Antibodies (Y shape) attack the coronavirus (computer model). When it became clear earlier this year that the mysterious disease in China was caused by a new coronavirus, scientists’ concern about the rapidly spreading epidemic in the country almost turned to despair. Members of this family usually do not develop strong immunity in those who have recovered: the same infection can be easily contracted again just six months to a year after recovery. However, the more we learn about the new virus, the more we are convinced that Sars-Cov-2 is very different from its predecessors. And so there is hope that the immune protection in recovered patients will be much longer lasting – possibly even lifelong (although the latter is unlikely). For now, however, these are only assumptions. There are still many unanswered questions that will determine the strategy to fight the epidemic.
The Russian service of the BBC reports that scientists have learned about immunity to Covid-19 in less than a year since the beginning of the pandemic and why the majority of virus researchers and vaccine developers are quite optimistic. Immunity is the body’s ability to resist infection, i.e., to protect against a specific disease by neutralizing its causative agent (virus, bacteria, or fungus). Immunity is divided into innate (which allows us to fight diseases in general) and acquired, which is formed after birth – as a result of encountering a specific infection (and protects only against it). The first time our body encounters the pathogen of any disease, it develops a strategy to fight it – and the next time it encounters it, it immediately develops effective resistance.
In fact, this means that after some time, our body is able to memorize and recognize previously experienced illnesses. Often immune memory, as scientists call this phenomenon, preserves memories of events that have long since been erased from normal memory. For example, you probably don’t remember having chickenpox or getting the BCG vaccine as a child, but your immune system remembers – and prevents you from getting sick again by neutralizing the varicella-zoster virus (or Koch’s bacillus) particles that enter your body.
Coronavirus: six months that changed the world. The duration of immune memory depends on the disease. Chickenpox, mumps, or measles often leave lifelong immunity, but this is the exception rather than the rule. Vaccinations against hepatitis A or yellow fever are recommended to be repeated every 10 years. Contrary to popular belief, seasonal flu does create a lasting immunity in a patient – but only to the specific strain of the virus from which they recovered. Flu mutates quickly, so a person can become infected again within a few months – but with a new version of the virus. In such cases, however, the illness is often mild because the patient is partially protected by antibodies from the previous infection (more details below).
The body selects the key to each new infection by trial and error. Imagine that each viral particle is a dagger with a blade of intricate shape, and your body is a factory for producing sheaths. Each conveyor in this factory produces sheaths of the same shape, but together they provide an almost infinite variety of sizes and configurations.
Testing people for antibodies is an important part of the fight against the pandemic. We explain quickly, simply, and clearly what happened, why it matters, and what happens next. эпизоды – Episodes End of story Podcast advertising As soon as one of the sheaths fits the dagger and the blade has been successfully concealed, all free conveyors in the factory immediately start mass producing sheaths of the same shape – and sooner or later all daggers are successfully secured. In this case, each conveyor is a specific type of blood cell (called a B lymphocyte), and the sheaths are protein molecules (antibodies) produced by that cell. When one of the molecules matches the shape of the “blade” (the spike on the surface of the coronavirus), it covers it, changing its shape and preventing the virus from entering the cell. But outside the cell, the virus cannot replicate: it needs its building blocks to make its own copies. Therefore, the growth rate of the virus particles slows down. At the same time, the body receives a signal that one of the antibodies has succeeded in neutralizing the invader – and initiates “mass production” of molecules of a similar shape. If it is possible to block the spikes on all the viral particles, a person will recover.
The antibody attack on the microscopic level against the virus looks something like this. This is not necessary, as the same dagger can be placed in different sheaths. Let’s say the scabbard can’t be significantly shorter than the blade, or have an opening narrower than its thickness. However, the scabbard can be slightly longer and the opening can be slightly wider than the blade itself. In the blood of Covid-19 patients, scientists discovered a total of more than 250 types of antibodies that the patient’s body produced in an attempt to stop the infection. Of these, 28 fought the disease most effectively (i.e., neutralized the virus). Unused cases “go to storage”: antibodies remain in the blood and continue to patrol the body in case of reinfection. This is why they can be found in recovered patients weeks, months, and even years later (see above). Some B lymphocytes transform into memory cells. Upon repeated encounter with an infection, they become activated and restart the already tuned conveyor, ensuring a rapid immune response. However, this does not mean that such a person is protected from reinfection. The strength of the immune response depends on many factors, primarily the severity of the illness and the time since recovery. Usually, the more severe the disease, the stronger the protection against reinfection. And the longer it has been since recovery, the less effective the immune response will be.
Applications against Covid-19: How carriers of the virus are tracked in Russia, China and Europe Theoretically, this is possible. As mentioned above, antibodies against coronaviruses (as well as other diseases) can come in different forms. Some of them may prove to be effective against not only one but several infections. Let’s say that in blood samples taken from healthy people long before the start of the epidemic (2015-2018), antibodies were found that could effectively neutralize the new coronavirus SARS-CoV-2. Even though they were originally produced to protect against another disease. The mere presence of such antibodies does not mean that people have effective immune protection. However, it does increase the likelihood that these individuals will have a milder form of Covid-19 infection. But the opposite is also possible. The thing is that the new virus itself is not very dangerous for the body – the main problems for the patient’s health are created by their own immune system. The most dangerous symptoms in seriously ill patients begin to manifest themselves after the production of antibodies against Covid-19. And it cannot be ruled out that the presence of similar antibodies in a person’s blood from previous diseases can only activate the infection, intensify the aggressive immune response (the so-called cytokine storm) and complicate the course of the disease.
The simple answer is no. In addition to antibodies, other agents are involved in the fight against infection, including T cells (or T lymphocytes), which also contribute to the formation of immune memory. In addition, recent data suggest that T cells play no less (and perhaps even more) of a role in immune defense against Covid-19 than antibodies to the coronavirus, since the lifespan of T lymphocytes is significantly longer. Like antibodies, which can provide partial protection against the new virus by lingering in the body after fighting another infection, T cells can have similar properties and effectively neutralize the Covid-19 pathogen even in individuals who have not previously encountered it. This could be one of the explanations why some patients experience such a severe form of the disease, while others have no symptoms of Covid-19 at all: some of us may be partially protected due to immunity from previously contracted diseases.
The thing is, the common cold is caused by four different viruses that belong to the same family called coronaviridae. So many people who have had a cold in the past few years are likely to have either antibodies to one of these coronaviruses or T-cells that remember the previous infection. This phenomenon is called cross-immunity, and some data suggest that such non-specific T cells are present in almost one in three people. Together with “cross-reactive” antibodies, up to half of the world’s population may have partial protection against Covid-19.
In addition to T lymphocytes and antibodies, there is another protective mechanism that scientists have been talking about more and more in recent years – so-called trained innate immunity. As mentioned above, inherited immunity is not specific, meaning it is not directed against a particular infection. However, a growing number of studies indicate that it also has the ability to remember past illnesses, although not for long. Macrophages, monocytes, and natural killer cells are more effective at neutralizing viruses and bacteria they have encountered before, and can even protect against other pathogens that are new to the body. In particular, according to some studies, the tuberculosis vaccine (BCG) may provide partial protection against yellow fever and some other viruses. People with trained immunity are less susceptible to infection, and if they do get infected, they are more likely to experience a mild form of illness and recover more quickly.
Myths about Covid-19 treatment Back in August, when the first version of this article was published, there was no information about any cases of reinfection – and it was hoped that immunity to Covid-19 would last a lifetime in recovered patients. Soon, however, recurrent cases began to appear. At the time of this writing, scientists know with certainty of about 30 cases of reinfection – that is, when several months after recovery confirmed by tests, the patient again tested positive for coronavirus, but with a different genetic variant. However, it should be noted that it is practically impossible to accurately calculate such cases, and the number of patients who cannot exclude reinfection has already exceeded 1500 (although even this is only 0.002% of the total number of recovered patients). After recovery from Covid-19, specific antibodies are found in the blood of most patients, even those who had asymptomatic cases. As with many other viral diseases, their concentration is higher in those who had a severe form of the virus, but it still declines by about half within three months, and sometimes even within a few weeks. Since Sars-Cov-2 was first detected less than a year ago, scientists do not have a precise answer to the question of how long acquired immunity lasts. According to a recent study conducted in Oxford, the average duration of immune protection is at least six months. The good news is that although the virus continues to mutate, it does so relatively slowly: on average, Sars-CoV-2 accumulates about two mutations per month. So scientists assure us that the vaccines being developed should protect against all existing mutations of the virus – and most likely against those that may emerge in the near future.