Covid-19.
What is “trained immunity”?
As mentioned above, hereditary immunity is not specific, that is, it is not directed against any particular infection. However, more and more research indicates that he, too, has the ability to remember past illnesses – albeit not for so long.
Macrophages, monocytes and natural killer cells are more effective in neutralizing viruses and bacteria with which they have already met before, and can even protect against other pathogens completely new to the body.
In particular, according to some studies, vaccination against tuberculosis (BCG) can provide the body with partial protection against yellow fever and some other viruses.
People with trained immunity are less susceptible to infection, and if they become infected, they are more likely to survive the disease in a mild form and recover faster.
So is it possible to get infected with Covid-19 again?
So far, no reliable case of re-infection with coronavirus is known. Scientists do not exclude that this is possible, but – in the context of all of the above – the likelihood of such an infection is not too high.
As in the case of many other viral diseases, their concentration is higher in those who have had the virus in a severe form, but in any case it drops by about half after three months , and sometimes after a few weeks .
Since Sars-Cov-2 was first discovered only seven months ago, scientists have so far refrained from predicting how long acquired immunity lasts and whether re-infection is possible in principle. However, some conclusions can be drawn from its more studied predecessors and close relatives – the SARS (atypical pneumonia) and MERS (Middle East respiratory syndrome) viruses.
In the blood of patients who recovered from SARS in 2002-2003, specific antibodies to the virus persisted for an average of two years (to be exact, from a year to three ). Although in people who had severe SARS, they were found even 12 years later.
Patients with Middle East respiratory syndrome (an average of every third infected die from it) retained specific antibodies to the MERS virus for at least 34 months .
And even if scientists do not yet know how long the immune defense against Covid-19 will last and how effective it will turn out to be, this very fact is already encouraging.
The longer they don’t know the answer to this question, the more likely it is that reinfection is impossible.