What Do We Know About Antibodies After SARS-CoV2 Infection? A Study in the Catalan Population

This text has been written by Marianna Karachaliou, Gemma Moncunill, Assistant Research Professor, Carlota Dobaño, head of the ISGlobal Malaria Immunology Group, and Manolis Kogevinas, Scientific Director of the ISGlobal Severo Ochoa distinction.

If you get COVID-19, for how long are you protected against a reinfection? Are children and their parents responding to SARS-CoV-2 infection in the same way? Is the immune response different in those with severe disease as compared to those with asymptomatic infections, or in smokers and non-smokers, or in obese and normal weight people? These are some of the questions we addressed in the large COVICAT epidemiological prospective study in Catalonia.

Photo: Mariona Gil / Barcelona City Council

Our body has the ability to develop powerful specific immunity against pathogens that can last a long period of time. This is thanks to antibodies and lymphocytes, which block, attack and destroy the specific invading microorganism. This kind of responses are also produced upon SARS-CoV-2 infection. Antibodies are not of only one type. They can target different parts of the virus (e.g. the spike protein which is the target of the vaccines currently administered in Catalonia) and they can play different roles in our body (e.g. IgA antibodies, present in mucous membranes, prevent the entry of pathogens into the circulation).

A high number of different antibodies produced by our body against SARS-CoV-2 were measured in blood samples from Catalan participants of the big COVICAT study

A high number of different antibodies produced by our body against SARS-CoV-2 were measured in blood samples from Catalan participants of the big COVICAT study. COVICAT was developed shortly after the beginning of the pandemic and was carried out with 10,000 volunteers who were already part of different population cohorts. This project is a collaboration between the GCAT Genomes for Life Project of the Germans Trias i Pujol Research Institute (IGTP) and two research groups from ISGlobal (epidemiology and immunology). In addition to providing a blood sample, the participants completed an online questionnaire on several aspects of their life during and after the lockdown. And for every person we already had data previous to the pandemic (genetic data, their health history, lifestyle etc.), which provided a key added value to this epidemiological study.

Photo: Curro Palacios / Barcelona City Council

We used the results of the antibody testing and the characteristics of our study participants to estimate that about 15% of the adult population in Catalonia had been infected with SARS-CoV-2 by early autumn 2020. This percentage was much higher than what other studies had estimated for the region. We believe that this is because our expanded antibody testing was better at detecting previous COVID-19 infections. For example, about four out of ten infected people did not report any symptoms during the previous months, and many infected people developed specific antibodies of one type (e.g. IgA). These infections would have remained undetected by other serological tests.

We used the results of the antibody testing and the characteristics of our study participants to estimate that about 15% of the adult population in Catalonia had been infected with SARS-CoV-2 by early autumn 2020

Among those infected, antibodies persisted up to 9 months after infection, the longest follow-up period we had in this study. Apart from measuring the presence of antibodies as an indicator of previous infection, we also examined the types of antibody people develop in order to better understand what is going on. We saw that adults who had experienced a more severe infection (admitted to hospital or in an intensive care unit) had more robust and richer responses, even long after infection. We also tested whether lifestyle and individual factors were associated with the way someone responds to infection. What we found is that smokers presented weaker antibody responses after COVID-19 infection. This was a very constant finding even after taking into account the severity of infection, the age and the sex of the individuals. Therefore, we speculate that the higher COVID-19 morbidity among smokers might be due to impaired immunity, as reflected in these lower antibody levels. But we need more studies to confirm this.

We saw that adults who had experienced a more severe infection (admitted to hospital or in an intensive care unit) had more robust and richer responses, even long after infection

COVICAT also examined differences in how children respond to the virus, as compared to adults. We contacted 260 families of the INMA-Environment and Childhood Project Sabadell mother-child cohort, and we found that, within one family, adolescents were less likely than their parents to get infected. Also, adolescents mostly produced IgG antibodies against the SARS-CoV-2 spike protein but lacked anti-nucleocapsid responses, something that might indicate a less widespread infection.

We are still following up the study population, which will allow us to know how long does the protection last. We will also be able to soon share results related to infection with the new SARS-CoV-2 variants as well as to protection by the COVID-19 vaccines.