The currently administered vaccinations against Covid-19 are intended to protect against the disease for as long as possible. However, how long the human immune system keeps contact with Sars-CoV-2 or its proteins in memory has so far been unclear. A US study is now providing encouraging data. According to this, important components of the immune memory are detectable in most patients even eight months after surviving an infection. This suggests that the protective immunization will last longer than initially feared. This is also good news for the expected vaccination effect.
When we become infected with the Sars-CoV-2 coronavirus, this triggers several defense reactions in our immune system. The most effective are specific antibodies that attach to the virus’s binding sites in order to neutralize it. This stops the virus from multiplying and with it the infection. “While complete immunity against viruses can only be achieved through high titers of neutralizing antibodies, successful protection against falling ill or dying can also be achieved through several other forms of immune memory,” explain Jennifer Dan from the La Jolla Institute for Immunology and her colleagues . This is ensured primarily by special memory cells in the immune system. B memory cells store the “building instructions” for the virus-specific antibodies and enable the production of large quantities of these antibodies quickly when the virus comes into contact with the virus. CD4 + and CD8 + T memory cells also “remember” certain proteins or protein parts of the virus and, in the event of a second infection, promote, among other things, the activation of T killer cells and T helper cells that release antiviral messenger substances.
Antibodies, memory B cells and T cells
Ideally, these components of the immune memory are retained for years, so that people remain protected for a long time after an initial infection or vaccination. Whether this is the case with Sars-CoV-2 and Covid-19 has so far been unclear. Studies indicate that some recovered Covid patients hardly have any neutralizing antibodies in their blood after a short time. In addition, there have been some cases of people who became infected again with the coronavirus a few weeks or months after an initial infection. This information raised the question of whether post-Covid-19 immunization might be short-lived. Since the newly developed corona vaccines also aim to cause the antibodies and T cells to react as long as possible, in extreme cases this could mean that a corona vaccination has to be repeated regularly.
To gain more clarity, the research team followed the immune events in 188 Covid-19 patients over eight months. The men and women had suffered Sars-CoV-2 infections of varying severity: some had hardly any symptoms, others had to be treated in intensive care. The scientists used blood samples to determine the content of the antibodies specific for the spike protein of Sars-CoV-2, of B memory cells, of CD4 + T cells and of CD8 + T memory cells. “As far as we know, this is the largest study ever conducted for an infection that examined all four of these components of immune memory,” says co-author Alessandro Sette of the University of California at San Diego.
Immune memory still after eight months
The evaluations showed that 95 percent of the patients still carried at least three components of the immune memory against Covid-19 five to eight months after surviving the infection. The amount of neutralizing antibodies decreased slowly over time in most of the test subjects, in some of the virus-specific immunoglobulin G antibodies were only detectable in small amounts after almost three months. Overall, however, 90 percent of the patients were still tested positive for neutralizing antibodies against Sars-CoV-2 even after six to eight months, as Dan and her team report. The defense and memory cells are also largely preserved for a long time. The memory B cells continued to gain weight even in the first four months after infection and then remained at a high level. “Based on our observations, B-cell memory is robust and probably long-lasting,” the researchers state.
The CD4 + T memory cells also proved to be similarly long-lived. They could be detected in 92 percent of the subjects six months after infection. In contrast, the virus-specific CD8 + T cells apparently last less long. After one month they were detectable in 70 percent of the participants, after six months only in half, as Dan and her team report. Their number slowly decreases over time. According to the researchers, the results overall suggest that the immune protection against Sars-CoV-2 lasts for at least six to eight months. “There is a good chance that people are protected for this time and probably beyond – at least against severe courses of Covid-19,” says Dan’s colleague Shane Crotty. That is also good news for the vaccinations. “It is quite possible that the immune memory lasts for a similar time after a vaccination, but we will only know for sure when we have the relevant data,” explains Dan’s colleague Daniela Weiskopf.
However, the study also confirmed that there are large individual differences in immune responses, which are then also reflected in immune memory. “It is therefore possible that a small proportion of those who have recovered with a weak immune memory will relatively quickly become susceptible to re-infection again,” the scientists write. In their study, however, this proportion was below five percent. Why the immune system reacts so weakly in some people is still unknown.
Source: Jennifer Dan (La Jolla Institute for Immunology, La Jolla) et al., Science, doi: 10.1126 / science.abf4063