When it became clear earlier this year that the causative agent of the mysterious disease in China was a new coronavirus, scientists’ anxiety about the epidemic was almost replaced by despair. Usually, members of this family of viruses do not form a strong immunity: in six months to a year after recovery, the same infection can easily be picked up a second time.
However, the more we learn about the new virus, the more convinced we are that Sars-Cov-2 is very different from its predecessors. This means that there is hope that the immune defenses of recovered patients will last longer – perhaps even for life.
However, so far this is only an assumption. The key questions on which the strategy for combating the epidemic depends remain unanswered.
The BBC says scientists have learned about immunity to Covid-19 in the six months since the pandemic began, and why most virus researchers and vaccine developers are quite optimistic about it.
What is immunity?
Immunity is the body’s ability to resist infection, ie to protect against a disease by neutralizing its causative agent (virus, bacterium or fungus).
Immunity can be innate (it allows us to fight disease in general) and acquired, ie formed after birth – as a result of meeting with a specific infection (and protects only from it).
When we first encounter the causative agent of any disease, our body forms a strategy to combat it – and during the next meeting immediately puts up effective resistance. In fact, it means that our body can remember and recognize the diseases over time.
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 can hardly remember how in early childhood you had chickenpox or BCG vaccinations, but your immune system remembers it – and does not get sick a second time, neutralizing the particles of chickenpox virus (or Koch’s disease), which got into your body.
How “live” immunity?
The duration of immune memory depends on the disease. Chickenpox, mumps, or measles often leave a lifelong immunity – but this is the exception rather than the rule.
Vaccinations to 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 the patient – however, only to the type (strain) of the virus with which he became ill.
The flu mutates quickly, so in a few months a person can become infected again. However, this time the disease is often mild, as partial protection of the patient is provided by antibodies to the previous infection.
What are antibodies and how do they work?
The key to any new infection is selected by the body through trial and error.
Imagine that each viral particle is a dagger with a whimsical blade, and your body is a plant for the production of vaginas. Each conveyor in this plant produces cases of the same shape, but together they provide an almost infinite variety of sizes and configurations.
As soon as one of the cases fits the dagger in shape and the blade is hidden, all the free conveyors of the plant immediately begin to stamp the scabbards 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 vagina is a protein molecule (antibody) produced by this cell. When one of the molecules approaches the shape of the “blade” (a spike on the surface of the coronavirus), it covers it with itself, changing its shape – and prevents the virus from entering the cell.
But outside the cell, the virus cannot reproduce: to stamp its own copies, it needs its building materials. Therefore, the growth of the number of viral particles slows down.
At the same time, the body receives a signal that one of the antibodies has managed to neutralize the invader – and launches molecules of a similar shape “into mass production.” When the spikes on all viral particles can be blocked, the person recovers.
Will the antibodies of all patients be the same?
Not necessarily, because the same dagger can be inserted into different vaginas.
For example, the case cannot be much shorter than the blade or have a hole that is too narrow. But the vagina may well be a little longer, and the hole – a little wider than the blade.
In the blood of patients with Covid-19 scientists found more than 250 types of antibodies that the body has produced to stop the infection. 28 of them fought the disease (ie neutralized the virus) most effectively.
What happens after recovery?
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 patients who have recovered in weeks, months and even years.
Some B-lymphocytes are transformed into so-called memory cells. During the re-encounter with the infection, they become active and restart the already established conveyor belt, providing a rapid 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 disease and the time that has elapsed since recovery.
Usually, the more severe the disease, the stronger the protection against re-infection. And the more time that has passed since recovery, the less effective the immune response will be.
Is it true that people who have not had Covid-19 may be immune to the virus?
Theoretically, yes. Antibodies to coronavirus (as well as to other diseases) can be of various forms. Some of them can be effective in fighting not one but several infections.
For example, in blood samples taken from healthy people long before the epidemic (2015-2018), antibodies were found that can effectively neutralize the new coronavirus Sars-Cov-2. Although they were intended to protect against some other disease.
The presence of such antibodies does not mean that people have effective immune defenses. However, this increases the likelihood that Covid-19 infection in such people will be milder.
However, the opposite is possible. The fact is that the new virus itself is not too dangerous for the body – the main health problems for patients are created by their own immune system.
The most dangerous symptoms in critically ill patients begin to appear just after the appearance of antibodies to Covid-19. And it is possible that the presence in the human blood of such antibodies to previously transmitted diseases can only intensify the infection, increasing the aggressive immune response (so-called cytokine storm) and complicating the course of the disease.
Is only antibodies immune to coronavirus?
The simple answer is no. In addition to antibodies, other agents are involved in the fight against infection, including so-called T cells (or T lymphocytes), which are also involved in the formation of immune memory.
In addition, as evidenced recent research, T-cells play no less, and perhaps more, role in immune protection against Covid-19 than antibodies to coronavirus, because the lifespan of T-lymphocytes is much longer.
As with antibodies that may provide partial protection against the new virus, despite remaining in the body after fighting another infection, T cells may have similar properties and quite effective to neutralize the pathogen Covid-19, even in people who have not encountered it.
This can be one of explanations why some patients are very ill, while others may not have any symptoms at all with Covid-19: some of us may be partially protected by the immunity left over from previous illnesses.
The fact is that the common cold is caused by four different viruses of the same family (coronaviridae). Therefore, many people who have caught a cold in the last few years are more likely to have 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 data, such nonspecific T cells are almost present every third. And along with “cross” antibodies, about half of the world’s population can have partial protection against Covid-19.
What is “trained immunity”?
In addition to T-lymphocytes and antibodies, there is another protective mechanism, which in recent years is increasingly common scientists say, – the so-called trained innate immunity.
Hereditary immunity is not specific, ie it is not directed against any specific infection. However, more and more research indicate that he also has the ability to remember illnesses – albeit not for long.
Macrophages, monocytes, and natural killers are more effective at neutralizing viruses and bacteria they have encountered before, as well as can protect from other, completely new to the body pathogens.
In particular, according to some studies, vaccination against tuberculosis (BCG) can partially protect the body from yellow fever and some other viruses.
People with trained immunity are less prone to infection, get sick in a mild form and recover faster.
So can Covid-19 be re-infected?
Studies show that after recovery from Covid-19 in the blood of most patients there are specific antibodies – even in those who became ill asymptomatically.
Since Sars-Cov-2 was first recorded only seven months ago, scientists are still refraining from predicting how long the acquired immunity lasts and whether re-infection is possible in principle. However, some conclusions can be drawn on the basis of its predecessors and close relatives – viruses SARS (SARS) and MERS (Middle Eastern Respiratory Syndrome).
In the blood of patients who relapsed into SARS in 2002-2003, specific antibodies to the virus persisted for an average of two years (to be precise, from one to three). Although people who have suffered from pneumonia in a particularly severe form, their found even 12 years later.
Patients who relapsed with Middle Eastern respiratory syndrome (from which an average of one in three died) retained specific antibodies to the MERS virus for at least 34 months.
And while scientists do not yet know how long the immune defense against Covid-19 will last and how effective it will be, this fact itself already inspires optimism.
The longer they do not have an answer to this question, the more likely it is that re-infection is impossible.