Protective antibodies against COVID-19 are maintained for a minimum of six months

Immunity to SARS-CoV-2 is, almost a year after the first cases of COVID-19, still a big unknown. Not all patients generate antibodies, and those that do are not certain that they are long-lasting. Now, research staff from IrsiCaixa, a center promoted jointly by "la Caixa" Foundation and the Department of Health of the Generalitat de Catalunya, confirms that COVID-19 patients generate neutralizing antibodies -capable of blocking the virus and, therefore, with a protective effect- for a minimum of six months from the moment of infection. These antibodies are kept at stable levels both in COVID-19 patients who are asymptomatic and in those who have required hospitalization. To obtain these results, the research staff has followed 210 people infected by SARS-CoV-2 during these six months and has grouped them according to the severity of the disease. Thus, they have also observed that in the case of the most severe patients the production of antibodies is always higher than in the mild cases. The study, published in BioRxiv and funded by Grifols, the Department of Health of the Generalitat de Catalunya, the ISCIII and the sponsorship campaigns #YoMeCorono, BonPreu/Esclat and Correos, is the result of a consortium created together with the Institute for AIDS Research (IrsiCaixa), the Center for Research in Animal Health (CReSA) at the Institute for Research and Technology in Agri-Food (IRTA) and the Barcelona Supercomputing Center-National Supercomputing Center (BSC-CNS), which is supported by Grifols.

Antibodies are an important part of the immune response generated specifically against a pathogen, such as a virus. In case of an infection, only neutralizing antibodies are able to block the virus and prevent it from entering the cells. "We have seen that the great majority of these antibodies block the S protein of the virus and this is what gives them the neutralizing capacity", explains Julià Blanco, main researcher at IrsiCaixa and the Germans Trias i Pujol Research Institute. "This information is very valuable. Knowing how the immune system is activated is making the process of developing a new vaccine much easier," adds Blanco. 

The study has included a group of 210 patients who had a positive CRP during the first or second wave of COVID-19; half of these patients passed the disease asymptomatically, and the other half required hospitalization due to respiratory problems. Thus, the research staff has been able to observe that hospitalized patients reach maximum antibody levels 17 days after presenting the first symptoms. From this peak and until the third month, the immune response declines in a pronounced way. "This decrease is expected and, in fact, necessary. Maintaining such a strong immune response for too long could be harmful, that is why the immune system partially decreases its activity," clarifies Edwards Pradenas, pre-doctoral researcher at IrsiCaixa and first author of the article. In spite of this decrease, scientists have demonstrated that the levels of antibodies remain high during the six months of the study.

In the case of asymptomatic or mildly symptomatic persons, the production of antibodies is also sustained, but, in this case, it is lower and does not show the initial peak or the decrease observed in severe patients. "Until now, the scientific community has not been able to clarify which is the minimum level of antibodies capable of protecting us from a reinfection. The results of this study, however, allow us to be optimistic and think that immunity, either in response to SARS-CoV-2 or provided by a vaccine, could be protective and long term," explains Benjamin Trinité, IrsiCaixa's associated researcher. 

The low levels of antibodies in mild or asymptomatic cases underline the importance of other mechanisms of the immune system, besides antibodies, in the protection of COVID-19, such as T cells. These cells are part of the innate immunity, the first line of defense of our body, and can play an important role in the absence of antibodies. 

"About 80% of people infected with SARS-CoV-2 have mild symptoms. It is essential that the scientific community study the course of the disease in this high percentage of the population. Only then can we better understand group immunity to this virus," says Blanco. In these cases, the research personnel propose several scenarios. In the first place, that the low levels of antibodies are enough to face the disease in the first days of the infection. Secondly, that they have pre-existing T cells activated by previous infections by other coronaviruses, such as the common cold virus, which also react against SARS-CoV-2. 

In this way, the research staff stresses the importance of studying the functioning of the immune response regulated by T cells in order to design and implement new strategies to fight COVID-19 in the absence of antibodies.