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The blood of the ideal person for the test was safely stored in a biological database., long before Maike Hofmann knew it. The molecular medicine specialist at the Freiburg University Hospital is investigating the immune memory of former Covid 19 patients together with other scientists. Their latest study, which was published recently in the journal Nature Medicine, shows that even those who only get mildly ill form an effective immune defense for at least months. The best test is from the unknown test person from the biobank.
The man or the woman, for data protection reasons, the researchers can not reveal the smallest detail that suggests the identity of the test person, had accidentally given blood samples before becoming infected with the coronavirus. A stroke of luck for science, because the sample preserved the immune system status of Sars-CoV-2. If the researchers found other immune cells in the test subject’s blood after a corona infection, they were most likely due to the corona virus. And so it happened.
Seven days after the subject had the first symptoms, the Freiburg researchers detected so-called memory T cells. These killer cells of the immune system specialize in the coronavirus. They recognize which cell is infected with the pathogen and kill it. Furthermore, T cells obviously form a permanent defense; they were still detectable more than three months after infection.
Based on analysis of T cells in connection with other viral diseases, the research team has no reason to assume that the value of these cells will suddenly drop drastically after a short time. “We know from other infectious diseases that such cells play a crucial role in preventing new infections with the same pathogen,” Hofmann says by phone. “Therefore, we are confident that this is also the case for Covid-19.” And that’s very likely with the vast majority of patients, even if they only had mild symptoms. The researchers found the memory cells not just in one test person, but in nearly 90 percent of the 26 test people they examined. They were all slightly ill.
“It looks promising.”
Molecular Medicine Specialist Maike Hofmann
How long exactly the immunity will last, no one can say with absolute certainty, the coronavirus has not been around long enough for that. “But so far we haven’t seen anything in the T cells we examined that should cause us concern,” says Hofmann. At the moment there is nothing to stop immunity from lasting for years.
On the other hand, it has been shown in several studies, including those from Germany, that antibodies are significantly more volatile than memory T cells. In several cases, specific antibodies to coronavirus could not be detected in test subjects just a few months after infection. This was also the case with the ideal test person from Freiburg. Antibody tests remained negative 79 days after onset of symptoms. For participants in previous studies, the tests didn’t even work, even though they had been shown to be infected with the coronavirus.
What does falling antibody levels mean?
Was the immunity short-lived? And what would that mean for a vaccine? Do you have to get vaccinated against the virus every few months or every year, similar to the flu?
“The fact that the amount of antibodies decreases after infection does not mean that it is not immune,” explains Hofmann. Covid-19 is by no means the only disease in which the amount of antibodies decreases so much after a short time that they can no longer be detected. “But that doesn’t mean that the immune response has completely disappeared,” says Hofmann.
The coronavirus normally attaches itself to the cells of the body through proteins in the outer shell. They give the virus its spiny appearance. These bumps are what we mean when we talk about spiny protein or spiny protein. Once penetrated, the pathogen causes the cell to make countless copies of the virus. The cell dies, the viruses break free and seek a new host. However, the antibodies block the spike protein and the virus can no longer penetrate the cell.
Once the infection has finished, you have done your main task for now. It is understandable that their numbers later decrease. Who would keep a highly specialized battalion on the front lines all the time when the battle is over?
Healthy this time
Since the enemy, in this case the virus, could reappear at any moment, so-called memory cells continue to patrol the body, including the T cells that Hofmann is investigating. If memory cells recognize the virus again, they attack themselves or become stronger. In the best of cases, the virus is stopped before it spreads. The human being does not notice anything of the battle in his body and this time he remains healthy.
In addition to T cells, so-called B cells also play an important role in long-term immunity. They have a kind of mugshot of the coronavirus that they match every intruder in the body they find. If they recognize the virus, they again produce personalized antibodies that turn off the pathogen. According to the results of the first study by American researchers, B cells can be detected in the blood of former Covid 19 patients for months. Unlike Freiburg’s work, the study by American researchers has not yet been published in a specialized journal and has not been reviewed by independent researchers. First, the New York Times reported on the results.
The research team from the La Jolla Institute of Immunology in the US state of California had repeatedly tracked the defenses in the blood of 38 former Covid 19 patients over a period of months, which the body had accumulated after becoming infected with the coronavirus. The antibody level fluctuated up to 200 times depending on the test person. Especially in seriously ill people, a large number of antibodies could be detected. In contrast, the value of B and T cells hardly differed, regardless of the severity of the disease of the affected person. According to the American researchers, the immune defense, which can still be demonstrated after eight months, should be sufficient to prevent a new infection. The protection is likely to last even longer.
The first results of an Oxford University study also show that immunity should last for at least six months. From April to November, the research team examined more than 12,000 UK healthcare workers known to have contracted the coronavirus. None of the study participants became ill again during this time.
Researchers do not yet know how high the level of antibodies and memory cells must be in order to safely assume immunity. Other coronaviruses that cause colds can apparently strike again quickly. According to a summer study, it can even become re-infected in a few months. However, these cold viruses also trigger significantly milder symptoms, while Covid-19 can be fatal.
Vaccination could be even more reliable than a real infection
Researchers suspect that immunity to serious diseases lasts much longer. After infection with a coronavirus, which causes severe symptoms in animals, the mice built up defenses that protected them from reinfection throughout their lives. Even with Sars and Mers corona diseases, which are serious for humans, specific antibodies could be detected in the blood of survivors for at least two years. In Sars, the researchers found specialized T cells in the blood 17 years after the actual infection.
How long immunity to Covid-19 will last, researchers can only say with certainty when there are more and more reinfections. There are already some reports of new infections. Compared to the millions of people around the world now verifiably infected with the virus, they have so far barely been significant.
However, one thing is remarkable in studies, whether from Freiburg or California: In some recovered people, there is no evidence of long-term immunity. “We still cannot say why this is the case,” says Hofmann. It is possible that those affected were only exposed to a few viruses that the immune system could quickly turn off. Therefore, the body’s own defense could have worked without tougher weapons.
On the contrary, a vaccine, the researchers hope, could trigger a similar immune response in all vaccinated people and, in certain cases, even protect them better than a real infection. Several candidate vaccines were able to show in initial studies that they not only stimulate the formation of antibodies, but also of T cells. “When I read about the high efficacy of the first vaccine candidates, I was very satisfied,” says Hofmann. “It can’t go any better. Of course, we will have to wait for the later results, but it looks promising.”
